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

Cellular Respiration and Glucose Transport Mechanisms

• Metabolism represents the comprehensive network of chemical reactions in living organisms designed to extract energy from ingested nutrients to execute cellular work.

• Carbohydrates, specifically glucose, represent the primary energy currency for most organisms. Glucose is rapidly broken down under both aerobic and anaerobic conditions to produce pyruvate and adenosine triphosphate ($\text{ATP}$).

• Cellular respiration completely oxidizes glucose into $\text{CO}_2$ and $\text{H}_2\text{O}$ across three main metabolic stages:

  • Glycolysis: A $10$-step cytosolic pathway that converts one $6$-carbon glucose molecule into two $3$-carbon pyruvate molecules, generating net $2\text{ ATP}$ and $2\text{ NADH}$.

  • TCA Cycle (Citric Acid Cycle / Krebs Cycle): Pyruvate is converted to $2$-carbon acetyl-CoA, which enters a cyclic mitochondrial pathway to produce $\text{CO}_2$, $NADH$, and $\text{FADH}_2$.

  • Electron Transport Chain (ETC) / Oxidative Phosphorylation: Electron carriers ($NADH$, $\text{FADH}_2$) transfer extracted electrons to a series of four mitochondrial enzymes. This process generates a proton motive force driving $\text{ATP}$ synthesis, with oxygen ($\text{O}_2$) acting as the final electron acceptor.

• Comparison of Aerobic vs. Anaerobic Respiration:

  • Aerobic Respiration: Requires $\text{O}_2$, completely oxidizes glucose to $\text{CO}_2$ and $\text{H}_2\text{O}$, and yields approximately $30\text{--}38\text{ ATP}$ per glucose molecule ($34\text{--}36\text{ ATP}$ via oxidative phosphorylation).

  • Anaerobic Respiration: Operates independently of $\text{O}_2$ in the cytosol. Pyruvate is reduced to lactate in humans or ethanol and $\text{CO}_2$ in yeast, regenerating $\text{NAD}^+$ to allow continuous glycolytic $\text{ATP}$ production.

  • Energetics & Kinetics: Aerobic glycolysis yields $30\text{--}38\text{ ATP}$ per glucose with a relative flux of $1$ (total energy output = $30\text{--}38$). Anaerobic glycolysis yields $2\text{ ATP}$ per glucose but operates at a relative flux of $100$ (total energy output = $200$), supplying rapid bursts of energy during high demand.

• Thermodynamics of Metabolic Pathways:

  • Pathways consist of irreversible and reversible enzymatic steps.

  • Irreversible steps possess large negative free energy changes ($\Delta G \ll 0$), making them exergonic, unidirectional, and primary sites of metabolic regulation.

  • Reversible steps operate near equilibrium ($\Delta G \approx 0$). Their direction is governed by reactant and product concentrations (mass action flux), allowing them to be shared by opposing pathways (e.g., glycolysis and gluconeogenesis).

• Glucose Absorption and Transport:

  • Epithelial Transport: In the intestinal lumen and kidney nephrons, glucose uptake across brush border microvilli occurs via secondary active transport through $\text{Na}^+$/glucose symporters (SGLTs), driven by high extracellular $\text{Na}^+$ concentration gradients. The basolateral $\text{Na}^+/K^+$ ATPase pump maintains this gradient by pumping $3\text{ Na}^+$ out and $2\text{ K}^+$ in using $\text{ATP}$.

  • Blood Efflux: Glucose exits epithelial cells into the bloodstream via the basolateral GLUT2 uniporter via facilitated diffusion, maintaining blood glucose levels around $5\,mM$.

• Classification of Glucose Transporters:

  • $\text{Na}^+$/Glucose Transporters (SGLT): Active transport, ion-dependent, located in intestinal epithelial cells and nephrons.

  • Passive / Facilitated Diffusion Transporters (Ion-independent, GLUT 1-4):

  • GLUT1 and GLUT3: Expressed in the central nervous system (brain and nerves); high-affinity transporters.

  • GLUT2: Expressed in liver hepatocytes, pancreatic $\alpha$ and $\beta$ cells, and intestinal basolateral membranes. Low-affinity, high-capacity transporter ($K_M \approx 10\,mM$), serving as a glucose sensor.

