Comprehensive Study Notes: Glycolysis, Pyruvate Fates, and Hexose Metabolism

Introduction to Metabolism and Cellular Respiration

• Metabolism encompasses the comprehensive set of chemical reactions that process dietary nutrients to extract energy required for cellular work.
• Carbohydrates represent one of the primary categories of nutrients utilized for rapid energy production.
• High-intensity athletic demands rely heavily on rapid glucose metabolism; for example, energy drinks like Glucon-D deliver an influx of pure glucose right before physical activity to rapidly generate energy.
• Glucose ($\text{C}_6\text{H}_{12}\text{O}_6$) is the most abundant carbohydrate and serves as a universal fuel source across most living organisms.
• Cells rapidly metabolize glucose under both aerobic and anaerobic conditions to produce pyruvate and synthesized adenosine triphosphate ($\text{ATP}$).
• Glycolysis is the central metabolic pathway that splits one $6$ -carbon glucose molecule into two $3$ -carbon pyruvate molecules, generating a net output of energy in the form of $\text{ATP}$ and reduced nicotinamide adenine dinucleotide ($\text{NADH}$).
• Cellular respiration harnesses the chemical energy stored within biomolecules through three interconnected stages:
  • Glycolysis: A $10$ -step cytosolic metabolic pathway that breaks down a $6$ -carbon glucose into two $3$ -carbon pyruvate molecules while producing $\text{ATP}$ and $\text{NADH}$.
  • TCA Cycle (Tricarboxylic Acid / Citric Acid / Krebs Cycle): Pyruvate is converted into the central $2$ -carbon metabolite acetyl-CoA, which undergoes complete oxidation to $\text{CO}_2$, producing reduced electron carriers $\text{NADH}$ and $\text{FADH}_2$ .
  • Electron Transport Chain (ETC) / Oxidative Phosphorylation: The electron carriers ($\text{NADH}$ and $\text{FADH}_2$ ) donate high-energy electrons to a series of four mitochondrial enzyme complexes. This transfer creates a proton motive force that drives large-scale $\text{ATP}$ synthesis.
• Physiological respiration involves breathing in oxygen ($\text{O}_2$ ) transported by hemoglobin to tissue cells, where it serves as the terminal electron acceptor ("sink") in oxidative phosphorylation.
• Glycolysis itself does not require oxygen to proceed; thus, it can function under both aerobic conditions (aerobic respiration) and oxygen-depleted conditions (anaerobic respiration).

Comparative Energetics and Thermodynamics of Respiration

• Respiration pathways differ substantially in energy yield depending on the presence or absence of oxygen:
  • Aerobic Respiration: In the presence of oxygen, pyruvate enters the mitochondria, converts to acetyl-CoA, and undergoes complete oxidation via the TCA cycle. Generated electron carriers feed into oxidative phosphorylation, yielding a total of $34\text{--}36\text{ ATP}$ per glucose molecule.
  • Anaerobic Respiration: In the absence of oxygen, pyruvate is converted to lactate (in humans/animals) or ethanol and $\text{CO}_2$  (in yeast). These pathways regenerate $\text{NAD}^+$, allowing glycolysis and substrate-level $\text{ATP}$ production to continue rapidly.
• Thermodynamic principles governing metabolic pathways:
  • Metabolic pathways must be overall irreversible to prevent products from spontaneously reverting back into substrates.
  • Pathways consist of a combination of irreversible and reversible enzymatic steps.
  • Irreversible steps exhibit large negative free energy changes ($\Delta G \ll 0$ ), making them highly thermodynamically favorable and serving as key regulatory checkpoints.
  • Reversible steps operate near equilibrium ($\Delta G \approx 0$ ), where mass action ratio and metabolic flux (reactant and product concentrations) dictate the net reaction direction. Reversible steps are often shared between opposing pathways (such as glycolysis and gluconeogenesis) operating in opposite directions.

