Unit 2: Carbohydrate Metabolism

1. Glycolysis and Regulation

Conceptual Overview and Definition

Glycolysis (Embden-Meyerhof-Parnas Pathway): A sequence of ten enzyme-catalyzed reactions occurring in the cytosol of eukaryotic and prokaryotic cells that converts one molecule of glucose (6-carbon) into two molecules of pyruvate (3-carbon), with the net production of 2 ATP and 2 NADH.

Glycolysis operates under both aerobic (oxygen present) and anaerobic (oxygen absent) conditions. It is the primary cytosolic pathway for carbohydrate catabolism and provides essential metabolic intermediates for synthetic pathways.

Subcellular Location: Cytosol

Tissue Location: All tissues (obligate energy source for mature erythrocytes, brain, renal medulla, and exercising skeletal muscle).

Sequence of Reactions in Glycolysis

Glycolysis is divided into two distinct phases: the Preparatory (Energy Investment) Phase and the Payoff (Energy Generation) Phase.

Phase I: Preparatory Phase (Steps 1–5)

Two molecules of ATP are consumed to phosphorylate hexose sugars, trapping glucose inside the cell and destabilizing it for cleavage.

1. Phosphorylation of Glucose:

   Glucose is phosphorylated at C-6 by ATP to form Glucose-6-phosphate (G-6-P), catalyzed by Hexokinase (in all tissues) or Glucokinase (Hexokinase IV, in liver and pancreatic β-cells). This reaction is irreversible and consumes 1 ATP.

   Glucose + ATP → Glucose-6-phosphate + ADP + H⁺

2. Isomerization of Glucose-6-Phosphate:

   G-6-P is isomerized to Fructose-6-phosphate (F-6-P) by Phosphohexose Isomerase (Phosphoglucose Isomerase). This reaction is reversible.

   Glucose-6-phosphate ⇄ Fructose-6-phosphate

3. Phosphorylation of Fructose-6-Phosphate (The Rate-Limiting Step):

   F-6-P is phosphorylated at C-1 by ATP to yield Fructose-1,6-bisphosphate (F-1,6-BP), catalyzed by Phosphofructokinase-1 (PFK-1). This reaction is irreversible, consumes 1 ATP, and is the principal committed step of glycolysis.

   Fructose-6-phosphate + ATP → Fructose-1,6-bisphosphate + ADP + H⁺

4. Cleavage of Fructose-1,6-Bisphosphate:

   F-1,6-BP is cleaved by Aldolase (Fructose-1,6-bisphosphate aldolase) into two 3-carbon triose phosphates: Glyceraldehyde-3-phosphate (GAP) and Dihydroxyacetone phosphate (DHAP). Reversible reaction.

   Fructose-1,6-bisphosphate ⇄ Glyceraldehyde-3-phosphate + Dihydroxyacetone phosphate

5. Isomerization of Triose Phosphates:

   DHAP is rapidly and reversibly isomerized into a second molecule of Glyceraldehyde-3-phosphate by Triose Phosphate Isomerase. Thus, one glucose molecule generates two molecules of Glyceraldehyde-3-phosphate.

   Dihydroxyacetone phosphate ⇄ Glyceraldehyde-3-phosphate

Phase II: Payoff Phase (Steps 6–10)

Each G-3-P molecule is converted to pyruvate. Because two trioses enter this phase per glucose, all yields are doubled.

6. Oxidation and Phosphorylation of Glyceraldehyde-3-Phosphate:

   GAP undergoes oxidation and phosphorylation by inorganic phosphate (P_i) catalyzed by Glyceraldehyde-3-Phosphate Dehydrogenase (GAPDH) to produce 1,3-Bisphosphoglycerate (1,3-BPG) and reduce NAD⁺ to NADH. Requires NAD⁺.

   Glyceraldehyde-3-phosphate + P_i + NAD⁺ ⇄ 1,3-Bisphosphoglycerate + NADH + H⁺

7. Substrate-Level Phosphorylation (First ATP Yield):

   High-energy phosphate group from 1,3-BPG is transferred to ADP by Phosphoglycerate Kinase, forming 3-Phosphoglycerate (3-PG) and 1 ATP per triose (2 ATP total). Reversible.

   1,3-Bisphosphoglycerate + ADP ⇄ 3-Phosphoglycerate + ATP

8. Isomerization of 3-Phosphoglycerate:

   3-PG is converted to 2-Phosphoglycerate (2-PG) by shifting the phosphate group from C-3 to C-2, catalyzed by Phosphoglycerate Mutase.

   3-Phosphoglycerate ⇄ 2-Phosphoglycerate

9. Dehydration of 2-Phosphoglycerate:

   2-PG loses H₂O to form Phosphoenolpyruvate (PEP), catalyzed by Enolase. This creates a compound with high phosphate-group transfer potential. Fluoride inhibits Enolase.