  • GLUT4: Expressed in skeletal muscle, cardiac muscle, and adipose tissues. High-affinity transporter ($K_M \approx 0.1\,mM$), regulated by insulin.

  • GLUT5: Specific for fructose transport across plasma membranes.

• Five-Step Mechanism of Insulin-Dependent GLUT4 Translocation:

  1. Hormone Activation: Insulin binds to cell-surface receptors.

  2. Signal Induction: Intracellular signaling cascades are triggered.

  3. Vesicle Transport: GLUT4-containing storage vesicles translocate to the plasma membrane.

  4. Fusion & Insertion: Vesicles fuse, embedding GLUT4 transporters into the plasma membrane.

  5. Glucose Uptake: Glucose rapidly enters muscle and adipose cells via facilitated diffusion.

Glycolysis Phase I: The Energy Investment Phase

• Phase Overview: Phase I converts one stable $6$–carbon glucose molecule into two $3$–carbon glyceraldehyde-3-phosphate (G3P) molecules through five enzymatic steps, requiring an investment of $2\text{ ATP}$.

• Rationale for Energy Investment:

  1. Increasing Chemical Reactivity: Free glucose is stable (uncatalyzed breakdown rate = $2.3 \times 10^{-10}\,s^{-1}$, half-life = $96\text{ years}$). $\text{ATP}$ phosphorylation yields high-energy phosphorylated intermediates.

  2. Cellular Trapping: Phosphorylation confers a negative charge, forming glucose-6-phosphate ($G6P$), for which no membrane export channels exist, trapping the sugar inside the cell.

• Reaction 1: Hexokinase / Glucokinase

  • Reaction Equation: $\text{Glucose} + \text{ATP} \xrightarrow{\text{Mg}^{2+}} \text{Glucose-6-Phosphate (G6P)} + \text{ADP} + \text{H}^+$

  • Thermodynamics: Irreversible and regulated. Standard $\Delta G'^\circ = -16.7\,kJ\,mol^{-1}$ ($\Delta G'^\circ = -20.9\,kJ\,mol^{-1}$ in heart muscle; cellular $\Delta G = -27.2\,kJ\,mol^{-1}$).

  • Cofactor Role: Requires $\text{Mg}^{2+}$ to coordinate $\text{ATP}^{4-}$ charges. The actual substrate is $\text{MgATP}^{2-}$.

  • Mechanism: Operates via a random sequential mechanism. The C6-hydroxyl group of glucose acts as a nucleophile attacking the $\gamma$-phosphate of $\text{ATP}$. Glucose binding induces a conformational change (induced fit) that closes the active site, excluding $\text{H}_2\text{O}$ and preventing non-productive $\text{ATP}$ hydrolysis.

  • Isoforms and Tissue Specificity:

  • Muscle Hexokinase (Isoforms I-III): High affinity for glucose ($K_M \approx 0.1\,mM$), operating near maximum velocity at normal blood glucose concentrations ($5\,mM$). Phosphorylates other hexoses (fructose, mannose) at lower affinities. Allosterically inhibited by its product, $G6P$ (feedback inhibition).

  • Liver Glucokinase (Hexokinase IV / GCK): Low affinity for glucose ($K_M \approx 10\,mM$), operating at rates proportional to blood glucose levels. Specific to glucose and not inhibited by $G6P$. Prevents the liver from trapping glucose needed by peripheral tissues during low blood sugar levels.

• Regulation of Glucokinase:

  • Sequestration: Glucokinase regulatory protein (GKRP) binds glucokinase in the nucleus, keeping it inactive. High cytosolic fructose-6-phosphate ($F6P$) promotes nuclear translocation and GKRP binding. High blood glucose levels cause GKRP dissociation, releasing active glucokinase into the cytosol.

  • Transcriptional Control: Insulin binds response elements on the GCK gene promoter to induce glucokinase synthesis. Glucagon induces cAMP-dependent signaling, increasing GKRP-mediated sequestration.