Overview of Glycolysis: Phases and Strategy

• The primary metabolic goal of glycolysis is the oxidation of one $6$ -carbon glucose molecule into two $3$ -carbon pyruvate molecules, producing a net yield of $2\text{ ATP}$ and $2\text{ NADH}$.
• Glycolysis is divided into two phases:
  • Phase I (Investment Phase): Consists of Reactions $1$ through $5$. Two molecules of $\text{ATP}$ are invested to phosphorylate and destabilize glucose, splitting it into two molecules of glyceraldehyde-3-phosphate ($\text{G3P}$/$\text{GAP}$ ).
  • Phase II (Payoff Phase): Consists of Reactions $6$ through $10$. Runs twice per original glucose molecule (once for each $\text{G3P}$ ). It extracts high-energy electrons to reduce $\text{NAD}^+$ to $\text{NADH}$ and generates $4\text{ ATP}$ via substrate-level phosphorylation, yielding a net profit of $2\text{ ATP}$.

Phase I: Investment Phase Reactions

Reaction 1: Hexokinase

• Reaction: Transfer of a phosphoryl group from $\text{ATP}$ to the carbon-6 ($\text{C6}$ ) hydroxyl group of glucose to yield glucose-6-phosphate ($\text{G6P}$ ).
• Enzyme: Hexokinase.
• Thermodynamics: Irreversible step with standard free energy change $\Delta G'^{\circ} = -16.7\text{ kJ}\,\text{mol}^{-1}$.
• Functional Significance:
  • Traps glucose inside the cell, as plasma membranes lack transport proteins for phosphorylated sugars like $\text{G6P}$.
  • Destabilizes glucose, raising its free energy and priming it for subsequent chemical transformations.
• Cofactor Requirement: Requires $\text{Mg}^{2+}$ ions, which complex with $\text{ATP}$ to neutralize negative charges and coordinate the $\gamma$ -phosphate in the active site. The true enzymatic substrate is $\text{MgATP}^{2-}$.
• Mechanism: Operates via a random sequential mechanism where substrates and products can bind/release in any order, but catalysis only occurs when both $\text{MgATP}^{2-}$ and glucose occupy the active site. The $\text{C6}$ hydroxyl nucleophilically attacks the $\gamma$ -phosphate of $\text{ATP}$.

• Metabolic Crossroads of G6P: $\text{G6P}$ is not exclusively committed to glycolysis. It serves as a precursor for:
  • Glycolysis and its reversal, gluconeogenesis.
  • Glycogen synthesis (via conversion to glucose-1-phosphate).
  • The Pentose Phosphate Pathway (generating $\text{NADPH}$ and $4$ -carbon, $5$ -carbon, and $7$ -carbon sugars).
  • Carbohydrate synthesis (via glucuronate) and glucosamine-6-phosphate synthesis.
  • Because $\text{G6P}$ feeds multiple pathways, Hexokinase is not the first committed step of glycolysis.

• Isoforms of Hexokinase:
  • Muscle Isoform (Hexokinase I-III): Possesses a high affinity for glucose (low $K_M$ value), operating at near-maximal velocity ($V_{\max}$ ) even at low physiological blood glucose concentrations. It can phosphorylate other hexoses (fructose, mannose) with lower affinity. It is subject to feedback inhibition by its product, $\text{G6P}$, preventing excessive glucose accumulation when cellular energy requirements are met.
  • Liver Isoform (Glucokinase / Hexokinase IV): Possesses a low affinity for glucose (high $K_M$ value) and is specific only to glucose. Glucokinase reaches significant activity only at high blood glucose levels (around $5\text{ mM}$ ). It is not inhibited by $\text{G6P}$, enabling the liver to clear excess blood glucose after meals to store it as glycogen or fat. At low blood glucose levels, low glucokinase activity prevents the liver from consuming glucose needed by peripheral tissues.

• Regulation of Glucokinase (Hexokinase IV):
  • Nuclear Sequestration: Glucokinase activity is controlled by a nuclear regulatory protein. At low glucose levels, fructose-6-phosphate ($\text{F6P}$ ) promotes binding of glucokinase to the regulatory protein, translocating and sequestering it inside the nucleus in an inactive state. At high glucose levels, glucose enters liver cytosol via $\text{GLUT2}$ transporters and binds glucokinase, causing its release from the regulatory protein back into the cytosol in its active form.
  • Hormonal and Transcriptional Control: The GCK gene contains an insulin-sensitive promoter. High insulin levels upregulate glucokinase transcription to enhance glucose uptake and utilization. Glucagon raises cyclic $\text{AMP}$ ($\text{cAMP}$ ), promoting regulatory protein activity and nuclear sequestration of glucokinase.

