Microbial Metabolism

Fundamentals of Microbial Metabolism

Metabolism refers to the sum of all chemical reactions occurring within a living cell, encompassing both the building up and breaking down of complex molecules. Every cell must acquire nutrients, which serve as the essential building blocks and energy sources required for life.

Core Metabolic Pathways

Metabolism is divided into two primary types of pathways:

• Catabolism (Catabolism Destroys): These are exergonic metabolic pathways that break down complex molecules into simpler ones. Exergonic reactions are spontaneous and release energy. During catabolism, large polymers like polysaccharides, lipids, nucleic acids, and proteins are broken down into their constituent monomers: monosaccharides, fatty acids, nucleotides, and amino acids. These units can then be used to build new polymers or degraded further into waste products while releasing energy.

• Anabolism (Anabolism Builds): These are endergonic metabolic pathways that convert simple molecules into more complex molecules. Endergonic reactions require an input of energy to proceed. Anabolism includes the biosynthesis of cellular macromolecules (proteins, lipids, nucleic acids, polysaccharides), vitamins, coenzymes, and structural compounds for the cell.

Major Organic Molecules Involved

The metabolic processes involve the four major classes of organic molecules:

1. Carbohydrates: Such as glucose (the primary focus of cellular catabolism).

2. Lipids: For example, components of the cell membrane or phospholipid bilayer.

3. Proteins: Including enzymes, which are structural and functional proteins.

4. Nucleic Acids: Such as DNA and RNA.

Oxidation-Reduction (Redox) Reactions

Energy is transferred throughout metabolic processes via the movement of electrons ($e^-$). In biological systems, energy is often moved in the form of hydrogen atoms ($H$), as each hydrogen consists of one electron and one proton.

Definitions and Concepts

• Oxidation: The loss of electrons by a molecule, atom, or ion.

• Reduction: The gain of electrons by a molecule, atom, or ion.

• Mnemonic Tool (OIL RIG): Oxidation Is Loss; Reduction Is Gain.

Examples of Redox Reactions

• Reduction of Zinc: $\text{Zn}^{2+} + 2e^- \rightarrow \text{Zn}$

• Oxidation of Zinc: $\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-$

• Formation of water: $\text{O}_2 + 4e^- + 4H^+ \rightarrow 2 ext{H}_2 ext{O}$

Energy and Electron Carriers

Living cells must manage the energy released during catabolism safely, storing it and releasing it only when needed.

Important Electron Carriers

Energy carriers are molecules that bind to and shuttle high-energy electrons between compounds in metabolic pathways. These molecules represent potential energy:

• Nicotinamide adenine dinucleotide ($\text{NAD}^+$): Reduces to $\text{NADH}$.

• Nicotinamide adenine dinucleotide phosphate ($\text{NADP}^+$): Reduces to $\text{NADPH}$.

• Flavine adenine dinucleotide ($\text{FAD}$): Reduces to $\text{FADH}_2$.

Adenosine Triphosphate (ATP)

ATP is the "energy currency" of the cell, used to fill any immediate energy need. It is produced during photosynthesis and cellular respiration and is necessary for the cell to "do work."

• Structure: Composed of an adenine molecule, a ribose sugar, and three phosphate groups.

• Precursors: include Adenosine monophosphate ($\text{AMP}$) with one phosphate and Adenosine diphosphate ($\text{ADP}$) with two.

• Energy Release: The bonds between phosphate groups are high-energy. Breaking these bonds releases the energy required to drive endergonic reactions.

Enzyme Structure and Function

Enzymes are proteins that act as catalysts to speed up chemical reactions without being consumed in the process.

Key Characteristics

• Active Site: A specific region on the enzyme where it binds to a specific substrate.

• Lowering Activation Energy: Enzymes reduce the amount of energy input required for a reaction to occur.

• Increasing Reaction Rate: By lowering the required energy, enzymes significantly decrease the time it takes for a reaction to reach completion.

Factors Influencing Enzyme Activity

1. Temperature: High temperatures can denature proteins, rendering them inactive.

2. pH: Altered pH levels can denature enzymes.

3. Substrate Concentration: The availability of reactants affects the pace of enzymatically catalyzed reactions.

4. Inhibitors: Substances that block or reduce enzyme activity.

Enzyme Regulation and Inhibition

Types of Inhibition

• Competitive Inhibition: An inhibitor molecule competes with the substrate for the active site. If the inhibitor fits into the active site, it prevents the substrate from binding ("If it fits, it sits").

• Non-competitive (Allosteric) Inhibition: An inhibitor binds to an allosteric site (a location on the enzyme other than the active site). This binding alters the shape of the active site, preventing the substrate from binding.

• Feedback Inhibition: An end product of a metabolic pathway acts as an inhibitor for an enzyme earlier in that same pathway. This allows the cell to stop production when enough energy or product has been accumulated (e.g., stopping glucose breakdown if energy levels are sufficient).

Oxygen Requirements in Microbial Metabolism

Microbes vary significantly in their requirement for and tolerance of oxygen ($\text{O}_2$). Microbial types can be identified by their growth patterns in liquid culture:

Microbial Classifications

1. Obligate Aerobes: Gather at the top of a test tube to absorb the maximum amount of oxygen.

2. Obligate Anaerobes: Gather at the bottom of the tube to avoid oxygen entirely.

3. Facultative Anaerobes: Gather mostly at the top because aerobic respiration is most beneficial, but can be found throughout the tube because the lack of oxygen does not harm them.

