Microbial Metabolism

Page 1: Introduction to Microbial Metabolism

Microbial metabolism encompasses the full spectrum of biochemical processes operating within microscopic organisms. Understanding these pathways provides the foundational basis for clinical microbiology, infectious disease pathology, taxonomy, and biotechnology.

Page 2: Definition and Ultimate Function of Metabolism

Metabolism is formally defined as the collection of controlled biochemical reactions that take place within a microbe.

The ultimate and singular biological function of metabolism is to reproduce the organism. Every metabolic pathway, regulatory feedback loop, and structural transformation serves to generate sufficient cellular components, energy, and genetic material to drive cell growth and division.

Page 3: The Eight Elementary Statements Guiding Metabolic Processes

All metabolic events across microbial life adhere to eight fundamental principles:

1. Every single cell acquires nutrients from its external environment.

2. Metabolism fundamentally requires energy, which cells obtain either from incident light or through the catabolism of acquired nutrients.

3. Chemical energy is concentrated and temporarily stored in high-energy phosphate bonds, predominantly within adenosine triphosphate ($\text{ATP}$).

4. Microbial cells catabolize nutrient molecules to assemble precursor metabolites.

5. Precursor metabolites, coupled with energy harvested from $\text{ATP}$ and directed by enzymes, are utilized in anabolic reactions.

6. Enzymes working alongside $\text{ATP}$ assemble precursor units into macromolecules.

7. Cells grow in mass and volume by systematically assembling macromolecules into cellular structures.

8. Cells reproduce via binary fission or related division mechanisms once they have doubled in size.

Page 4: Overview of Metabolic Cycling and Energy Interdependence

Metabolism operates as an integrated cycle alternating between the degradation of organic matter and the biosynthesis of complex macromolecules.

• Catabolism breaks down complex molecules into simple building blocks, releasing free energy.

• Anabolism takes simple building blocks and synthesizes complex structures, consuming cellular energy.

• The central energetic intermediary linking these two opposing arms is the adenosine diphosphate ($\text{ADP}$) and adenosine triphosphate ($\text{ATP}$) cycle, where phosphorylation stores energy and hydrolysis releases it:

$\text{ADP} + \text{P}_i + \text{energy} \rightleftharpoons \text{ATP} + \text{H}_2\text{O}$

Page 5: Catabolism Versus Anabolism

Metabolic pathways divide into two overarching classes based on bioenergetics and structural transformation:

Catabolic Pathways

• Function: Degrade larger, complex molecules into smaller, simpler end products.

• Thermodynamic profile: Exergonic, meaning they release net free energy.

• Biological role: Generate available free energy to phosphorylate $\text{ADP}$ into $\text{ATP}$, supply precursor metabolites, and release heat.

Anabolic Pathways

• Function: Synthesize complex macromolecules from the smaller products and precursor metabolites provided by catabolism.

• Thermodynamic profile: Endergonic, meaning they require more energy input than they release.

• Biological role: Build cellular architecture, drive motility, power active transport, and sustain cell division.

Page 6: Energy Flow, Precursor Pool, and Cellular Architecture

The coordinated relationship between catabolic and anabolic pathways directs cellular economy and material flow.

1. External nutrients enter the cell across boundary membranes.

2. Catabolic breakdown of these nutrients releases energy, which is captured by phosphorylating $\text{ADP}$ to $\text{ATP}$, while unavoidable thermodynamic inefficiency leads to energy lost as heat.

3. Catabolism simultaneously yields a core pool of precursor molecules.

4. Anabolic pathways harness energy stored in $\text{ATP}$ (converting it back to $\text{ADP}$ and $\text{P}_i$) and utilize precursor molecules to generate larger building blocks.

5. Building blocks polymerize into cellular macromolecules, including polysaccharides, polypeptides, polynucleotides, and complex lipids.

6. Macromolecules are assembled into cellular structures (such as lipid membranes, ribosomes, cell walls, and flagella) or routed into cellular processes (cell growth, DNA replication, and cell division), with excess chemical energy directed into energy storage polymers (carbohydrates, lipids).

Page 7: Oxidation and Reduction Reactions in Microbial Metabolism

Metabolic energy generation relies fundamentally on oxidation-reduction (redox) reactions. A redox reaction consists of the simultaneous transfer of electrons from an electron donor molecule to an electron acceptor molecule.

• Oxidation: The loss of one or more electrons by a chemical entity.

• Reduction: The gain of one or more electrons by a chemical entity.

• Co-occurrence: Oxidation and reduction reactions always occur concurrently; an electron cannot exist in isolation in biological solution and must be accepted immediately by a partner molecule.

• Hydrogen transfer: In biological systems, electron transfers frequently involve the transfer of an entire hydrogen atom ($\text{H}$), which comprises one proton ($\text{H}^+$) and one electron ($e^-$). Therefore, biological oxidation is frequently a dehydrogenation reaction.

Major Cellular Electron Carriers

Cells utilize specialized organic coenzymes as soluble electron carriers to shuttle reducing power between metabolic steps:

1. Nicotinamide adenine dinucleotide ($\text{NAD}^+$): Operates primarily in catabolic breakdown pathways.

2. Nicotinamide adenine dinucleotide phosphate ($\text{NADP}^+$): Operates primarily in anabolic biosynthetic pathways and photosynthesis.

3. Flavin adenine dinucleotide ($\text{FAD}$): Serves as a tightly bound electron shuttle in pathways such as the Krebs cycle and fatty acid catabolism.

Page 8: Mechanisms of Electron Transfer

During a redox pair interaction, the electron donor becomes oxidized upon transferring electrons, whereas the electron acceptor becomes reduced.

The process follows the standard thermodynamic transfer pathway:

$\text{Electron Donor (reduced)} + \text{Electron Acceptor (oxidized)} \rightarrow \text{Donor (oxidized)} + \text{Acceptor (reduced)}$

Page 9: Electron Carriers and Reducing Power Dynamics

Electron carrier cofactors alternate between oxidized and reduced states to transfer energy throughout the cytoplasm:

$\text{NAD}^+ + 2\,e^- + 2\,\text{H}^+ \rightarrow \text{NADH} + \text{H}^+$

$\text{NADP}^+ + 2\,e^- + 2\,\text{H}^+ \rightarrow \text{NADPH} + \text{H}^+$

$\text{FAD} + 2\,e^- + 2\,\text{H}^+ \rightarrow \text{FADH}_2$

These reduced coenzymes carry high-potential electrons that are subsequently harvested during oxidative phosphorylation to yield large quantities of $\text{ATP}$.

Page 10: ATP Production and Phosphorylation Mechanisms

Energy extracted from nutrient oxidation is concentrated within the phosphoanhydride bonds of $\text{ATP}$. Phosphorylation refers to the addition of an inorganic phosphate group ($\text{P}_i$) to an organic substrate.

Microbial cells phosphorylate $\text{ADP}$ to form $\text{ATP}$ through three distinct mechanisms:

1. Substrate-Level Phosphorylation: Direct enzymatic transfer of a phosphate group from a phosphorylated organic metabolic intermediate directly to $\text{ADP}$. Occurs during glycolysis and the Krebs cycle.

2. Oxidative Phosphorylation: Synthesis of $\text{ATP}$ driven by energy released from a series of redox reactions within an electron transport chain, creating a proton electrochemical gradient that powers $\text{ATP}$ synthase.

3. Photophosphorylation: Synthesis of $\text{ATP}$ driven by light-activated electron transport chains in photosynthetic cells, coupling photon absorption to proton pumping and $\text{ATP}$ synthase activity.

Anabolic pathways access this conserved chemical potential by hydrolyzing the terminal phosphate bond of $\text{ATP}$, yielding $\text{ADP}$, $\text{P}_i$, and free energy.

Page 11: Nature and Catalytic Properties of Enzymes

Enzymes are organic catalysts that dramatically increase the rate and likelihood of chemical reactions without being permanently altered or consumed during the process.

• Thermodynamic neutrality: Enzymes do not alter the overall free energy change ($\Delta G$) of a reaction; they cannot make an endergonic reaction proceed spontaneously without energy coupling.

• Specificity: Enzymes exhibit structural and chemical specificity for their specific substrates.

Page 12: Activation Energy and Catalytic Acceleration

Chemical transformations require an initial input of energy, termed the activation energy ($E_a$), to destabilize chemical bonds within reactants and push them into an unstable transition state.

• Uncatalyzed reactions: Possess a tall activation energy barrier. At physiological temperatures, few reactant molecules possess sufficient thermal kinetic energy to overcome this threshold, resulting in extremely slow reaction rates.

• Catalyzed reactions: The enzyme interacts with the reactant, stabilizing the transition state and drastically lowering the activation energy required. Consequently, a vast majority of reactant molecules readily convert to products at physiological temperatures.

Page 13: Functional Categories of Metabolic Enzymes

Enzymes are classified into six primary functional categories based on the specific type of chemical transformation they catalyze:

1. Hydrolases: Catalyze the cleavage of chemical bonds through the addition of water (hydrolysis reactions; predominantly catabolic).

2. Isomerases: Catalyze the structural rearrangement of atoms within a single molecule (neither strictly catabolic nor anabolic).