Reaction 2: Phosphoglucoisomerase (Phosphohexose Isomerase)

• Reaction: Reversible isomerization of the aldose glucose-6-phosphate ($\text{G6P}$ ) to the ketose fructose-6-phosphate ($\text{F6P}$ ).
• Enzyme: Phosphoglucoisomerase ($\text{PGI}$ ) / Phosphohexose Isomerase.
• Thermodynamics: Standard free energy change $\Delta G'^{\circ} = 1.7\text{ kJ}\,\text{mol}^{-1}$; under intracellular conditions, $\Delta G = -2.9\text{ kJ}\,\text{mol}^{-1}$. Driven forward by the accumulation of $\text{G6P}$ .
• Mechanism: Involves ring opening, acid-base catalyzed isomerization shifting the anomeric carbonyl from $\text{C1}$ to $\text{C2}$ via a cis-enediol intermediate, followed by ring closure.
• Significance: Repositioning the carbonyl group to $\text{C2}$ creates a hemiketal that is a better nucleophile and sets up a symmetrical structure necessary for $\text{C-C}$ aldolytic cleavage in Reaction $4$ .

Reaction 3: Phosphofructokinase-1 (PFK-1)

• Reaction: Irreversible phosphorylation of fructose-6-phosphate ($\text{F6P}$) at carbon-1 ($\text{C1}$ ) using $\text{ATP}$ to produce fructose-1,6-bisphosphate ($\text{F-1,6-BP}$ ).
• Enzyme: Phosphofructokinase-1 ($\text{PFK-1}$ ).
• Thermodynamics: Highly exergonic and irreversible step with $\Delta G'^{\circ} = -14.2\text{ kJ}\,\text{mol}^{-1}$. Requires $\text{Mg}^{2+}$ as a cofactor.
• First Committed Step: Once $\text{F-1,6-BP}$ is synthesized, it is committed to undergoing glycolysis. Thus, $\text{PFK-1}$ is the primary rate-limiting and most heavily regulated enzyme of the pathway.
• Quaternary Structure: Homotetrameric enzyme that exhibits sigmoidal kinetics with respect to substrate $\text{F6P}$, alternating between an inactive T-state (tense) and an active R-state (relaxed).

• Allosteric Regulation of PFK-1:
  • Energy State Control:
  • High $\text{ATP}$ levels allosterically inhibit $\text{PFK-1}$ by binding to a low-affinity regulatory site, stabilizing the T-state and shifting the kinetic curve to the right.
  • High $\text{ADP}$ and $\text{AMP}$ levels signal low cellular energy, relief of $\text{ATP}$ inhibition, and allosteric activation of $\text{PFK-1}$ (stabilizing the active R-state).
  • Metabolic State Control:
  • Citrate: High concentrations of citrate (first intermediate of the TCA cycle) indicate sufficient biosynthetic precursors and energy supply, exerting allosteric feedback inhibition on $\text{PFK-1}$.
  • Fructose-2,6-Bisphosphate (F-2,6-BP): A potent, positive allosteric effector that overrides $\text{ATP}$ inhibition and activates $\text{PFK-1}$.

• Regulation by the Bifunctional Enzyme PFK-2/FBPase-2:
  • $\text{F-2,6-BP}$ is synthesized and broken down by a single bifunctional protein containing two enzymatic domains: Phosphofructokinase-2 ($\text{PFK-2}$) and Fructose-2,6-Bisphosphatase ($\text{FBPase-2}$ ).
  • High Blood Glucose (Insulin signaling): Insulin triggers protein phosphatase activation, dephosphorylating the bifunctional enzyme. This activates $\text{PFK-2}$ and inactivates $\text{FBPase-2}$, raising $\text{[F-2,6-BP]}$ and stimulating $\text{PFK-1}$ to drive glycolysis.
  • Low Blood Glucose (Glucagon signaling): Glucagon triggers $\text{cAMP}$-dependent protein kinase (PKA) activation, phosphorylating the bifunctional enzyme. This inactivates $\text{PFK-2}$ and activates $\text{FBPase-2}$, reducing $\text{[F-2,6-BP]}$ and suppressing $\text{PFK-1}$ activity.