• Branch Points of $G6P$: $G6P$ is a substrate for Glycolysis, Gluconeogenesis, Glycogenesis (liver and muscle energy storage), Pentose Phosphate Pathway (producing $NADPH$ and Ribose-5-phosphate), and Hexosamine Biosynthesis (producing UDP-GlcNAc). Because $G6P$ serves multiple pathways, Hexokinase is NOT the first committed step of glycolysis.

  • Reaction 2: Phosphoglucoisomerase (PGI / Phosphohexose Isomerase)

• Reaction Equation: $\text{Glucose-6-Phosphate (G6P)} \rightleftharpoons \text{Fructose-6-Phosphate (F6P)}$

• Thermodynamics: Reversible, near equilibrium. Standard $\Delta G'^\circ = +1.67\,kJ\,mol^{-1}$ ($+2.2\,kJ\,mol^{-1}$ in heart muscle; cellular $\Delta G = -1.4\,kJ\,mol^{-1}$ to $-2.92\,kJ\,mol^{-1}$).

• Rationale: Isomerizes an aldose (pyranose) into a ketose (furanose), moving the carbonyl from C1 to C2. Phosphorylating the C1 primary alcohol in Step 3 is chemically easier than phosphorylating a hemiacetal C1. Converts $G6P$ into an almost symmetrical intermediate, positioning C3 and C4 for aldolytic cleavage in Step 4. The hemiketal of $F6P$ is a stronger nucleophile than the hemiacetal of $G6P$.

• Mechanism: Involves ring opening, base abstraction of a proton by active-site Glu to generate a cis-enediol intermediate, and ring closure to form $F6P$.

• Reaction 3: Phosphofructokinase-1 (PFK-1)

  • Reaction Equation: $\text{Fructose-6-Phosphate (F6P)} + \text{ATP} \xrightarrow{\text{Mg}^{2+}} \text{Fructose-1,6-Bisphosphate (F-1,6-BP)} + \text{ADP} + \text{H}^+$

  • Thermodynamics: Irreversible and rate-limiting. Standard $\Delta G'^\circ = -14.2\,kJ\,mol^{-1}$ ($-17.2\,kJ\,mol^{-1}$ in heart muscle; cellular $\Delta G = -18.8\,kJ\,mol^{-1}$ to $-25.9\,kJ\,mol^{-1}$).

  • Significance: The FIRST COMMITTED STEP of glycolysis.

  • Structural Allosteric Regulation: Homotetrameric enzyme exhibiting sigmoidal kinetics ($T$-state vs. $R$-state transitions).

  • High [$\text{ATP}$]: Allosterically inhibits PFK-1, stabilizing the low-affinity $T$-state and shifting the activity curve to the right.

  • High [$\text{ADP}$] / [$\text{AMP}$]: Allosterically activates PFK-1, signaling low cellular energy charge and restoring the high-affinity $R$-state.

  • Citrate: Allosterically inhibits PFK-1, signaling an abundance of TCA cycle intermediates.

  • Fructose-2,6-Bisphosphate ($F\text{-}2,6\text{-BP}$): Allosteric activator that relieves $\text{ATP}$ inhibition.

• Regulation by Bifunctional Enzyme PFK-2/FBPase-2:

  • PFK-2 activity synthesizes $F\text{-}2,6\text{-BP}$ from $F6P$; FBPase-2 activity hydrolyzes $F\text{-}2,6\text{-BP}$ to $F6P$. High [$F6P$] stimulates PFK-2 kinase activity.

  • Insulin: Activates phosphoprotein phosphatase, dephosphorylating the bifunctional enzyme. Dephosphorylated PFK-2 is active (FBPase-2 inactive), increasing [$F\text{-}2,6\text{-BP}$] and activating PFK-1 to stimulate glycolysis.

  • Glucagon: Activates cAMP-dependent protein kinase A (PKA), phosphorylating the bifunctional enzyme. Phosphorylated PFK-2 is inactive (FBPase-2 active), decreasing [$F\text{-}2,6\text{-BP}$] and inhibiting PFK-1, shifting metabolism toward gluconeogenesis.