3. Ligases (Polymerases): Catalyze the joining of two or more distinct chemical compounds using energy derived from $\text{ATP}$ cleavage (anabolic).

4. Lyases: Catalyze the splitting of chemical bonds without utilizing water or oxidation-reduction mechanisms (catabolic).

5. Oxidoreductases: Catalyze the transfer of electrons or hydrogen atoms between donor and acceptor molecules in oxidation-reduction pathways.

6. Transferases: Catalyze the transfer of functional groups (such as methyl, phosphate, or acetyl groups) from one molecule to another (often anabolic).

Page 14: Systematic Classification of Enzymes (Table 5.1)

Page 15: Structural Architecture of Enzymes

Enzymes possess distinct structural compositions essential for catalytic competency:

• Simple enzymes: Consist entirely of one or more polypeptide protein chains that are catalytically functional without nonprotein components.

• Conjugated enzymes: Composed of protein and nonprotein components.

  • Apoenzyme: The pure protein portion of a conjugated enzyme; catalytically inactive on its own.

  • Cofactor: The nonprotein chemical component required for enzyme activity. Cofactors may be inorganic ions (such as metal cations) or organic molecules.

  • Coenzyme: An organic cofactor, typically synthesized from vitamin precursors.

  • Holoenzyme: The active, intact biochemical complex formed by the binding of an apoenzyme to its required cofactor(s) and/or coenzyme(s).

• Ribozymes: Non-protein catalytic RNA molecules. Ribozymes process RNA molecules and catalyze peptide bond formation within ribosomes.

Page 16: Holoenzyme Architecture and Component Assembly

The assembly of an active holoenzyme requires precise molecular association:

• The apoenzyme provides the three-dimensional protein scaffold and forms the physical active site pocket.

• The inorganic cofactor (e.g., metal ion) coordinates substrate orientation or stabilizes electrostatic charges.

• The coenzyme (organic cofactor) directly participates in group transfer or electron shuttling.

• Binding of both cofactors to the apoenzyme creates the functional active site capable of accommodating and transforming the substrate.

Page 17: Coenzymes, Inorganic Cofactors, and Vitamin Precursors (Table 5.2)

Page 18: Enzymatic Energy Profiling and Activation Energy Lowering

Thermodynamic coordination of enzymatic catalysts operates by restructuring bond strain:

• The uncatalyzed pathway requires substantial kinetic collisions to disrupt stable ground-state bonds, establishing a large activation energy requirement ($E_a$).

• The enzyme-catalyzed pathway provides an alternative reaction coordinate via transient intermediate states, reducing the activation energy ceiling while preserving identical free-energy levels for initial reactants and final products.

Page 19: Induced-Fit Model of Enzyme-Substrate Binding

Enzyme specificity relies upon complementary structural geometry between the active site and substrate molecules.

• Lock-and-key model: Posited a completely rigid, unyielding active site matching the substrate shape prior to binding.

• Induced-fit model: The active site possesses a shape roughly complementary to the substrate. As the substrate approaches and initiates contact, the enzyme undergoes conformational changes, molding the active site precisely around the substrate to generate the enzyme-substrate complex.

• Strain induction: This conformational shift places mechanical and electrostatic strain on specific chemical bonds within the substrate, positioning catalytic amino acid residues in proximity to accelerate bond cleavage or synthesis.

Page 20: Catalytic Mechanism of Fructose-1,6-Bisphosphate Aldolase

The enzymatic cleavage of fructose 1,6-bisphosphate by fructose-1,6-bisphosphate aldolase exemplifies lyase function:

1. Free enzyme aldolase presents an unoccupied active site.

2. Substrate fructose 1,6-bisphosphate enters the active site pocket.

3. Formation of the enzyme-substrate complex induces conformational fit, stressing the carbon-carbon bond between carbons 3 and 4.

4. Aldolase cleaves the six-carbon backbone without the addition of water, releasing two distinct phosphorylated three-carbon products: dihydroxyacetone phosphate ($\text{DHAP}$) and glyceraldehyde 3-phosphate ($\text{G3P}$).

5. The unchanged enzyme regenerates its original active site configuration to engage subsequent substrate molecules.

Page 21: Stepwise Progression of Enzymatic Catalysis

Enzymatic turnover cycles through four sequential events:

1. Substrate approach: Substrates ($\text{A}$ and $\text{B}$ or compound $\text{AB}$) diffuse into the catalytic cleft of the enzyme.

2. Complex formation: Substrates bind to complementary residues, forming the transient enzyme-substrate complex.

3. Chemical reorganization: Bonds within the substrates are broken, rearranged, or synthesized via general acid-base catalysis, nucleophilic attack, or charge stabilization.

4. Product dissociation: Products ($\text{A} + \text{B}$) have decreased affinity for the active site and dissociate away into the surrounding cytoplasm, restoring the free enzyme.

Page 22: Environmental Factors Influencing Enzyme Kinetics

The catalytic efficiency of any enzyme is governed by environmental variables and regulatory molecules:

• Temperature: Governs molecular collision rates and internal conformational stability.

• pH: Governs the ionization state of acidic and basic amino acid side chains within the active site.

• Enzyme concentration: Linearly regulates maximal catalytic capacity under saturating conditions.

• Substrate concentration: Governs the proportion of active sites occupied at any given time.

• Presence of inhibitors: Molecules that interact specifically with the enzyme to diminish or abolish catalytic output without causing generalized denaturation.

Page 23: Temperature, pH, and Substrate Saturation Curves

Temperature Kinetics

• Increasing temperature elevates kinetic energy, increasing collision frequency between enzyme and substrate, which raises reaction velocity.

• Thermal optimum: Human pathogens possess an optimal temperature around $37\,^\circ\text{C}$.

• Thermal denaturation: Temperatures exceeding the optimum cause excessive molecular vibration, disrupting noncovalent bonds (hydrogen bonds, ionic interactions), causing the active site to collapse. Activity drops sharply toward zero by $50\,^\circ\text{C}$.

pH Kinetics

• Acidic or alkaline extremes alter the protonation state of catalytic residues (such as carboxylate or amino groups), abolishing ionic bonds and catalytic ability.

• Enzymes exhibit distinct bell-shaped curves with an optimal pH range (typically between pH $7.0$ and $8.0$ for neutrophilic microorganisms).

Substrate Saturation Kinetics

• At low substrate concentrations, active sites are unoccupied; increasing substrate concentration yields a linear rise in reaction velocity.

• Saturation point: When every active site of every enzyme molecule is continuously occupied ($100\%$ saturation), the reaction reaches its maximum velocity ($V_{\max}$). Adding further substrate cannot increase the reaction velocity.

Page 24: Protein Denaturation and Loss of Catalytic Integrity

• Functional protein: Possesses an organized, three-dimensional native structure containing alpha-helices, beta-pleated sheets, and compact tertiary folds that align critical catalytic residues.

• Denatured protein: Exposure to high heat, extreme pH, or chaotropic chemicals unfolds the polypeptide chain. Secondary and tertiary structures unravel into random coils, destroying the active site geometry and causing irreversible loss of biological activity.

Page 25: Mechanisms of Competitive Inhibition

Enzyme inhibitors are chemical agents that diminish catalytic activity without denaturing the enzyme protein.

• Competitive inhibitors: Exhibit structural similarity to the natural substrate.

• Mechanism: The inhibitor directly binds to the active site pocket, physically blocking substrate entry.

• Reversibility: Reversible competitive inhibition can be overcome by increasing the substrate concentration, which outcompetes the inhibitor for unoccupied active sites.

Page 26: Antibacterial Action of Sulfanilamide via Competitive Inhibition

The antibacterial mechanism of sulfonamide drugs illustrates competitive inhibition:

• Natural substrate: Para-aminobenzoic acid ($\text{PABA}$), an essential intermediate required by bacteria to synthesize folic acid ($\text{Vitamin } \text{B}_9$), which is necessary for purine and pyrimidine nucleotide production.

• Chemical structure of $\text{PABA}$: Contains a benzene ring substituted with a primary amino group ($-\text{NH}_2$) and a carboxyl group ($-\text{COOH}$).

• Competitive inhibitor: Sulfanilamide.

• Chemical structure of sulfanilamide: Contains a benzene ring substituted with an identical primary amino group ($-\text{NH}_2$) and a sulfonamide moiety ($-\text{SO}_2\text{NH}_2$) in place of the carboxyl group.

• Consequence: Sulfanilamide binds into the active site of the bacterial enzyme (dihydropteroate synthase), halting folic acid biosynthesis. Because humans obtain folic acid directly from dietary sources and lack this enzymatic pathway, sulfanilamide exhibits selective toxicity against bacteria.

Page 27: Allosteric Regulation: Inhibition and Activation

Allosteric regulation controls enzyme velocity via binding events outside the catalytic active site:

• Allosteric site: A regulatory binding cleft spatially separate from the active site.

• Noncompetitive (Allosteric) Inhibition: Binding of an allosteric inhibitor alters the tertiary conformation of the enzyme, distending the active site so that the natural substrate can no longer bind or undergo catalysis.