Reaction 4: Aldolase

• Reaction: Reversible aldol cleavage of the $6$ -carbon sugar fructose-1,6-bisphosphate ($\text{F-1,6-BP}$) between carbons $\text{C3}$ and $\text{C4}$, generating two $3$ -carbon triose phosphates:
  • Dihydroxyacetone phosphate ($\text{DHAP}$ ), a ketotriose.
  • Glyceraldehyde-3-phosphate ($\text{G3P}$/$\text{GAP}$ ), an aldotriose.
• Enzyme: Fructose-1,6-bisphosphate aldolase.
• Thermodynamics: Standard free energy change is highly endergonic ($\Delta G'^{\circ} = 23.8\text{ kJ}\,\text{mol}^{-1}$ ). Under intracellular conditions, rapid consumption of $\text{G3P}$ in subsequent steps and high concentrations of $\text{F-1,6-BP}$ pull the actual free energy change to a reversible $\Delta G = -0.23\text{ kJ}\,\text{mol}^{-1}$.
• Mechanistic Classes:
  • Class I Aldolases (Animals and Plants): Catalysis proceeds via a covalent Schiff base intermediate formed between an active-site Lysine residue ($\text{Lys}$ ) and the carbonyl carbon of $\text{F-1,6-BP}$, stabilizing the carbanion transition state.
  • Class II Aldolases (Fungi and Bacteria): Do not form a covalent intermediate; instead, they use divalent metal cofactors ($\text{Zn}^{2+}$ or $\text{Fe}^{2+}$ ) to coordinate the carbonyl oxygen and stabilize the enolate intermediate.

Reaction 5: Triose Phosphate Isomerase (TIM)

• Reaction: Reversible isomerization of dihydroxyacetone phosphate ($\text{DHAP}$) to glyceraldehyde-3-phosphate ($\text{G3P}$ ).
• Enzyme: Triose Phosphate Isomerase ($\text{TIM}$ ).
• Thermodynamics: Standard free energy change $\Delta G'^{\circ} = 7.5\text{ kJ}\,\text{mol}^{-1}$ . The reaction is driven forward because $\text{G3P}$ is continuously depleted by Phase II reactions.
• Catalytic Perfection: $\text{TIM}$ is a kinetically perfect enzyme; its reaction rate is diffusion-limited ($k_{\text{cat}}/K_M \approx 10^8\text{--}10^9\text{ M}^{-1}\,\text{s}^{-1}$ ).
• Mechanism: General acid-base catalysis via an enediol intermediate:
  1. Active-site $\text{Glu165}$ acts as a general base to abstract a proton from $\text{C1}$ of $\text{DHAP}$.
  2. Active-site $\text{His95}$ acts as a general acid, donating a proton to the oxygen at $\text{C2}$, forming the enediol intermediate.
  3. $\text{His95}$ (now acting as a base) abstracts a proton from the $\text{C1}$ hydroxyl, while $\text{Glu165}$ donates a proton to $\text{C2}$, yielding $\text{G3P}$.
• Summary of Phase I: One glucose molecule ($6$  carbons) + $2\text{ ATP}$ consumed $\rightarrow$ two molecules of $\text{G3P}$ ($3$ carbons each) + $2\text{ ADP}$.

Phase II: Payoff Phase Reactions

• The Payoff Phase processes both $\text{G3P}$ molecules generated in Phase I, effectively running twice per starting glucose molecule.
• Converts two $\text{G3P}$ molecules into two pyruvate molecules while generating $2\text{ NADH}$ and $4\text{ ATP}$ (yielding a net profit of $2\text{ ATP}$ and $2\text{ NADH}$ per glucose).