• Reaction 4: Aldolase (Fructose-1,6-Bisphosphate Aldolase)

  • Reaction Equation: $\text{Fructose-1,6-Bisphosphate} \rightleftharpoons \text{Dihydroxyacetone Phosphate (DHAP)} + \text{Glyceraldehyde-3-Phosphate (G3P)}$

  • Thermodynamics: Standard $\Delta G'^\circ = +23.8\,kJ\,mol^{-1}$ to $+23.9\,kJ\,mol^{-1}$ ($+22.8\,kJ\,mol^{-1}$ in heart muscle). Cellular $\Delta G = -0.23\,kJ\,mol^{-1}$ to $-5.9\,kJ\,mol^{-1}$ (spontaneous and reversible) due to high substrate [$F\text{-}1,6\text{-BP}$] and continuous removal of G3P in Phase II.

  • Carbon Mapping: C1, C2, and C3 of $F\text{-}1,6\text{-BP}$ yield DHAP (ketotriose); C4, C5, and C6 yield G3P (aldotriose).

  • Mechanistic Classes:

  • Class I Aldolases (Animals, Plants): Uses an active-site Lysine residue to form a covalent, protonated Schiff base intermediate with the substrate C2 carbonyl. This delocalizes electrons to drive C3-C4 bond cleavage.

  • Class II Aldolases (Fungi, Bacteria): Uses active-site divalent metal ions ($\text{Zn}^{2+}$ or $\text{Fe}^{2+}$) to coordinate the carbonyl oxygen non-covalently and stabilize the enolate intermediate.

• Reaction 5: Triose Phosphate Isomerase (TIM / TPI)

  • Reaction Equation: $\text{Dihydroxyacetone Phosphate (DHAP)} \rightleftharpoons \text{Glyceraldehyde-3-Phosphate (G3P)}$

  • Thermodynamics: Reversible, near equilibrium. Standard $\Delta G'^\circ = +7.5\,kJ\,mol^{-1}$ ($+7.9\,kJ\,mol^{-1}$ in heart muscle; cellular $\Delta G = +2.7\,kJ\,mol^{-1}$ or $\sim 0\,kJ\,mol^{-1}$).

  • Catalytic Efficiency: TIM is a catalytically perfect enzyme ($k_{cat}/K_M$ approaches diffusion limit).

  • Mechanism: Employs acid-base catalysis via an enediol intermediate. Active-site Glu165 acts as a general base, abstracting the C1 proton of DHAP, while His95 acts as a general acid, donating a proton to the C2 carbonyl oxygen. The intermediate collapses into an aldehyde to form G3P.

Glycolysis Phase II: The Energy Payoff Phase

• Phase Overview: Phase II runs TWICE for every glucose molecule (once per G3P). It extracts energy from $3$–carbon intermediates to generate $4\text{ ATP}$ ($2\text{ ATP}$ net yield) and $2\text{ NADH}$.

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

  • Reaction Equation: $\text{G3P} + \text{NAD}^+ + \text{P}_i \rightleftharpoons \text{1,3-Bisphosphoglycerate (1,3-BPG)} + \text{NADH} + \text{H}^+$

  • Thermodynamics: Standard $\Delta G'^\circ = +6.3\,kJ\,mol^{-1}$. Couples exergonic aldehyde oxidation to endergonic phosphorylation, generating 1,3-BPG (a high-energy mixed acyl-phosphate anhydride).

  • Mechanism: An active-site Cysteine thiolate ($\text{S}^-$) exhibits a reduced $pK_a$ ($5.5$ vs. $8$ when $\text{NAD}^+$ is bound). The thiolate attacks the C1 aldehyde of G3P, forming a covalent thiohemiacetal intermediate. Hydride ($\text{H}^-$) transfer to $\text{NAD}^+$ forms $NADH$. $NADH$ leaves and is replaced by $\text{NAD}^+$. Inorganic phosphate ($\text{P}_i$) attacks the thioester linkage (phosphorolysis), releasing 1,3-BPG and restoring the active-site Cysteine.

• Reaction 7: Phosphoglycerate Kinase (PGK)

  • Reaction Equation: $\text{1,3-BPG} + \text{ADP} \xrightarrow{\text{Mg}^{2+}} \text{3-Phosphoglycerate (3PG)} + \text{ATP}$

  • Thermodynamics: Substrate-level phosphorylation. Standard $\Delta G'^\circ = -18.9\,kJ\,mol^{-1}$. Reversible under cellular conditions (cellular $\Delta G = +0.1\,kJ\,mol^{-1}$ in erythrocytes; $-1.4\,kJ\,mol^{-1}$ in heart muscle). Exergonic $\text{ATP}$ formation drives the preceding endergonic GAPDH reaction. Named after the reverse reaction.