• Allosteric Activation: Occurs in enzymes that remain naturally inactive due to an improperly configured active site. Binding of an allosteric activator to the allosteric site induces a conformational shift that converts the active site into a functional configuration, permitting substrate binding and catalysis.

Page 28: Kinetics of Noncompetitive Inhibition

Noncompetitive inhibition displays unique kinetics relative to competitive inhibition:

• Binding site: Noncompetitive inhibitors bind to allosteric sites rather than competing for the active site.

• Independence from substrate concentration: Because the inhibitor and substrate occupy different physical locations, increasing substrate concentration cannot displace the allosteric inhibitor.

• Net kinetic outcome: Reduces the effective concentration of functional enzyme, lowering the theoretical maximum velocity ($V_{\max}$) of the reaction.

Page 29: End-Product Feedback Inhibition

Feedback inhibition (negative feedback) regulates multi-step biosynthetic pathways to prevent metabolic waste:

1. Initial pathway state: Substrate enters the pathway and is transformed by Enzyme 1 into Intermediate A, then by Enzyme 2 into Intermediate B, and finally by Enzyme 3 into the designated End-product.

2. Threshold accumulation: As the end-product accumulates beyond the immediate metabolic requirements of the cell, its concentration rises.

3. Allosteric shutdown: The end-product binds to an allosteric site on the first pacemaker enzyme (Enzyme 1).

4. Pathway arrest: Allosteric inhibition of Enzyme 1 stops the generation of Intermediate A, halting all downstream reactions in the sequence.

5. Pathway reactivation: When the cell consumes its supply of the end-product, the allosteric sites on Enzyme 1 empty, restoring catalytic function and reopening the biosynthetic pathway.

Page 30: Carbohydrate Catabolism Overview

Carbohydrates represent the primary source of cellular energy for synthesizing $\text{ATP}$ in heterotrophic organisms:

• Glucose is the primary and most ubiquitous hexose catabolized by microbes.

• Glucose catabolism proceeds via two main pathways:

  1. Cellular Respiration: Complete oxidation of glucose to carbon dioxide ($\text{CO}_2$) and water ($\text{H}_2\text{O}$), utilizing glycolysis, the Krebs cycle, and an electron transport chain with inorganic terminal electron acceptors.

  2. Fermentation: Incomplete oxidation of glucose utilizing cellular organic molecules as terminal electron acceptors, yielding lower $\text{ATP}$ amounts without requiring an electron transport chain.

Page 31: Comparative Architecture of Glucose Catabolism

• Common entry point: Glucose ($6\text{C}$) undergoes glycolysis to yield two molecules of pyruvic acid ($3\text{C}$), generating a net output of $2\,\text{ATP}$ and $2\,\text{NADH}$.

• Divergence point:

  • Respiration route: Pyruvic acid is converted into acetyl-CoA, which enters the Krebs cycle. Extracted high-energy electrons are shuttled via $\text{NADH}$ and $\text{FADH}_2$ to the electron transport chain, generating a proton gradient that drives substantial $\text{ATP}$ production.

  • Fermentation route: Pyruvic acid or a derivative serves as an electron acceptor to reoxidize $\text{NADH}$ back to $\text{NAD}^+$, releasing organic fermentation products without producing additional $\text{ATP}$.

Page 32: Principles and Stoichiometry of Glycolysis

Glycolysis (the Embden-Meyerhof-Parnas [EMP] pathway) is a ubiquitous metabolic pathway occurring within the cytoplasm of most living cells:

• Substrate: One molecule of the six-carbon sugar glucose ($\text{C}_6\text{H}_{12}\text{O}_6$).

• Cleavage event: Splitting of the six-carbon backbone into two identical three-carbon molecules of pyruvic acid ($\text{CH}_3\text{COCOOH}$).

• Phosphorylation method: Produces $\text{ATP}$ exclusively through substrate-level phosphorylation.

• Net stoichiometry per molecule of oxidized glucose:

  • $2\,\text{ATP}$ consumed (investment)

  • $4\,\text{ATP}$ synthesized (payoff)

  • Net yield = $2\,\text{ATP}$

  • Net reducing power = $2\,\text{NADH}$

  • Precursor metabolites = $2\,\text{pyruvic acid}$

Page 33: Overview of the Glycolytic Stages

Glycolysis is organized into two overarching phases: the Preparatory Stage (energy investment) and the Energy-Conservation Stage (energy payoff).

• Preparatory Stage: Consumes two molecules of $\text{ATP}$ to activate the hexose ring, converting glucose into fructose 1,6-bisphosphate, which is then cleaved into two interconvertible three-carbon intermediates: dihydroxyacetone phosphate ($\text{DHAP}$) and glyceraldehyde 3-phosphate ($\text{G3P}$).

• Energy-Conservation Stage: Both three-carbon units (as $\text{G3P}$) are oxidized and phosphorylated, generating two molecules of $\text{NADH}$ and synthesizing four molecules of $\text{ATP}$ via substrate-level phosphorylation, ending with two molecules of pyruvic acid.

Page 34: The Three Functional Divisions of Glycolysis

Biochemically, glycolysis comprises ten distinct enzymatic steps divided into three functional stages:

1. Energy-Investment Stage (Steps 1–3): Two phosphate groups derived from two $\text{ATP}$ molecules are transferred to glucose, forming fructose 1,6-bisphosphate.

2. Lysis Stage (Steps 4–5): Fructose 1,6-bisphosphate is cleaved into $\text{G3P}$ and $\text{DHAP}$, followed by the isomerization of $\text{DHAP}$ into a second $\text{G3P}$.

3. Energy-Conserving Stage (Steps 6–10): Oxidation of $\text{G3P}$ coupled to inorganic phosphorylation produces high-energy acyl-phosphates, which donate phosphate groups to $\text{ADP}$ to generate four $\text{ATP}$ and two pyruvic acid molecules.

Page 35: The Ten Enzymatic Steps of Glycolysis (Figure 5.13)

Stage 1: Energy Investment

• Step 1: Glucose is phosphorylated by $\text{ATP}$ via hexokinase to form glucose 6-phosphate (consumes $1\,\text{ATP}$, trapping glucose within the cell).

• Step 2: Glucose 6-phosphate is isomerized by phosphoglucoisomerase to form fructose 6-phosphate.

• Step 3: Fructose 6-phosphate is phosphorylated by $\text{ATP}$ via phosphofructokinase to form fructose 1,6-bisphosphate (consumes $1\,\text{ATP}$; key regulatory step).

Stage 2: Lysis

• Step 4: Fructose 1,6-bisphosphate is cleaved by aldolase into glyceraldehyde 3-phosphate ($\text{G3P}$) and dihydroxyacetone phosphate ($\text{DHAP}$).

• Step 5: $\text{DHAP}$ is isomerized by triose phosphate isomerase into a second molecule of $\text{G3P}$. All subsequent steps occur twice per initial glucose.

Stage 3: Energy Conservation

• Step 6: Two molecules of $\text{G3P}$ are oxidized while adding inorganic phosphate ($\text{P}_i$) via glyceraldehyde-3-phosphate dehydrogenase, reducing two $\text{NAD}^+$ to two $\text{NADH}$ and producing two molecules of 1,3-bisphosphoglyceric acid ($1,3\text{-BPG}$).

• Step 7: Phosphoglycerate kinase transfers a high-energy phosphate group from each $1,3\text{-BPG}$ to $\text{ADP}$, yielding two molecules of 3-phosphoglyceric acid ($3\text{-PGA}$) and synthesizing $2\,\text{ATP}$ via substrate-level phosphorylation.

• Step 8: Phosphoglycerate mutase relocates the phosphate group from carbon 3 to carbon 2, converting $3\text{-PGA}$ to 2-phosphoglyceric acid ($2\text{-PGA}$).

• Step 9: Enolase removes a water molecule from each $2\text{-PGA}$ (dehydration), introducing a high-energy enol-phosphate bond to form two molecules of phosphoenolpyruvic acid ($\text{PEP}$).

• Step 10: Pyruvate kinase transfers the high-energy phosphate from each $\text{PEP}$ to $\text{ADP}$, generating $2\,\text{ATP}$ via substrate-level phosphorylation and leaving two molecules of pyruvic acid ($3\text{C}$).

Page 36: Oxidation and Electron Harvest in Glycolysis

Step 6 of glycolysis constitutes the critical redox reaction of the pathway:

$\text{Glyceraldehyde 3-phosphate} + \text{NAD}^+ + \text{P}_i \xrightarrow{\text{G3P Dehydrogenase}} 1,3\text{-Bisphosphoglyceric acid} + \text{NADH} + \text{H}^+$

• Oxidation: The aldehyde carbon of $\text{G3P}$ is oxidized to a carboxylic acid derivative.

• Electron transfer: High-energy electrons and a proton are transferred to $\text{NAD}^+$, reducing it to $\text{NADH}$.

• Conservation of energy: The exergonic oxidation drives the incorporation of an inorganic phosphate ion ($\text{P}_i$) to establish a high-energy mixed anhydride bond in $1,3\text{-BPG}$.

Page 37: Substrate-Level Phosphorylation via Phosphoenolpyruvate

Substrate-level phosphorylation relies on phosphorylated organic intermediates possessing higher phosphate transfer potential than $\text{ATP}$.