Reaction 6: Glyceraldehyde-3-Phosphate Dehydrogenase (GAPDH)

• Reaction: Oxidation and phosphorylation of glyceraldehyde-3-phosphate ($\text{G3P}$) using inorganic phosphate ($\text{P}_i$) and $\text{NAD}^+$ to produce 1,3-bisphosphoglycerate ($\text{1,3-BPG}$) and $\text{NADH} + \text{H}^+$ .
• Enzyme: Glyceraldehyde-3-phosphate dehydrogenase ($\text{GAPDH}$ ).
• Thermodynamics: Standard free energy change $\Delta G'^{\circ} = 6.3\text{ kJ}\,\text{mol}^{-1}$ . The exergonic oxidation of the aldehyde group is coupled to the endergonic attachment of inorganic phosphate to form a high-energy mixed anhydride (acyl phosphate).
• Mechanism:
  1. $\text{NAD}^+$ binds to the enzyme active site, lowering the $pK_a$ of an active-site Cysteine residue ($\text{Cys}$) from $8$ to $5.5$, creating a reactive thiolate ion ($\text{S}^-$ ).
  2. The thiolate nucleophilically attacks the $\text{C1}$ carbonyl of $\text{G3P}$, forming a covalent thiohemiacetal intermediate.
  3. Oxidation occurs as a hydride ion ($\text{H}^-$) is transferred from the thiohemiacetal intermediate to $\text{NAD}^+$, generating $\text{NADH}$ and a covalent thioester intermediate.
  4. $\text{NADH}$ leaves the active site and is replaced by a fresh molecule of $\text{NAD}^+$.
  5. Phosphorolysis: Inorganic phosphate ($\text{P}_i$) nucleophilically attacks the thioester bond, releasing the high-energy product, $\text{1,3-BPG}$, and regenerating the cysteine thiolate.

Reaction 7: Phosphoglycerate Kinase (PGK)

• Reaction: Transfer of the high-energy acyl phosphate group from $\text{1,3-BPG}$ to $\text{ADP}$, forming 3-phosphoglycerate ($\text{3PG}$) and $\text{ATP}$.
• Enzyme: Phosphoglycerate Kinase ($\text{PGK}$ ).
• Thermodynamics: Highly exergonic under standard conditions ($\Delta G'^{\circ} = -18.9\text{ kJ}\,\text{mol}^{-1}$); operates near equilibrium under cellular conditions ($\Delta G = 0.1\text{ kJ}\,\text{mol}^{-1}$ ) in red blood cells.
• First Substrate-Level Phosphorylation: Produces $2\text{ ATP}$ per original glucose molecule, exactly breaking even with the $2\text{ ATP}$ invested in Phase I.
• Alternative Pathway (2,3-BPG Shunt): In red blood cells (erythrocytes), $\text{1,3-BPG}$ can be converted to 2,3-bisphosphoglycerate ($\text{2,3-BPG}$) by bisphosphoglycerate mutase. $\text{2,3-BPG}$ is a critical allosteric regulator that binds hemoglobin, stabilizing its T-state and promoting oxygen delivery to peripheral tissues. $\text{2,3-BPG}$ is subsequently hydrolyzed to $\text{3PG}$ by 2,3-bisphosphoglycerate phosphatase.

Reaction 8: Phosphoglycerate Mutase (PGM)

• Reaction: Reversible isomerization relocating the phosphate group from carbon-3 ($\text{C3}$) to carbon-2 ($\text{C2}$), converting 3-phosphoglycerate ($\text{3PG}$) to 2-phosphoglycerate ($\text{2PG}$ ).
• Enzyme: Phosphoglycerate Mutase ($\text{PGM}$ ).
• Thermodynamics: Standard free energy change $\Delta G'^{\circ} = 4.4\text{ kJ}\,\text{mol}^{-1}$ .
• Mechanism:
  1. The active site contains a Phosphoenzyme state with a phosphorylated Histidine residue ($\text{His}$ ).
  2. The phosphoenzyme transfers its phosphate to $\text{C2}$ of $\text{3PG}$, generating a transient $\text{2,3-BPG}$ intermediate bound in the active site.
  3. The phosphate at $\text{C3}$ is transferred back to the active-site Histidine residue, releasing $\text{2PG}$ and regenerating the phosphoenzyme.
  4. Trace amounts of $\text{2,3-BPG}$ are required to initially phosphorylate the active-site histidine.