• 1,3-BPG Bypass Shunt & Hemoglobin Regulation in Erythrocytes:

  • Erythrocytes contain Bisphosphoglycerate Mutase, which converts 1,3-BPG into 2,3-Bisphosphoglycerate ($2,3\text{-BPG}$). 2,3-Bisphosphoglycerate Phosphatase subsequently hydrolyzes $2,3\text{-BPG}$ to 3PG.

  • $2,3\text{-BPG}$ binds the central cavity of hemoglobin, stabilizing the $T$-state and promoting $\text{O}_2$ release in peripheral tissues.

  • Enzymatic Deficiencies:

  • Hexokinase Deficiency: Blocks early glycolysis, decreasing [$2,3\text{-BPG}$]. Hemoglobin shifts to the left (higher $\text{O}_2$ affinity, reduced unloading).

  • Pyruvate Kinase Deficiency: Blocks late glycolysis, leading to accumulation of upstream intermediates and increased [$2,3\text{-BPG}$]. Hemoglobin shifts to the right (lower $\text{O}_2$ affinity, enhanced unloading).

• Reaction 8: Phosphoglycerate Mutase (PGM)

  • Reaction Equation: $\text{3-Phosphoglycerate (3PG)} \rightleftharpoons \text{2-Phosphoglycerate (2PG)}$

  • Thermodynamics: Reversible isomerization. Standard $\Delta G'^\circ = +4.4\,kJ\,mol^{-1}$ ($+4.7\,kJ\,mol^{-1}$ in heart muscle; cellular $\Delta G = -0.6\,kJ\,mol^{-1}$ to $+0.83\,kJ\,mol^{-1}$).

  • Mechanism: Requires an active-site Histidine phosphorylated by trace amounts of $2,3\text{-BPG}$. The phosphoenzyme donates its phosphate to C2 of 3PG, forming a $2,3\text{-BPG}$ intermediate. The C3 phosphate is then transferred back to the Histidine, releasing 2PG.

• Reaction 9: Enolase

  • Reaction Equation: $\text{2-Phosphoglycerate (2PG)} \rightleftharpoons \text{Phosphoenolpyruvate (PEP)} + \text{H}_2\text{O}$

  • Thermodynamics: Reversible dehydration. Standard $\Delta G'^\circ = +1.8\,kJ\,mol^{-1}$ ($-3.2\,kJ\,mol^{-1}$ in heart muscle; cellular $\Delta G = -2.4\,kJ\,mol^{-1}$ to $+1.1\,kJ\,mol^{-1}$).

  • Cofactor & Inhibition: Requires $\text{Mg}^{2+}$ to stabilize the enolate intermediate. Fluoride ($\text{F}^-$) inhibits enolase by forming fluorophosphates that complex with $\text{Mg}^{2+}$.

  • Purpose: Converts a low-energy phosphoester (2PG) into a high-energy enol phosphate (PEP).

• Reaction 10: Pyruvate Kinase (PK)

  • Reaction Equation: $\text{Phosphoenolpyruvate (PEP)} + \text{ADP} + \text{H}^+ \xrightarrow{\text{Mg}^{2+}, \text{K}^+} \text{Pyruvate} + \text{ATP}$

  • Thermodynamics: Irreversible substrate-level phosphorylation. Standard $\Delta G'^\circ = -31.4\,kJ\,mol^{-1}$ to $-31.7\,kJ\,mol^{-1}$ ($-23.0\,kJ\,mol^{-1}$ in heart muscle; cellular $\Delta G = -13.9\,kJ\,mol^{-1}$).

  • Mechanism: Phosphate transfer to $\text{ADP}$ leaves an enolpyruvate intermediate, which spontaneously tautomerizes into the stable keto form of pyruvate. Exergonic keto-enol tautomerization drives $\text{ATP}$ synthesis.