4. Microaerophiles: Require oxygen but at low concentrations ($\text{O}_2$); they gather in the upper part of the tube but not at the very surface.

5. Aerotolerant Bacteria: Spread evenly throughout the tube as they are unaffected by the presence of oxygen.

Detoxification of Oxygen

Organisms using aerobic pathways must be able to handle toxic oxygen derivatives. They utilize specific enzymes for this purpose:

• Catalase

• Peroxidase

• Superoxide dismutase

Carbohydrate Catabolism: Aerobic Cellular Respiration

Glucose is the most commonly used monosaccharide for energy. Aerobic cellular respiration involves the complete breakdown of glucose into carbon dioxide ($\text{CO}_2$) and water ($\text{H}_2 ext{O}$), yielding a large amount of ATP (approximately $36$ molecules of ATP per glucose molecule).

The Four Major Steps of Aerobic Respiration

1. Step 1: Glycolysis     - Location: Occurs in the cytoplasm of most cells.     - Process: Splits one six-carbon glucose molecule into two three-carbon molecules known as Pyruvic Acid (pyruvate).     - Yield: Produces a net gain of $2$ ATP and $2$ NADH.

2. Step 2: Synthesis of Acetyl-CoA     - Requirement: This must occur before the Krebs Cycle can begin.     - Yield: Results in two molecules of Acetyl-CoA, two molecules of $\text{CO}_2$ (the source of the carbon dioxide we exhale), and two molecules of NADH.

3. Step 3: The Krebs Cycle     - Function: The primary goal is to reduce coenzymes (transfer electrons to carriers like $\text{NAD}^+$ and $\text{FAD}$).     - Location: Cytoplasm of prokaryotes; mitochondrial matrix of eukaryotes.     - Cycle Count: Because one glucose produces two molecules of Acetyl-CoA, the cycle turns twice per glucose molecule.     - Total Yield (after 2 cycles): Two molecules of ATP, two molecules of $\text{FADH}_2$, six molecules of NADH, and four molecules of $\text{CO}_2$.

4. Step 4: Electron Transport Chain (ETC) and Chemiosmosis     - Function: Where most of the ATP is produced via a series of redox reactions.

The Electron Transport Chain (ETC) and Chemiosmosis

Mechanism of Action

1. Oxidation-Reduction Reactions: Electron carriers (NADH and $\text{FADH}_2$) bring electrons and protons ($H^+$) to the ETC. Membrane-bound carrier molecules pass electrons along the chain to a final electron acceptor (oxygen in aerobic respiration).

2. Creation of a Proton Gradient: As electrons move down the chain, their energy is used to pump protons ($H^+$) across the membrane (to the exterior of prokaryotes or the intermembrane space of mitochondria).

3. Chemiosmosis: This gradient creates potential energy. Protons flow back down their gradient through a protein channel called ATP Synthase. This flow provides the energy to phosphorylate ADP into ATP.

Yield Comparison

• Glycolysis: $+2$ ATP (substrate-level phosphorylation).

• Krebs Cycle: $+2$ ATP (substrate-level phosphorylation).

• ETC/Chemiosmosis: Approximately $+32$ ATP (chemiosmotic phosphorylation).

• Total Net Yield: Approximately $36$ ATP molecules per glucose (though some figures suggest a maximum of $38$ depending on cellular efficiency).

Anaerobic Metabolism: Respiration and Fermentation

Cells must have alternative methods to acquire energy when oxygen is not available.

Anaerobic Respiration

• Mechanisms: Uses the Electron Transport Chain but employs an inorganic molecule other than oxygen as the final electron acceptor (e.g., nitrate, sulfate, or carbon dioxide).

• Efficiency: Not all of the ETC is used, resulting in less ATP produced than aerobic respiration.

• Products: If $\text{CO}_2$ is the acceptor, the product is methane ($\text{CH}_4$) or acetic acid. Methane is produced by organisms in the gut, in cows, or in swamps.

Fermentation

• Mechanisms: Occurs when there is no final electron acceptor for the ETC. It allows Glycolysis to continue by providing a way to regenerate $\text{NAD}^+$ from NADH.

• Essential Function: Oxidize NADH into $\text{NAD}^+$ so that glycolysis can repeat.

• Process: Electrons and hydrogen ions from NADH are donated to an organic molecule (like pyruvate).

• Yield: Only $2$ ATP per molecule of glucose.

• Common Byproducts:     - Lactic Acid: (e.g., in yogurt or muscle cells).     - Ethanol and $\text{CO}_2$: (e.g., in yeast and bread).

Summary Table of Metabolism Types

• Aerobic Respiration: Uses $O_2$; complete breakdown of glucose; high ATP yield (up to $38$ ATP).

• Anaerobic Respiration: No $O_2$; uses alternative inorganic acceptors; moderate ATP yield.

• Fermentation: No $O_2$; regenerates $\text{NAD}^+$; converts pyruvate to organic waste; low ATP yield ($2$ ATP).