Reaction 9: Enolase

• Reaction: Reversible dehydration of 2-phosphoglycerate ($\text{2PG}$) to form phosphoenolpyruvate ($\text{PEP}$ ), a compound with a high phosphate group transfer potential.
• Enzyme: Enolase.
• Thermodynamics: Standard free energy change $\Delta G'^{\circ} = 7.5\text{ kJ}\,\text{mol}^{-1}$ . Removal of a water molecule redistributes energy within the molecule, creating an enol phosphate ester.
• Cofactor: Requires $\text{Mg}^{2+}$ ions to coordinate and stabilize the enolate intermediate.
• Inhibition: Fluoride ions ($\text{F}^-$) inhibit enolase by forming fluorophosphate complexes with $\text{Mg}^{2+}$ in the enzyme active site, blocking glycolytic flux.

Reaction 10: Pyruvate Kinase (PK)

• Reaction: Transfer of the high-energy phosphoryl group from phosphoenolpyruvate ($\text{PEP}$) to $\text{ADP}$, yielding pyruvate and $\text{ATP}$.
• Enzyme: Pyruvate Kinase ($\text{PK}$ ).
• Thermodynamics: Highly exergonic and irreversible step with $\Delta G'^{\circ} = -31.4\text{ kJ}\,\text{mol}^{-1}$. Requires $\text{Mg}^{2+}$ and $\text{K}^+$ as cofactors.
• Tautomerization Drive: Transfer of phosphate yields an unstable enolpyruvate intermediate, which undergoes rapid, spontaneous keto-enol tautomerization to the stable keto form of pyruvate. This tautomerization drives the large overall negative free energy change.
• Second Substrate-Level Phosphorylation: Yields $2\text{ ATP}$ per original glucose molecule ($4\text{ ATP}$ total generated in Phase II, resulting in a net profit of $2\text{ ATP}$ ).

• Regulation of Pyruvate Kinase:
  • Allosteric Control (All Tissues):
  • Activation: $\text{AMP}$ (signals low cellular energy) and fructose-1,6-bisphosphate ($\text{F-1,6-BP}$ , feed-forward activation linking Phase I to Phase II).
  • Inhibition: $\text{ATP}$ (high energy status), acetyl-CoA, long-chain fatty acids (alternate fuels available), and alanine (amino acid synthesized directly from pyruvate skeleton).
  • Covalent Regulation (Liver Isoform - Pyruvate Kinase L):
  • Glucagon (low blood glucose signal) triggers $\text{cAMP}$-dependent protein kinase A (PKA), which phosphorylates Pyruvate Kinase L, inactivating it and making it more sensitive to $\text{ATP}$ and alanine inhibition. This prevents the liver from consuming glucose during starvation.
  • High blood glucose causes protein phosphatase (PP) to dephosphorylate and reactivate Pyruvate Kinase L.

Thermodynamics Summary and Free Energy Changes

• Net Reaction Equation of Glycolysis: $\text{Glucose} + 2\text{ NAD}^+ + 2\text{ ADP} + 2\text{ P}_i \rightarrow 2\text{ Pyruvate} + 2\text{ NADH} + 2\text{ H}^+ + 2\text{ ATP} + 2\text{ H}_2\text{O}$
• Out of the $10$ enzymatic steps in glycolysis:
  • $7$ steps are reversible ($\Delta G \approx 0$ ): Phosphoglucoisomerase, Aldolase, TIM, GAPDH, PGK, PGM, Enolase.
  • $3$ steps are irreversible regulatory checkpoints ($\Delta G \ll 0$ ): Hexokinase (Step 1), Phosphofructokinase-1 (Step 3), Pyruvate Kinase (Step 10).