  • Allosteric Regulation (All Tissues):

  • Activators: $\text{AMP}$ (low energy signal) and Fructose-1,6-bisphosphate (feed-forward activation).

  • Inhibitors: $\text{ATP}$, Acetyl-CoA, long-chain fatty acids, and Alanine (transamination product of pyruvate).

  • Covalent Regulation (Liver PK-L Isoform): Glucagon activates PKA, which phosphorylates and inactivates PK-L. Protein Phosphatase (PP) dephosphorylates and activates PK-L.

Thermodynamics, Regulation, and Energetics of Glycolysis

• Thermodynamic Parameters in Heart Muscle (Table 15-1):

  • Reaction 1 (Hexokinase): $\Delta G'^\circ = -20.9\,kJ\,mol^{-1}$, $\Delta G = -27.2\,kJ\,mol^{-1}$.

  • Reaction 2 (PGI): $\Delta G'^\circ = +2.2\,kJ\,mol^{-1}$, $\Delta G = -1.4\,kJ\,mol^{-1}$.

  • Reaction 3 (PFK): $\Delta G'^\circ = -17.2\,kJ\,mol^{-1}$, $\Delta G = -25.9\,kJ\,mol^{-1}$.

  • Reaction 4 (Aldolase): $\Delta G'^\circ = +22.8\,kJ\,mol^{-1}$, $\Delta G = -5.9\,kJ\,mol^{-1}$.

  • Reaction 5 (TIM): $\Delta G'^\circ = +7.9\,kJ\,mol^{-1}$, $\Delta G \approx 0\,kJ\,mol^{-1}$.

  • Reactions 6 + 7 (GAPDH + PGK): $\Delta G'^\circ = -16.7\,kJ\,mol^{-1}$, $\Delta G = -1.1\,kJ\,mol^{-1}$.

  • Reaction 8 (PGM): $\Delta G'^\circ = +4.7\,kJ\,mol^{-1}$, $\Delta G = -0.6\,kJ\,mol^{-1}$.

  • Reaction 9 (Enolase): $\Delta G'^\circ = -3.2\,kJ\,mol^{-1}$, $\Delta G = -2.4\,kJ\,mol^{-1}$.

  • Reaction 10 (PK): $\Delta G'^\circ = -23.0\,kJ\,mol^{-1}$, $\Delta G = -13.9\,kJ\,mol^{-1}$.

• Principles of Flux Regulation: Three steps (Steps 1, 3, and 10) display large negative cellular $\Delta G$ values, making them irreversible flux-controlling checkpoints. The remaining seven steps operate near equilibrium ($\Delta G \approx 0$) and are shared with gluconeogenesis.

• Net Chemical Equation of Glycolysis:   $\text{Glucose} + 2\,\text{NAD}^+ + 2\,\text{ADP} + 2\,\text{P}_i \rightarrow 2\,\text{Pyruvate} + 2\,\text{NADH} + 2\,\text{H}^+ + 2\,\text{ATP} + 2\,\text{H}_2\text{O}$

Metabolic Fates of Pyruvate and Anaerobic Fermentation Pathways

• Fate 1: Aerobic Respiration (Mitochondria)

  • Fed State: Pyruvate Dehydrogenase converts pyruvate into acetyl-CoA, which enters the TCA cycle to produce $\text{NADH}$ and $\text{FADH}_2$ for $\text{ATP}$ synthesis via oxidative phosphorylation.

  • Fasting State (Liver): Pyruvate Carboxylase converts pyruvate into oxaloacetate to initiate gluconeogenesis, producing glucose for export into the bloodstream.

• Fate 2: Lactic Acid Fermentation (Anaerobic - Animals / Erythrocytes)

  • Reaction Equation: $\text{Pyruvate} + \text{NADH} + \text{H}^+ \xrightleftharpoons{\text{Lactate Dehydrogenase}} \text{L-Lactate} + \text{NAD}^+$

  • Thermodynamics: Standard $\Delta G'^\circ = -25.1\,kJ\,mol^{-1}$.

  • Mechanism: Lactate Dehydrogenase transfers a hydride ($\text{H}^-$) from $NADH$ to the C2 carbonyl of pyruvate, while His195 donates a proton. Active-site Arg109 and Arg171 residues coordinate charges.