• The conversion of phosphoenolpyruvate ($\text{PEP}$) to pyruvic acid demonstrates this mechanism:

  • The active site of pyruvate kinase coordinates $\text{PEP}$ and $\text{ADP}$ via an essential divalent magnesium cation ($\text{Mg}^{2+}$).

  • The high-energy enol-phosphate bond in $\text{PEP}$ undergoes nucleophilic attack by the terminal oxygen of $\text{ADP}$.

  • The phosphate group transfers directly to $\text{ADP}$, yielding $\text{ATP}$ while tautomerizing $\text{PEP}$ into pyruvic acid.

Page 38: Cellular Respiration Overview

Following glycolysis, cellular respiration completely oxidizes the resulting pyruvic acid to generate $\text{ATP}$ through sequential redox reactions.

Respiration consists of three interconnected stages:

1. Synthesis of Acetyl-CoA (the transition step or bridge reaction).

2. The Krebs cycle (citric acid cycle or tricarboxylic acid [TCA] cycle).

3. Final series of redox reactions within an electron transport chain ($\text{ETC}$), coupled to chemiosmosis.

Page 39: Synthesis of Acetyl-CoA from Pyruvic Acid

Before entering the Krebs cycle, pyruvic acid must be converted into acetyl-coenzyme A via the multi-enzyme pyruvate dehydrogenase complex:

1. Decarboxylation: The carboxyl group ($-\text{COOH}$) of pyruvic acid ($3\text{C}$) is removed as carbon dioxide ($\text{CO}_2$), leaving a two-carbon acetate group ($\text{CH}_3\text{CHO}$).

2. Oxidation: The two-carbon acetate group is oxidized, transferring two electrons and a proton to $\text{NAD}^+$ to form $\text{NADH} + \text{H}^+$.

3. Attachment of Coenzyme A: Coenzyme A ($\text{CoA}$) attaches to the remaining two-carbon acetyl moiety via a high-energy sulfur-carbon thioester bond, forming acetyl-CoA ($\text{CH}_3\text{CO-CoA}$).

Page 40: Stoichiometry of the Bridge Reaction

Because glycolysis yields two molecules of pyruvic acid per glucose molecule, the synthesis of acetyl-CoA operates twice per initial hexose:

$2\,\text{Pyruvic acid} + 2\,\text{CoA} + 2\,\text{NAD}^+ \rightarrow 2\,\text{Acetyl-CoA} + 2\,\text{CO}_2 + 2\,\text{NADH} + 2\,\text{H}^+$

• Carbon output: Releases two molecules of $\text{CO}_2$.

• Reducing output: Yields two molecules of $\text{NADH}$.

• Carbon input for Krebs: Delivers two two-carbon units (as acetyl-CoA) to the Krebs cycle.

Page 41: Localization and Nature of the Krebs Cycle

The Krebs cycle is a cyclic sequence of eight enzymatic steps that harvests the remaining chemical potential residing within the acetyl groups of acetyl-CoA:

• Energy retention: A significant amount of chemical energy remains locked within the thioester bonds and carbon backbone of acetyl-CoA.

• Energy transfer: The cycle systematically transfers this energy to electron-carrier coenzymes ($\text{NAD}^+$ and $\text{FAD}$).

• Cellular localization:

  • In prokaryotes: Takes place directly in the cytosol.

  • In eukaryotes: Takes place within the matrix of the mitochondria.

Page 42: Reaction Classes in the Krebs Cycle

The eight steps of the Krebs cycle utilize six distinct classes of biochemical reactions:

1. Anabolism: Synthesis of six-carbon citric acid from two-carbon acetyl-CoA and four-carbon oxaloacetic acid.

2. Isomerization: Rearrangement of citrate to isocitrate.

3. Redox reactions: Four specific dehydrogenation steps reducing $\text{NAD}^+$ or $\text{FAD}$.

4. Decarboxylations: Two oxidative decarboxylations releasing carbon atoms as $\text{CO}_2$.

5. Substrate-level phosphorylation: Cleavage of a high-energy thioester bond coupled to $\text{GTP}$ and $\text{ATP}$ synthesis.

6. Hydration reaction: Addition of water across double bonds (fumarate to malate).

Page 43: Complete Cyclic Pathway of the Krebs Cycle (Figure 5.16)

1. Condensation (Step 1): Acetyl-CoA ($2\text{C}$) combines with oxaloacetic acid ($4\text{C}$) via citrate synthase to form citric acid ($6\text{C}$); coenzyme A is released.

2. Isomerization (Step 2): Citric acid is isomerized to isocitric acid ($6\text{C}$) by aconitase.

3. First Oxidative Decarboxylation (Step 3): Isocitric acid is oxidized and decarboxylated by isocitrate dehydrogenase to form $\alpha$-ketoglutaric acid ($5\text{C}$), releasing one $\text{CO}_2$ and producing one $\text{NADH}$.

4. Second Oxidative Decarboxylation (Step 4): $\alpha$-Ketoglutaric acid is oxidized and decarboxylated by the $\alpha$-ketoglutarate dehydrogenase complex with $\text{CoA}$ addition, producing succinyl-CoA ($4\text{C}$), releasing one $\text{CO}_2$, and producing one $\text{NADH}$.

5. Substrate-Level Phosphorylation (Step 5): Succinyl-CoA synthetase cleaves the thioester bond, transferring energy to synthesize $\text{GTP}$ from $\text{GDP} + \text{P}_i$. $\text{GTP}$ then phosphorylates $\text{ADP}$ to $\text{ATP}$, releasing $\text{CoA}$ and succinic acid ($4\text{C}$).

6. Dehydrogenation via FAD (Step 6): Succinic acid is oxidized to fumaric acid ($4\text{C}$) by succinate dehydrogenase, reducing $\text{FAD}$ to $\text{FADH}_2$.

7. Hydration (Step 7): Fumarase adds a water molecule across the double bond of fumaric acid, converting it to malic acid ($4\text{C}$).

8. Regeneration Dehydrogenation (Step 8): Malic acid is oxidized by malate dehydrogenase to regenerate oxaloacetic acid ($4\text{C}$), producing the third $\text{NADH}$.

Page 44: Detailed Isomerization and Early Harvest in the Krebs Cycle

During the initial turn of the cycle:

• The conversion of symmetrical citric acid into asymmetrical isocitric acid by the lyase aconitase repositions a hydroxyl group ($-\text{OH}$) to carbon 2, making the molecule susceptible to subsequent oxidative decarboxylation.

• Each $\text{NADH}$ formed preserves reducing power equivalent to approximately $3\,\text{ATP}$ molecules when processed via oxidative phosphorylation.

Page 45: Energy Transduction via Succinyl-CoA Cleavage

Step 5 of the Krebs cycle harnesses energy directly into nucleotide triphosphates:

$\text{Succinyl-CoA} + \text{GDP} + \text{P}_i \rightarrow \text{Succinic acid} + \text{GTP} + \text{CoA}$

$\text{GTP} + \text{ADP} \rightleftharpoons \text{GDP} + \text{ATP}$

This substrate-level phosphorylation preserves the high chemical bond energy of the intermediate without relying on membrane-bound complexes.

Page 46: Net Output and Energy Harvest of the Krebs Cycle

Because two molecules of acetyl-CoA result from one initial glucose molecule, the Krebs cycle turns twice per glucose, yielding:

• $2\,\text{ATP}$ (via substrate-level phosphorylation from $\text{GTP}$\,\rightarrow\,\text{ATP})\n- 2\,\text{FADH}2\n- 6\,\text{NADH}\n- 4\,\text{CO}_2\n\nCombined with the transition reaction, all six carbons of the original glucose molecule are released as six molecules of \text{CO}_2.\n\n\n# Page 47: Electron Transport Chain Architecture and Proton Pumping\n\nThe electron transport chain (\text{ETC}) is a membrane-embedded sequence of redox-active carrier proteins that transfers electrons down an electrochemical gradient:\n\n- Most significant ATP yield: The vast majority of \text{ATP} synthesized during respiration is generated through the electron transport chain coupled to chemiosmosis.\n- Mechanism: Electrons delivered by \text{NADH}$and$\text{FADH}_2 pass sequentially through carriers characterized by increasing reduction potentials.\n- Proton pumping: Energy released during exergonic electron transfer steps powers the active translocation of protons (\text{H}^+) across the membrane, establishing an electrochemical proton gradient (proton motive force).\n- Cellular membrane location:\n - In prokaryotes: Embedded in the cytoplasmic membrane.\n - In eukaryotes: Embedded in the inner mitochondrial membrane (cristae).\n\n\n# Page 48: The Stepwise Progression of Electron Carriers\n\nElectrons traverse the \text{ETC} in a defined order determined by their standard reduction potentials:\n\n\n\n1. \text{NADH}$donates two electrons to Flavin mononucleotide ($\text{FMN}$), returning to its oxidized state ($\text{NAD}^+).\n2. \text{FMN}$passes electrons to Iron-Sulfur ($\text{FeS}) proteins, pumping two protons across the membrane.\n3. Electrons move to Coenzyme Q (\text{CoQ}$, ubiquinone), which also accepts electrons directly from$\text{FADH}_2$(bypassing$\text{FMN}).\n4. Electrons flow through Cytochromes (\text{Cyt } b$,$\text{Cyt } c_1$,$\text{Cyt } c$,$\text{Cyt } a$,$\text{Cyt } a_3), triggering additional proton pumping cycles.\n5. Electrons reduce the terminal electron acceptor (\text{O}_2 in aerobic respiration).\n\n\n# Page 49: Structural Organization and Proton Redistribution\n\nAs electrons pass down the chain, directional proton movement polarizes the membrane:\n\n\n\n- The internal side of the membrane (cytoplasm in prokaryotes, matrix in eukaryotes) becomes depleted of protons, developing a net negative electrical charge and basic pH.\n- The external side (periplasmic space in bacteria, intermembrane space in mitochondria) accumulates protons, developing a net positive electrical charge and acidic pH.\n- This electrochemical gradient represents potential energy known as the proton motive force (\text{PMF}).\n\n\n# Page 50: Classes of Electron Carrier Molecules and Respiration Modes\n\nElectron transport chains utilize four primary classes of carrier molecules:\n\n1. Flavoproteins: Integral membrane proteins containing flavin prosthetic groups (such as \text{FMN}). They accept both electrons and protons but donate only electrons downstream.\n2. Ubiquinones (Coenzyme Q): Non-protein, lipid-soluble cofactors capable of rapid lateral diffusion within the hydrophobic core of the phospholipid bilayer.\n3. Metal-Containing Proteins: Proteins containing coordinated iron, sulfur, or copper atoms (\text{Fe-S}$clusters or$\text{Cu} centers) that cycle between reduced and oxidized states.\n4. Cytochromes: Integral proteins possessing iron-containing heme prosthetic groups that cycle between reduced (\text{Fe}^{2+}$) and oxidized ($\text{Fe}^{3+}) states.\n\n### Respiration Categories Based on Terminal Electron Acceptors\n- Aerobic Respiration: Molecular oxygen (\text{O}_2) acts as the final electron acceptor, reducing to water:\n\n\frac{1}{2}\,\text{O}_2 + 2\,\text{H}^+ + 2\,e^- \rightarrow \text{H}_2\text{O}\n\n- Anaerobic Respiration: An inorganic compound other than oxygen (such as nitrate [\text{NO}_3^-$], sulfate [$\text{SO}_4^{2-}$], or carbonate [$\text{CO}_3^{2-}]) acts as the terminal electron acceptor.\n\n\n# Page 51: Spatial Organization of Electron Transport (Figure 5.18)\n\n\n\n- In bacteria: Carriers reside in the cytoplasmic membrane; protons are translocated out into the periplasmic space between the cell membrane and cell wall.\n- In mitochondria: Carriers reside in the inner mitochondrial membrane; protons are translocated out into the intermembrane space between the inner and outer membranes.\n- Electron flow pathway: Complex I (\text{NADH}$dehydrogenase/$\text{FMN}$)$\rightarrow$Ubiquinone$\rightarrow$Complex III ($\text{Cyt } b/c_1$)$\rightarrow$Cytochrome$c$$\rightarrow$Complex IV ($\text{Cyt } a/a_3$)$\rightarrow$$\text{O}_2.\n\n\n# Page 52: Ubiquinone Function and Electron Shuttling\n\nUbiquinone (Coenzyme Q) functions as a mobile electron nexus within the phospholipid membrane:\n\n- It accepts electrons originating from \text{NADH}$via$\text{FMN} and Iron-Sulfur complexes.\n- It accepts electrons originating from \text{FADH}_2 through the succinate dehydrogenase complex (Complex II), bypassing Complex I.\n- Because electrons from \text{FADH}_2$enter downstream of the first proton-pumping complex, they drive fewer proton translocations, yielding less$\text{ATP}$per molecule than$\text{NADH}.\n\n\n# Page 53: Inhibitors, Uncouplers, and Factors Influencing ATP Yield\n\nChemical agents can disrupt oxidative phosphorylation via distinct mechanisms:\n\n- Cyanide and Azide: Bind specifically to the heme iron in Cytochrome oxidase (Complex IV), blocking electron transfer to oxygen. Electron transport halts, the proton gradient collapses, and cellular \text{ATP} production ceases.\n- Lack of final electron acceptor: In the absence of oxygen (in obligate aerobes), carriers back up in their reduced states, halting \text{NADH} oxidation and the Krebs cycle.\n- Uncoupling proteins: Provide an alternative membrane channel that allows protons to flow back down their electrochemical gradient without passing through \text{ATP}$synthase. Rather than synthesizing$\text{ATP}, the potential energy of the proton gradient is dissipated directly as heat (used for thermoregulation).\n\n\n# Page 54: Chemiosmosis and Oxidative Phosphorylation Mechanics\n\nChemiosmosis describes the synthesis of \text{ATP} driven by the passive flow of protons down their electrochemical gradient:\n\n- Proton motive force: The accumulation of protons outside the membrane creates both an electrical gradient (voltage difference) and a chemical gradient (pH difference).\n- ATP Synthase structure: A complex rotary nanomachine consisting of a membrane-spanning channel (F_0$subunit) and a catalytic head ($F_1 subunit) directed into the cytoplasm or mitochondrial matrix.\n- Catalysis: As protons flow passively through the F_0$channel, their passage drives the physical rotation of internal rotor proteins. This mechanical rotation induces conformational changes in the catalytic subunits of$F_1$, driving the condensation of$\text{ADP}$and$\text{P}_i$into$\text{ATP}.\n- Name origin: Designated oxidative phosphorylation because the generation of the proton gradient is driven by the oxidation of electron carriers.\n\n\n# Page 55: Maximum Theoretical ATP Yield in Prokaryotic Respiration (Table 5.3)\n\n| Pathway | ATP Produced | ATP Used | NADH Produced | FADH_2 Produced |\n| :--- | :--- | :--- | :--- | :--- |\n| **Glycolysis** | 4 (substrate-level) | 2 | 2 | 0 |\n| **Synthesis of acetyl-CoA and Krebs cycle** | 2 (substrate-level) | 0 | 8 (2 from bridge, 6 from Krebs) | 2 |\n| **Electron transport chain** | 34 (oxidative) | 0 | 0 | 0 |\n| **Total** | 40 | 2 | | |\n| **Net total** | **38** | | | |\n\n### Energetic Equivalence Calculations\n- Each \text{NADH}$entering the electron transport chain powers sufficient proton pumping to yield$3\,\text{ATP}.\n- Each \text{FADH}_2$entering downstream yields$2\,\text{ATP}.\n- Yield from reduced coenzymes:\n - 10\,\text{NADH} \times 3\,\text{ATP} = 30\,\text{ATP}\n - 2\,\text{FADH}_2 \times 2\,\text{ATP} = 4\,\text{ATP}\n - Total oxidative yield = 34\,\text{ATP}\n- Substrate-level phosphorylation yields: 2\,\text{ATP}$(net from glycolysis)$+ 2\,\text{ATP}$(from Krebs cycle)$= 4\,\text{ATP}.\n- Total net prokaryotic yield: 34 + 4 = 38\,\text{ATP} per glucose.\n- Eukaryotic yield difference: Eukaryotic aerobic respiration nets 36\,\text{ATP}$because two molecules of$\text{ATP}$are expended to transport the two cytoplasmic$\text{NADH} from glycolysis across the mitochondrial membranes into the matrix.\n\n\n# Page 56: Alternatives to Glycolysis\n\nSome microorganisms utilize alternative pathways to metabolize glucose. These alternative catabolic pathways produce fewer \text{ATP} molecules than standard glycolysis, but they generate specialized reduced coenzymes and essential precursor metabolites required for anabolism:\n\n1. Pentose phosphate pathway\n2. Entner-Doudoroff pathway\n\n\n# Page 57: The Pentose Phosphate Pathway (Figure 5.19)\n\nThe pentose phosphate pathway operates alongside glycolysis to generate reducing power and five-carbon sugars:\n\n\n\n- Glucose oxidation: Glucose is phosphorylated to glucose 6-phosphate, then oxidized and decarboxylated to form ribulose 5-phosphate.