Metabolic Fates of Pyruvate

Fate 1: Aerobic Respiration

• Under oxygenated conditions, pyruvate is transported into the mitochondrial matrix and converted into acetyl-CoA by the pyruvate dehydrogenase complex.
• Acetyl-CoA enters the TCA cycle, undergoing complete oxidation to $\text{CO}_2$ .
• High-energy electrons donated to $\text{NADH}$ and $\text{FADH}_2$ flow through the mitochondrial Electron Transport Chain (ETC) to $\text{O}_2$, regenerating $\text{NAD}^+$ and driving oxidative phosphorylation ($\approx 2.5\text{ ATP}$ per $\text{NADH}$, $\approx 1.5\text{ ATP}$ per $\text{FADH}_2$ ).
• Fed vs. Fasting Liver Dynamics:
  • Fed State: High glucose availability drives pyruvate conversion to acetyl-CoA for TCA oxidation and fatty acid synthesis.
  • Fasting State: Liver directs pyruvate toward gluconeogenesis via Pyruvate Carboxylase (converting pyruvate to oxaloacetate) to generate free glucose for release into the bloodstream.

Fate 2: Anaerobic Respiration in Animals — Lactic Acid Fermentation

• Under hypoxic/anaerobic conditions (or in cells lacking mitochondria like mature erythrocytes), electron transport stops, causing $\text{NADH}$ accumulation and $\text{NAD}^+$ depletion.
• To maintain glycolytic $\text{ATP}$ production at Reaction $6$ ($\text{GAPDH}$ ), cells must regenerate $\text{NAD}^+$ anaerobically.
• Reaction: Lactate Dehydrogenase ($\text{LDH}$) catalyzes the reduction of pyruvate to L-lactate, oxidizing $\text{NADH}$ back to $\text{NAD}^+$ .
• Mechanism: $\text{NADH}$ transfers a hydride ion ($\text{H}^-$) to the $\text{C2}$ carbonyl carbon of pyruvate, while an active-site Histidine residue ($\text{His195}$) donates a proton to the oxygen atom, yielding L-lactate.

• The Cori Cycle:
  1. Vigorous skeletal muscle contraction generates lactate anaerobically.
  2. Lactate accumulates, causing localized drop in pH and muscle discomfort, before diffusing into the bloodstream.
  3. The liver takes up circulating lactate and oxidizes it back to pyruvate via Lactate Dehydrogenase.
  4. The liver converts pyruvate into glucose via gluconeogenesis ($\text{ATP}$ consuming process).
  5. Glucose is released into the bloodstream and returned to active muscle tissue.

Fate 3: Anaerobic Respiration in Yeast — Ethanol Fermentation

• Yeast and microorganisms ferment pyruvate to ethanol and $\text{CO}_2$ in a two-step process:
  • Step 1: Pyruvate Decarboxylase:
  • Irreversibly decarboxylates pyruvate to produce acetaldehyde and $\text{CO}_2$ .
  • Requires $\text{Mg}^{2+}$ and the cofactor Thiamine Pyrophosphate (TPP).
  • Mechanism: The thiazolium ring of TPP forms a nucleophilic ylid carbanion that attacks the $\text{C2}$ carbonyl of pyruvate. $\text{CO}_2$ is eliminated, forming a resonance-stabilized carbanion intermediate. Protonation yields hydroxyethyl-TPP, which releases toxic acetaldehyde.
  • Note: Pyruvate Decarboxylase is absent in humans/animals.
  • Step 2: Alcohol Dehydrogenase (ADH):
  • Reduces acetaldehyde to ethanol, oxidizing $\text{NADH}$ to $\text{NAD}^+$ .
  • Contains an active-site $\text{Zn}^{2+}$ ion that polarizes the carbonyl oxygen of acetaldehyde.
  • $\text{NADH}$ donates a hydride ion to acetaldehyde, followed by proton uptake from the medium to yield ethanol.

Entry of Other Dietary Hexoses into Glycolysis

• Dietary sugars beyond glucose include disaccharides like sucrose (table sugar: glucose + fructose linked via an $\alpha(1\rightarrow 2)$ glycosidic bond) and lactose (milk sugar: glucose + galactose).