• Muscle Fiber Physiology:

  • Slow-Twitch Fibers: Adapted for endurance (marathon runners). Utilize aerobic respiration, containing abundant mitochondria, high myoglobin, and dense capillary networks for slow, sustained energy production.

  • Fast-Twitch Fibers: Adapted for speed and strength (sprinters). Rely on anaerobic glycolysis, containing fewer mitochondria, low myoglobin, and sparse blood vessels for rapid bursts of energy.

• The Cori Cycle: Lactate generated in active muscle travels through the blood to the liver. The liver oxidizes lactate back to pyruvate via Lactate Dehydrogenase and uses gluconeogenesis to synthesize glucose (costing $6\text{ ATP}$ equivalents). The synthesized glucose is then released back into the bloodstream for muscle use. Discovered by Drs. Carl and Gerty Cori.

• Clinical Aspect - Lactic Acidosis: Tissue hypoxia or impaired hepatic lactate clearance causes blood lactate accumulation and a drop in blood pH ($\downarrow pH$). Associated with liver failure, type 2 diabetes, and cardiac arrest. Symptoms include muscle pain, tenderness, and fatigue.

  • Fate 3: Ethanol Fermentation (Anaerobic - Yeast / Microorganisms)

• Step 1: Pyruvate Decarboxylase (PDC)     $\text{Pyruvate} \xrightarrow{\text{TPP}, \text{Mg}^{2+}} \text{Acetaldehyde} + \text{CO}_2$

  • Requires $\text{Mg}^{2+}$ and Thiamine Pyrophosphate (TPP, derived from Vitamin B1).

  • TPP Structure: Consists of an aminopyrimidine ring linked to a thiazolium ring. The acidic C2 proton on the thiazolium ring deprotonates to form a nucleophilic ylide (carbanion).

  • Mechanism: The ylide attacks the C2 carbonyl of pyruvate. Decarboxylation releases $\text{CO}_2$, and TPP stabilizes the resulting carbanion via resonance. Protonation yields hydroxyethyl-TPP, which eliminates toxic acetaldehyde and regenerates the TPP ylide. PDC is present in microorganisms but absent in humans.

• Step 2: Alcohol Dehydrogenase (ADH)     $\text{Acetaldehyde} + \text{NADH} + \text{H}^+ \xrightleftharpoons{\text{Zn}^{2+}} \text{Ethanol} + \text{NAD}^+$

  • Active-site $\text{Zn}^{2+}$ polarizes the acetaldehyde carbonyl oxygen. Hydride transfer from $NADH$ reduces acetaldehyde to ethanol, regenerating $\text{NAD}^+$. Present in yeast and human liver.

  • Clinical and Toxicity Aspects:

• Vitamin B1 (Thiamine) Deficiency: Prevents TPP synthesis, impairing decarboxylation reactions. Causes Dry Beriberi, characterized by emaciation, confusion, loss of tendon reflexes, burning/tingling, numbness of feet, foot drop, wrist drop, weakness, and inability to speak.

• Hepatic Alcohol Processing: Ingested ethanol travels to the liver and is converted by ADH into acetaldehyde, and then to acetate. This produces excess $NADH$ and elevates glycerol-3-phosphate levels, promoting fatty acid and triacylglycerol synthesis. Ethanol also inhibits $\beta$-oxidation and VLDL transport, causing lipid accumulation leading to Fatty Liver Disease.

• Medication Warning: Avoid combining Tylenol (acetaminophen) with alcohol, as ethanol alters clearance pathways, forming hepatotoxic byproducts. Advil is recommended instead.

Feeder Pathways and Alternative Hexose Metabolism

• Entry of Alternative Carbohydrates:

• Endogenous Glycogen/Starch: Cleaved by Glycogen Phosphorylase to Glucose-1-phosphate (G1P), which Phosphoglucomutase converts to $G6P$.

• Dietary Polysaccharides: Hydrolyzed by $\alpha$-amylase to free glucose, which is phosphorylated to $G6P$ by hexokinase.