\n- Reducing power: Yields two molecules of \text{NADPH}$, which provides reducing power for anabolic biosynthetic pathways (such as fatty acid and steroid synthesis) rather than fueling the$\text{ETC}.\n- Intermediate synthesis:\n - Ribulose 5-phosphate: Feeds into the Calvin-Benson cycle of photosynthesis.\n - Ribose 5-phosphate: Serves as the fundamental backbone for RNA, DNA, and \text{ATP} nucleotides.\n - Erythrose 4-phosphate: Used as a precursor to synthesize aromatic amino acids (phenylalanine, tryptophan, tyrosine).\n - Xylulose 5-phosphate and Sedoheptulose 7-phosphate: Interconvert with \text{G3P} and fructose 6-phosphate, re-entering glycolysis.\n- Net yield: Generates only 1\,\text{ATP} per glucose molecule via substrate-level phosphorylation.\n\n\n# Page 58: The Entner-Doudoroff Pathway (Figure 5.20)\n\nThe Entner-Doudoroff pathway represents an alternative catabolic route utilized by select Gram-negative bacteria (such as *Pseudomonas aeruginosa* and *Enterococcus faecalis*):\n\n- Initial steps: Glucose is phosphorylated to glucose 6-phosphate, then oxidized to 6-phosphogluconic acid, reducing one \text{NADP}^+$to$\text{NADPH}.\n- Characteristic intermediate: 6-Phosphogluconic acid is dehydrated to form 2-keto-3-deoxy-6-phosphogluconic acid (\text{KDPG}).\n- Cleavage: \text{KDPG}$is cleaved by a specialized aldolase into one molecule of pyruvic acid and one molecule of$\text{G3P}.\n- Processing: The pyruvic acid proceeds to respiration or fermentation; the \text{G3P}$enters steps 6–10 of glycolysis, yielding$\text{NADH}$and two$\text{ATP}.\n- Stoichiometry per glucose: Consumes 1\,\text{ATP}$, produces$2\,\text{ATP}$(net$1\,\text{ATP}$),$1\,\text{NADH}$, and$1\,\text{NADPH}.\n\n\n# Page 59: Rationale and Mechanism of Fermentation\n\nWhen a cell lacks an appropriate external electron acceptor (such as oxygen) or lacks an operational electron transport chain, cellular respiration ceases:\n\n- The critical metabolic crisis: Glycolysis requires a continuous supply of oxidized \text{NAD}^+$to perform step 6 ($\text{G3P}$oxidation). If all cytoplasmic$\text{NAD}^+$is reduced to$\text{NADH}$, glycolysis halts, resulting in cell death from lack of$\text{ATP}.\n- Fermentation function: Provides an alternative mechanism to reoxidize \text{NADH}$back to$\text{NAD}^+ without an electron transport chain.\n- Definition: Fermentation is the partial oxidation of sugar (or other organic metabolites) to release energy, utilizing an endogenous organic molecule derived from within the cell as the terminal electron acceptor.\n\n\n# Page 60: Chemical Pathways of Lactic Acid and Ethanol Fermentation\n\n\n\n### Lactic Acid Fermentation\n- Direct reduction: Pyruvic acid acts directly as the terminal electron acceptor.\n- Reaction: Lactic acid dehydrogenase transfers two electrons and a proton from \text{NADH}$directly to pyruvic acid ($\text{CH}_3\text{COCOOH}$), producing lactic acid ($\text{CH}_3\text{CHOHCOOH}$) and regenerating$\text{NAD}^+.\n\n### Alcohol (Ethanolic) Fermentation\n- Two-step reduction:\n 1. Decarboxylation: Pyruvate decarboxylase removes a carboxyl group from pyruvic acid, releasing \text{CO}_2$and producing acetaldehyde ($\text{CH}_3\text{CHO}).\n 2. Reduction: Alcohol dehydrogenase reduces acetaldehyde with electrons from \text{NADH}$, yielding ethanol ($\text{CH}_3\text{CH}_2\text{OH}$) and regenerating$\text{NAD}^+.\n\n\n# Page 61: Industrial Fermentation Products and Microorganisms (Figure 5.22)\n\n\n\n| Representative Microorganisms | Fermentation End Products | Commercial / Food Applications |\n| :--- | :--- | :--- |\n| *Propionibacterium* | Carbon dioxide (\text{CO}_2), propionic acid | Swiss cheese (gas forms the holes; propionic acid imparts flavor) |\n| *Aspergillus*, *Lactobacillus*, *Streptococcus* | Lactic acid | Cheddar cheese, yogurt, soy sauce |\n| *Saccharomyces* (yeast) | Carbon dioxide (\text{CO}_2), ethanol | Wine, beer, bread leavening |\n| *Clostridium* | Acetone, isopropanol | Industrial solvents, nail polish remover, rubbing alcohol |\n\n\n# Page 62: Redox Balance and Continuation of Glycolysis\n\nFermentation produces zero additional \text{ATP} beyond what is synthesized during glycolysis. Its sole biological value is recycling reducing power:\n\n\text{NADH} + \text{Pyruvic Acid (or derivative)} \rightarrow \text{NAD}^+ + \text{Fermentation Product}\n\nThis regeneration enables the cell to maintain continuous substrate-level phosphorylation via glycolysis, sustaining minimal viability under anaerobic conditions.\n\n\n# Page 63: Systematic Comparison of Respiration and Fermentation (Table 5.4)\n\n| Characteristic | Aerobic Respiration | Anaerobic Respiration | Fermentation |\n| :--- | :--- | :--- | :--- |\n| **Oxygen required** | Yes | No | No |\n| **Type of phosphorylation** | Substrate-level and oxidative | Substrate-level and oxidative | Substrate-level only |\n| **Final electron acceptor** | Molecular oxygen (\text{O}_2$) | Inorganic ions ($\text{NO}_3^-$,$\text{SO}_4^{2-}$,$\text{CO}_3^{2-}) or externally acquired organic molecules | Cellular organic molecules (e.g., pyruvate, acetaldehyde) |\n| **Potential molecules of ATP produced per glucose** | **38** in prokaryotes, **36** in eukaryotes | **2–36** (variable depending on the reduction potential of the acceptor) | **2** |\n\n\n# Page 64: Catabolism of Non-Carbohydrate Substrates\n\nWhen carbohydrates are limited, microbial cells catabolize lipids and proteins:\n\n- Lipids and proteins store substantial chemical potential within their covalent bonds.\n- Microbes produce extracellular enzymes to break these large polymers down into subunits that can enter the cell.\n- These subunits are converted into precursor metabolites that feed directly into standard catabolic pathways (glycolysis and the Krebs cycle).\n\n\n# Page 65: Triglyceride Hydrolysis and Beta-Oxidation of Fatty Acids\n\nLipid catabolism proceeds via extracellular hydrolysis followed by intracellular beta-oxidation:\n\n- Hydrolysis: Extracellular lipases cleave ester bonds in triglycerides, yielding one molecule of glycerol and three fatty acid chains.\n- Glycerol utilization: Glycerol (3\text{C}$) is phosphorylated by$\text{ATP}$to form glycerol 3-phosphate, then oxidized to dihydroxyacetone phosphate ($\text{DHAP}), entering Step 5 of glycolysis.\n- Beta-oxidation: Fatty acid chains are processed inside the cell through repeated cycles of beta-oxidation:\n 1. Enzymes attach Coenzyme A to the carboxyl end of the fatty acid.\n 2. Dehydrogenation and hydration reactions cleave off two-carbon acetyl groups while reducing one \text{FAD}$to$\text{FADH}_2$and one$\text{NAD}^+$to$\text{NADH}.\n 3. Each cleaved two-carbon acetyl unit forms an acetyl-CoA molecule, which feeds into the Krebs cycle.\n 4. The shortened fatty acid repeats the cycle until the entire chain is converted into acetyl-CoA.\n\n\n# Page 66: Extracellular Proteolysis and Amino Acid Deamination\n\nProteins are too large to traverse microbial cell walls and cytoplasmic membranes intact:\n\n- Extracellular proteases: Microbes secrete proteases into the external environment to hydrolyze peptide bonds, breaking proteins down into short peptides and free amino acids.\n- Transport: Amino acids are transported into the cytoplasm via specific transport proteins.\n- Deamination: Within the cell, enzymes remove the amine group (-\text{NH}_2$) from the amino acid, releasing ammonia ($\text{NH}_3$) or ammonium ($\text{NH}_4^+).\n- Carbon skeleton entry: The remaining organic acid enters the central metabolic network as pyruvic acid, acetyl-CoA, or specific intermediates of the Krebs cycle (\alpha-ketoglutarate, succinyl-CoA, fumarate, oxaloacetate).\n\n\n# Page 67: Fundamentals of Photosynthesis\n\nPhotosynthesis is the biochemical process by which phototrophic organisms capture radiant light energy and use it to synthesize energy-rich organic carbohydrates from inorganic carbon dioxide (\text{CO}_2$) and water ($\text{H}_2\text{O}):\n\n6\,\text{CO}_2 + 12\,\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}{12}\text{O}6 + 6\,\text{H}_2\text{O} + 6\,\text{O}_2\n\n\n# Page 68: Structural Organization of Photosynthetic Metabolism\n\nPhotosynthesis divides into two interdependent biochemical phases:\n\n1. Light-Dependent Reactions: Capture photon energy, drive light-dependent electron flow, pump protons across membranes, and generate \text{ATP}$(via photophosphorylation) and$\text{NADPH}.\n2. Light-Independent Reactions (Calvin-Benson Cycle): Consume the \text{ATP}$and$\text{NADPH}$synthesized in the light-dependent reactions to reduce inorganic$\text{CO}_2 into sugars.