Fructose Metabolism

• In Muscle Tissue: Hexokinase can phosphorylate fructose directly to fructose-6-phosphate ($\text{F6P}$ ), which enters directly into Reaction $3$ of glycolysis.
• In Liver Tissue (Fructolysis): Glucokinase cannot phosphorylate fructose. Liver metabolizes fructose via the Fructolysis pathway:
  1. Fructokinase: Phosphorylates fructose to fructose-1-phosphate ($\text{F1P}$ ) using $\text{ATP}$.
  2. Fructose-1-Phosphate Aldolase (Type B Aldolase): Cleaves $\text{F1P}$ into glyceraldehyde and dihydroxyacetone phosphate ($\text{DHAP}$ ).
  3. Triose Phosphate Isomerase: Converts $\text{DHAP}$ to $\text{G3P}$.
  4. Glyceraldehyde Kinase: Phosphorylates glyceraldehyde to $\text{G3P}$ using $\text{ATP}$, allowing entry into glycolysis.
  5. Alternate Glycerol Route: Alcohol dehydrogenase can reduce glyceraldehyde to glycerol (consuming $\text{NADH}$ ), which is phosphorylated by Glycerol Kinase to glycerol-3-phosphate, then oxidized by Glycerol Phosphate Dehydrogenase to $\text{DHAP}$.

• Health Implications of High Fructose Consumption:
  • Hepatic fructolysis bypasses the main regulatory step of glycolysis ($\text{PFK-1}$ ).
  • Unregulated fructose flux leads to high production of glycerol-3-phosphate and acetyl-CoA.
  • Glycerol forms the esterified backbone for triacylglycerols (triglycerides). Excessive fructose consumption directly promotes hepatic fatty acid synthesis, leading to Non-Alcoholic Fatty Liver Disease (NAFLD).

Alcohol Toxicity and Hepatic Pathology

• Alcohol consumed orally goes directly to the liver via portal circulation ("the liver is the trash can of the body").
• In hepatocytes, Alcohol Dehydrogenase ($\text{ADH}$) converts ethanol into toxic acetaldehyde, which is converted to acetate, producing excess $\text{NADH}$.
• Elevated $\text{NADH}$ levels increase glycerol-3-phosphate production and promote fatty acid synthesis while suppressing fatty acid breakdown ($\beta$-oxidation).
• Ethanol inhibits Very Low Density Lipoproteins ($\text{VLDL}$) that export fats out of liver cells, causing triacylglycerols to accumulate, resulting in fatty liver disease and cirrhosis.
• Pharmacological Warning: Never take acetaminophen (Tylenol) when drinking alcohol, as ethanol alters hepatic cytochrome P450 metabolism, producing toxic reactive intermediates that cause acute liver failure. Use ibuprofen (Advil) instead.

Galactose Metabolism

• Galactose is a $\text{C4}$-epimer of glucose derived from lactose hydrolysis.
• Hepatic Conversion Pathway to Glycolytic Intermediates:
  1. Galactokinase: Phosphorylates galactose using $\text{ATP}$ to produce galactose-1-phosphate.
  2. Galactose-1-Phosphate Uridylyltransferase: Exchanges galactose-1-phosphate with $\text{UDP-glucose}$, yielding glucose-1-phosphate ($\text{G1P}$) and $\text{UDP-galactose}$.
  3. UDP-Glucose-4-Epimerase: Converts $\text{UDP-galactose}$ back to $\text{UDP-glucose}$ via an $\text{NAD}^+$-dependent epimerization at $\text{C4}$.
  4. Phosphoglucomutase: Isomerizes $\text{G1P}$ to glucose-6-phosphate ($\text{G6P}$ ), which enters glycolysis.

• Galactosemia Pathology:
  • Deficiency in Galactose-1-Phosphate Uridylyltransferase leads to Classic Galactosemia.
  • Accumulated galactose is reduced by aldose reductase to the toxic sugar alcohol galactitol.
  • Galactitol accumulation in the eye lens causes osmotic swelling and cataracts. Other manifestations include developmental delays, liver jaundice, motor abnormalities, and severe neonatal sepsis.
  • Treatment: Strict dietary restriction eliminating galactose and lactose sources.