• Disaccharide Cleavage: Lactose is hydrolyzed by Lactase into Galactose and Glucose; Sucrose is hydrolyzed by Sucrase into Fructose and Glucose ($\alpha(1\rightarrow 2)$ linkage); Trehalose is hydrolyzed by Trehalase into Glucose.

  • Fructose Metabolism Pathways:

• In Muscle / Adipose Tissue: Hexokinase directly phosphorylates Fructose to Fructose-6-phosphate ($F6P$) using $\text{ATP}$, entering glycolysis at Step 3.

• In Liver Tissue (Fructolysis): Glucokinase cannot phosphorylate fructose. Fructose is processed through the liver-specific fructolytic pathway:

  1. Fructokinase (Ketohexokinase / KHK): $\text{Fructose} + \text{ATP} \rightarrow \text{Fructose-1-phosphate (F1P)} + \text{ADP}$.

  2. Fructose-1-Phosphate Aldolase (Aldolase B / ALDOB): Cleaves F1P into Dihydroxyacetone Phosphate (DHAP) and Glyceraldehyde.

  3. Triose Phosphate Isomerase: Converts DHAP to Glyceraldehyde-3-phosphate (G3P).

  4. Glyceraldehyde Kinase (Triokinase): $\text{Glyceraldehyde} + \text{ATP} \rightarrow \text{G3P}$, entering glycolysis.

  5. Secondary Glyceraldehyde Branch: Alcohol Dehydrogenase reduces Glyceraldehyde to Glycerol. Glycerol Kinase phosphorylates Glycerol to Glycerol-3-phosphate. Glycerol Phosphate Dehydrogenase oxidizes Glycerol-3-phosphate to DHAP.

• Inborn Errors of Fructose Metabolism:

  • Essential Fructosuria: Fructokinase deficiency. Benign condition where unphosphorylated fructose is excreted in urine.

  • Hereditary Fructose Intolerance (HFI): Aldolase B deficiency. F1P accumulates, trapping inorganic phosphate ($\text{P}_i$), depleting $\text{ATP}$, and inhibiting gluconeogenesis and glycogenolysis. Causes severe Hypoglycemia and liver dysfunction.

• High Fructose Corn Syrup (HFCS) and Nonalcoholic Fatty Liver Disease (NAFLD): HFCS (50% fructose, 42% glucose) bypasses the rate-limiting PFK-1 regulatory step. Unregulated fructolysis in the liver generates excessive Glycerol-3-phosphate and Acetyl-CoA, driving de novo Lipogenesis (triglyceride synthesis) and leading to NAFLD. $\text{ATP}$ depletion from rapid fructokinase activity also increases $\text{AMP}$ degradation to uric acid, contributing to hyperuricemia.

  • Galactose Metabolism (The Leloir Pathway):

• Galactose is the C4 epimer of Glucose.

• Galactokinase: $\text{Galactose} + \text{ATP} \rightarrow \text{Galactose-1-phosphate} + \text{ADP}$.

• Galactose-1-phosphate Uridylyltransferase: $\text{Galactose-1-phosphate} + \text{UDP-Glucose} \rightleftharpoons \text{UDP-Galactose} + \text{Glucose-1-phosphate (G1P)}$.

• UDP-Glucose-4-Epimerase: $\text{UDP-Galactose} \rightleftharpoons \text{UDP-Glucose}$ (uses $\text{NAD}^+$).

• Phosphoglucomutase: $\text{G1P} \rightleftharpoons \text{Glucose-6-phosphate (G6P)}$, entering glycolysis.

• Clinical Aspect - Galactosemia: Deficiency in Galactose-1-phosphate Uridylyltransferase (Step 2). Accumulated galactose is reduced by Aldose Reductase into toxic galactitol. Causes cataracts (galactitol accumulation in lens tissue), jaundice, severe liver damage, developmental delays, motor abnormalities, and sepsis. Treated with a strict galactose- and lactose-free diet.

  • Mannose Metabolism:

• Hexokinase: $\text{Mannose} + \text{ATP} \rightarrow \text{Mannose-6-phosphate} + \text{ADP}$.

• Phosphomannose Isomerase: $\text{Mannose-6-phosphate} \rightleftharpoons \text{Fructose-6-phosphate (F6P)}$, entering glycolysis at Step 3.