\n\n\n# Page 69: Energy Transduction in Organelles and Microbes\n\nAerobic eukaryotic respiration relies on mitochondria, whereas photosynthetic eukaryotes utilize chloroplasts. In prokaryotes, these energy-transducing pathways reside within the cytoplasmic membrane and specialized internal membranous invaginations.\n\n\n# Page 70: Chemical Structure of Chlorophylls\n\nPhotosynthetic light capture depends on pigment molecules known as chlorophylls:\n\n- Porphyrin ring: Contains a light-absorbing active site centered around a coordinated magnesium ion (\text{Mg}^{2+}). This structure is structurally related to the iron-centered heme rings of cytochromes.\n- Hydrocarbon tail: A hydrophobic phytol chain that anchors the chlorophyll molecule into the photosynthetic lipid membrane.\n- Wavelength diversity: Slight variations in side groups distinguish chlorophyll a$, chlorophyll$b, and bacteriochlorophylls, altering their light absorption spectra to exploit different wavelengths of the electromagnetic spectrum.\n\n\n# Page 71: Photosystems, Thylakoid Architecture, and Grana\n\nChlorophylls and accessory pigments associate with structural proteins to form functional light-harvesting matrices termed photosystems:\n\n- Thylakoids: Cellular lipid membranes housing embedded photosystems and photosynthetic electron transport chains.\n - In prokaryotes: Formed by invaginations of the cytoplasmic membrane.\n - In eukaryotes: Formed from the inner membrane of chloroplasts.\n- Grana: Tightly stacked, disc-like arrangements of thylakoids found in chloroplasts.\n- Stroma: The fluid-filled space surrounding the grana within the chloroplast inner membrane, analogous to the mitochondrial matrix or bacterial cytoplasm.\n\n\n# Page 72: Molecular Architecture of Photosystems\n\nWithin a photosystem, dozens of pigment molecules are arranged to maximize photon capture:\n\n- Antenna pigments: Multiple pigment molecules absorb photons and rapidly pass the absorbed excitation energy from molecule to molecule via resonance energy transfer.\n- Reaction center: A specialized chlorophyll molecule located at the core of the photosystem. When excitation energy reaches the reaction center, an electron is boosted to a higher energy level and transferred to a primary electron acceptor, initiating electron transport.\n\n\n# Page 73: Classes of Photosystems and Reaction Phases\n\nPhototrophic cells utilize two functional classes of photosystems:\n\n- Photosystem I (\text{PS I}$): Possesses a reaction center optimized for longer wavelengths (e.g.,$P{700}).\n- Photosystem II (\text{PS II}$): Possesses a reaction center optimized for shorter wavelengths (e.g.,$P_{680}).\n\nBoth photosystems coordinate light capture to power two photophosphorylation routes: cyclic and noncyclic.\n\n\n# Page 74: Light-Dependent Reactions and Photophosphorylation\n\nAs electrons flow through the photosynthetic electron transport chain, their energy drives proton translocation across the thylakoid membrane into the thylakoid space:\n\n- Proton motive force: Protons accumulate inside the thylakoid lumen (or outside the prokaryotic cytoplasmic membrane).\n- ATP generation: Protons flow down their electrochemical gradient through \text{ATP}$synthase back into the stroma (or prokaryotic cytoplasm), driving photophosphorylation of$\text{ADP}$to$\text{ATP}.\n\n\n# Page 75: Light Harvesting and Energy Transfer in the Reaction Center\n\nIncident light hits antenna pigments, and energy transfers along a pathway through intermediate pigments until it reaches the reaction center chlorophyll, which ejects high-energy electrons into the photosynthetic electron transport chain.\n\n\n# Page 76: Cyclic Versus Noncyclic Photophosphorylation (Figure 5.27)\n\n\n\n### Cyclic Photophosphorylation\n- Operates using Photosystem I (\text{PS I}) alone.\n- Electron trajectory: Light energizes \text{PS I}$; excited electrons travel through an electron transport chain ($\text{cytochromes}$,$\text{Fe-S}$, and copper-containing plastocyanin) that pumps protons, and then return to the original$\text{PS I} reaction center.\n- Output: Produces \text{ATP}$only; no$\text{NADPH}$is synthesized, and no oxygen ($\text{O}_2) is evolved.\n\n### Noncyclic Photophosphorylation\n- Operates using both Photosystem II (\text{PS II}$) and Photosystem I ($\text{PS I}).\n- Electron trajectory: Photons strike \text{PS II}$, ejecting electrons into an electron transport chain (quinones, cytochromes) to generate a proton gradient and synthesize$\text{ATP}.\n- Photolysis of water: The electron void in \text{PS II} is filled by the enzymatic photolysis of water:\n\n\text{H}_2\text{O} \rightarrow 2\,\text{H}^+ + 2\,e^- + \frac{1}{2}\,\text{O}_2\n\n- Movement to PS I: Electrons leaving the intermediate \text{ETC}$pass into$\text{PS I}, where light absorption re-energizes them.\n- NADPH synthesis: Electrons transfer through ferredoxin to \text{NADP}^+$reductase ($\text{NADPase}$), reducing$\text{NADP}^+$to$\text{NADPH}.\n- Output: Produces \text{ATP}$,$\text{NADPH}$, and molecular oxygen ($\text{O}_2).\n\n\n# Page 77: Molecular Dynamics of Cyclic Photophosphorylation\n\nIn cyclic photophosphorylation, the closed circuit of electron flow provides a mechanism for cells to generate additional \text{ATP}$without accumulating$\text{NADPH}$or oxygen. This allows phototrophs to adjust their$\text{ATP}:\text{NADPH} production ratio to meet the metabolic demands of the Calvin-Benson cycle.\n\n\n# Page 78: Electron Flow and Reduction in Noncyclic Photophosphorylation\n\nNoncyclic photophosphorylation provides both the chemical energy (\text{ATP}$) and the reducing power ($\text{NADPH}) required for carbon fixation. The process requires a continuous supply of water molecules as electron donors, releasing oxygen as a metabolic byproduct.\n\n\n# Page 79: The Calvin-Benson Cycle and Light-Independent Reactions\n\nThe light-independent reactions (Calvin-Benson cycle) fix inorganic carbon into organic sugars without directly requiring photon absorption:\n\n- Energy consumption: Consumes \text{ATP}$and$\text{NADPH} produced during the light-dependent reactions.\n- Key enzyme: Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO).\n- The cycle proceeds through three distinct phases:\n 1. Carbon Fixation\n 2. Reduction\n 3. Regeneration of RuBP\n\n\n# Page 80: The Three Phases of the Calvin-Benson Cycle (Figure 5.28)\n\n\n\n### Phase 1: Carbon Fixation\n- Three molecules of carbon dioxide (\text{CO}_2$) combine with three molecules of the five-carbon acceptor ribulose 1,5-bisphosphate ($\text{RuBP}) via RuBisCO.\n- This yields three unstable six-carbon intermediates that immediately split into six molecules of 3-phosphoglyceric acid (3\text{-PGA}$,$3\text{C}).\n\n### Phase 2: Reduction\n- Six \text{ATP}$donate phosphate groups to the six$3\text{-PGA}$, forming six molecules of 1,3-bisphosphoglyceric acid ($1,3\text{-BPG}).\n- Six \text{NADPH}$reduce the six$1,3\text{-BPG}$, releasing six inorganic phosphates to form six molecules of glyceraldehyde 3-phosphate ($\text{G3P}$,$3\text{C}).\n\n### Phase 3: Regeneration of RuBP\n- One of the six \text{G3P} molecules exits the cycle as net product, serving as a building block for glucose synthesis.\n- The remaining five \text{G3P}$molecules ($5 \times 3\text{C} = 15\text{C}$) undergo a complex sequence of rearrangements consuming three$\text{ATP}$to regenerate three molecules of$\text{RuBP}$($3 \times 5\text{C} = 15\text{C}), completing the cycle.\n\n\n# Page 81: Enzymatic Machinery and Recycling in the Calvin-Benson Cycle\n\nTo synthesize one net molecule of hexose glucose (6\text{C}), the Calvin-Benson cycle requires:\n\n- Input: 6\,\text{CO}_2$,$18\,\text{ATP}$, and$12\,\text{NADPH}.\n- Net output: Two molecules of \text{G3P}, which condense to form one molecule of fructose 1,6-bisphosphate and ultimately glucose.\n- The enzyme cascade driving intermediate shuffling in Phase 3 includes: triose phosphate isomerase, aldolase, transketolase, phosphatase, ribose phosphate isomerase, phosphopentose epimerase, and phosphoribulokinase.\n\n\n# Page 82: Principles of Anabolism and Amphibolic Pathways\n\nAnabolic reactions are endergonic synthetic processes requiring chemical energy (from \text{ATP}) and carbon skeletons (from precursor metabolites):\n\n- Origin of components: Energy derives from catabolic \text{ATP} production; carbon precursors derive from central glycolysis, the pentose phosphate pathway, and the Krebs cycle.\n- Pathway reversibility: Many anabolic pathways share enzymes and operate as the functional reverse of corresponding catabolic pathways.\n- Amphibolic pathways: Metabolic pathways that can function in both catabolic (degradative) and anabolic (biosynthetic) capacities.\n\n\n# Page 83: The Twelve Precursor Metabolites (Table 5.6)\n\nAll cellular structures and macromolecules across microbial life are synthesized from twelve core precursor metabolites (characterized in *Escherichia coli*):\n\n| Precursor Metabolite | Generating Pathway | Examples of Synthesized Macromolecules | Functional Cellular Role |\n| :--- | :--- | :--- | :--- |\n| **Glucose 6-phosphate** | Glycolysis | Lipopolysaccharide | Outer membrane of Gram-negative cell walls |\n| **Fructose 6-phosphate** | Glycolysis | Peptidoglycan | Bacterial cell wall peptidoglycan layer |\n| **Glyceraldehyde 3-phosphate (G3P)** | Glycolysis | Glycerol portion of lipids | Fat storage, cytoplasmic membranes |\n| **Phosphoglyceric acid (3-PGA)** | Glycolysis | Amino acids: cysteine, selenocysteine, glycine, serine | Structural proteins, enzymes |\n| **Phosphoenolpyruvic acid (PEP)** | Glycolysis | Aromatic amino acids: phenylalanine, tryptophan, tyrosine | Structural proteins, enzymes |\n| **Pyruvic acid** | Glycolysis | Amino acids: alanine, leucine, valine | Structural proteins, enzymes |\n| **Ribose 5-phosphate** | Pentose phosphate pathway | DNA, RNA, ATP, histidine | Genome structure, protein synthesis |\n| **Erythrose 4-phosphate** | Pentose phosphate pathway | Aromatic amino acids: phenylalanine, tryptophan, tyrosine | Structural proteins, enzymes |\n| **Acetyl-CoA** | Bridge reaction / Krebs | Fatty acid portion of lipids | Cytoplasmic membrane phospholipids |\n| **\alpha-Ketoglutaric acid** | Krebs cycle | Amino acids: arginine, glutamic acid, glutamine, proline | Structural proteins, enzymes |\n| **Succinyl-CoA** | Krebs cycle | Heme | Cytochromes, electron transport carriers |\n| **Oxaloacetate** | Krebs cycle | Amino acids: aspartic acid, asparagine, isoleucine, lysine, methionine, threonine | Structural proteins, enzymes |\n\n\n# Page 84: Gluconeogenesis and Biosynthesis of Carbohydrates (Figure 5.29)\n\nGluconeogenesis is the anabolic synthesis of glucose and related carbohydrates from non-hexose precursors:\n\n\n\n- Substrate sources: Pyruvic acid, oxaloacetic acid (from amino acid deamination), and glycerol (from fat hydrolysis).\n- Pathway mechanics: Shares seven reversible enzymes with glycolysis. Three irreversible steps of glycolysis (hexokinase, phosphofructokinase, pyruvate kinase) are bypassed by distinct gluconeogenic enzymes consuming \text{ATP}$and$\text{GTP}.\n- Biosynthetic destinations:\n - Glucose 6-phosphate: Precursor for lipopolysaccharide and glycogen storage polymers.\n - Fructose 6-phosphate: Combines with amino sugars to form peptidoglycan polymers (N$-acetylglucosamine and$N-acetylmuramic acid).\n - Polysaccharides: Subunits condense to assemble bacterial starch and cellulose.\n\n\n# Page 85: Lipid Biosynthesis (Figure 5.30)\n\nTriglycerides and phospholipids are synthesized through distinct anabolic routes:\n\n\n\n- Glycerol backbone: Derived from glycolysis or the Calvin-Benson cycle via glyceraldehyde 3-phosphate (\text{G3P}$), which is isomerized to$\text{DHAP} and then reduced to glycerol.\n- Fatty acid tails: Assembled through the stepwise condensation of two-carbon acetyl units from acetyl-CoA via an acyl-carrier protein complex, essentially reversing beta-oxidation while consuming \text{ATP}$and$\text{NADPH}.\n- Condensation: Dehydration synthesis linkages join fatty acids to glycerol to yield membrane phospholipids and storage fats.\n\n\n# Page 86: Synthesis of Amino Acids via Amination and Transamination (Figure 5.31)\n\nAmino acid biosynthesis relies on two primary biochemical mechanisms:\n\n\n\n### Amination\n- Direct addition of an inorganic amine/ammonia group (\text{NH}_3) to a carbon keto acid:\n\n\text{Oxaloacetic acid (from Krebs)} + \text{NH}_3 \rightarrow \text{Aspartic acid}\n\n### Transamination\n- Transfer of an amino group from an existing amino acid to a carbon acceptor keto acid:\n\n\text{Glutamic acid} + \text{Oxaloacetic acid} \xrightleftharpoons{\text{Transaminase (PLP)}} \alpha\text{-Ketoglutaric acid} + \text{Aspartic acid}\n\n- Enzyme cofactor: Transaminases strictly require pyridoxal phosphate (\text{PLP}$, derived from$\text{Vitamin } \text{B}_6) to shuttle the amino group between substrates.\n\n\n# Page 87: Biosynthesis of Nucleotides (Figure 5.32)\n\nNucleotides (purines and pyrimidines) are complex molecules synthesized from multiple metabolic intermediates:\n\n\n\n- Five-carbon sugar: Ribose 5-phosphate, produced by the pentose phosphate pathway, forms the ribose/deoxyribose core.\n- Purine ring assembly (Adenine, Guanine): Built atom-by-atom directly onto the ribose 5-phosphate scaffold using glutamine (derived from Krebs cycle \alpha-ketoglutarate), aspartic acid (from oxaloacetate), glycine (from 3-PGA), and carbon units carried by folic acid.\n- Pyrimidine ring assembly (Cytosine, Thymine, Uracil): The six-membered pyrimidine ring is assembled first from aspartic acid and carbamoyl phosphate, then attached to ribose 5-phosphate.\n- Clinical relevance: Sulfanilamide blocks the conversion of \text{PABA} to folic acid in bacteria, halting nucleotide biosynthesis and preventing DNA replication and RNA transcription.\n\n\n# Page 88: Cellular Strategies for Metabolic Regulation and Economy\n\nMicrobial survival requires tight metabolic regulation to prevent the futile expenditure of energy and materials. Cells employ eight primary strategies:\n\n1. Regulation of transport: Cells synthesize channel and transport proteins only when specific substrates are present in the environment; excess or unneeded transport channels are degraded.\n2. Substrate-induced synthesis: Enzymes are frequently synthesized only when their specific substrate is physically available (e.g., the *lac* operon).\n3. Catabolite preference: When presented with two energy sources, cells catabolize the more energy-efficient nutrient first (catabolite repression, such as utilizing glucose before lactose).\n4. Biosynthetic feedback: Cells synthesize metabolites only when they are scarce; if an end-product is available externally, internal biosynthesis ceases.\n\n\n# Page 89: Compartmentalization, Allosteric Control, and Coenzyme Partitioning\n\nCells further regulate metabolic economy through physical, kinetic, and chemical partitioning:\n\n5. Compartmentalization: Eukaryotes segregate opposing metabolic pathways within membrane-bound organelles (e.g., fatty acid catabolism occurs inside mitochondria, whereas fatty acid synthesis occurs in the cytoplasm). Prokaryotes achieve functional isolation via protein microcompartments.\n6. Allosteric control: Key pacemaker enzymes possess allosteric sites that modulate activity instantaneously in response to changing effector concentrations.\n7. Feedback inhibition: End-products bind allosterically to early enzymes in their biosynthetic pathway to shut down unnecessary synthesis.\n8. Coenzyme partitioning: Cells segregate catabolic and anabolic pathways by requiring different coenzymes; catabolic pathways predominantly utilize \text{NAD}^+/\text{NADH}$, whereas anabolic pathways predominantly utilize$\text{NADP}^+/\text{NADPH}.\n\n\n# Page 90: Regulation of Gene Expression Versus Control of Metabolic Expression\n\nMetabolic control operates at two distinct organizational levels:\n\n1. Control of Gene Expression: Regulates the absolute quantity of enzymes produced.\n - Mechanism: Operates at the transcriptional and translational levels (e.g., operon induction or repression).\n - Dynamics: Slower response time (minutes to hours); governs long-term adaptation by controlling how many enzyme molecules are synthesized or degraded.\n2. Control of Metabolic Expression: Regulates the catalytic activity of existing enzyme molecules.\n - Mechanism: Operates post-translationally via allosteric activation, competitive/noncompetitive inhibition, and covalent modification (e.g., phosphorylation).\n - Dynamics: Immediate response time (milliseconds to seconds); fine-tunes metabolic flux in real time.\n\n\n# Page 91: Global Integration of Microbial Metabolism (Figure 5.33)\n\n\n\n- The central metabolic pathways (glycolysis, pentose phosphate pathway, Krebs cycle) form an interconnected metabolic core.\n- Anabolic pathways branch out from this core to polymerize amino acids into proteins, nucleotides into nucleic acids, simple sugars into polysaccharides, and fatty acids/glycerol into lipids.\n- Catabolic inputs from carbohydrates, lipids, and proteins converge into this core to supply energy, reducing power, and precursor metabolites.\n\n\n# Page 92: Integrated Map of Catabolic and Anabolic Pathways\n\n\n\nMicrobial metabolism functions as a unified biochemical system where catabolic pathways and anabolic pathways are linked through the cycling of \text{ATP}/\text{ADP}$,$\text{NADH}/\text{NAD}^+$, and$\text{NADPH}/\text{NADP}^+$$, directing raw nutrients into the synthesis of new cells.