   2-Phosphoglycerate ⇄ Phosphoenolpyruvate + H₂O

10. Substrate-Level Phosphorylation (Second ATP Yield):

    PEP transfers its high-energy phosphate group to ADP via Pyruvate Kinase, yielding Pyruvate and 1 ATP per triose (2 ATP total). Irreversible step.

    Phosphoenolpyruvate + ADP + H⁺ → Pyruvate + ATP

Overall Reaction and Net Yield

Net Aerobic Reaction:
Glucose + 2 NAD⁺ + 2 ADP + 2 P_i → 2 Pyruvate + 2 NADH + 2 H⁺ + 2 ATP + 2 H₂O

• ATP Consumed: 2 ATP (Steps 1 and 3)
• ATP Produced: 4 ATP (Steps 7 and 10)
• Net ATP Yield: 2 ATP per glucose molecule
• Reducing Equivalent Yield: 2 NADH per glucose molecule

Regulation of Glycolysis

Glycolysis is regulated at three irreversible key enzyme steps:

• 1. Hexokinase / Glucokinase:
  • Hexokinase (Types I–III): Inhibited allosterically by its product, Glucose-6-phosphate. High affinity for glucose (low K_m).
  • Glucokinase (Hexokinase IV): Located in liver and pancreatic β-cells. Low affinity for glucose (high K_m), not inhibited by G-6-P; regulated by Glucokinase Regulatory Protein (GKRP).
• 2. Phosphofructokinase-1 (PFK-1) — Primary Rate-Limiting Enzyme:
  • Allosteric Activators: AMP, ADP, Fructose-2,6-bisphosphate (F-2,6-BP).
  • Allosteric Inhibitors: ATP, Citrate, H⁺ (low pH).
  • Hormonal control: Insulin increases F-2,6-BP levels (stimulates PFK-1); Glucagon decreases F-2,6-BP levels (inhibits PFK-1).
• 3. Pyruvate Kinase:
  • Allosteric Activator: Fructose-1,6-bisphosphate (feed-forward activation).
  • Allosteric Inhibitors: ATP, Acetyl-CoA, Alanine.
  • Covalent Regulation: Phosphorylated (inactivated) by Protein Kinase A under glucagon influence in liver; dephosphorylated (activated) by insulin.

2. Fate of Pyruvate

Pyruvate occupies a key metabolic junction. Its metabolic destination depends on cellular oxygen availability, tissue type, and energy state.

1. Aerobic Fate: Oxidative Decarboxylation to Acetyl-CoA

In oxygenated conditions with functional mitochondria, pyruvate enters the mitochondrial matrix via a specific transporter and undergoes oxidative decarboxylation to produce Acetyl-CoA, catalyzed by the multi-enzyme Pyruvate Dehydrogenase Complex (PDH).

Pyruvate + NAD⁺ + CoA-SH → Acetyl-CoA + CO₂ + NADH + H⁺

Required Coenzymes for PDH Complex:

1. Thiamine pyrophosphate (TPP, Vitamin B₁)
2. Lipoic acid / Lipoamide
3. Coenzyme A (CoA-SH, Vitamin B₅)
4. Flavin adenine dinucleotide (FAD, Vitamin B₂)
5. Nicotinamide adenine dinucleotide (NAD⁺, Vitamin B₃)

2. Anaerobic Fate in Animals: Reduction to Lactate

In hypoxic tissues (e.g., active skeletal muscle) or cells lacking mitochondria (e.g., mature erythrocytes), pyruvate is reduced to lactate by Lactate Dehydrogenase (LDH). This regenerates cytosolic NAD⁺ required to keep GAPDH (Step 6 of glycolysis) functioning for continuous anaerobic ATP generation.

Pyruvate + NADH + H⁺ ⇄ Lactate + NAD⁺

Cori Cycle Connection: Lactate produced in red blood cells or skeletal muscle enters the blood stream, travels to the liver, and is converted back to pyruvate for gluconeogenesis.

3. Anaerobic Fate in Yeast/Microorganisms: Ethanol Fermentation

In yeast and certain bacterial microorganisms under anaerobic conditions, pyruvate is converted to ethanol and carbon dioxide in two steps:

1. Decarboxylation of pyruvate to acetaldehyde by Pyruvate Decarboxylase (requires TPP and Mg²⁺).
2. Reduction of acetaldehyde to ethanol by Alcohol Dehydrogenase, regenerating NAD⁺.

Pyruvate → Acetaldehyde + CO₂
Acetaldehyde + NADH + H⁺ ⇄ Ethanol + NAD⁺

4. Carboxylation to Oxaloacetate (Anaplerotic / Gluconeogenic Pathway)

Pyruvate can undergo ATP-dependent carboxylation into Oxaloacetate (OAA) by Pyruvate Carboxylase (requires Biotin). This reaction is essential for gluconeogenesis and for replenishing TCA cycle intermediates.

Pyruvate + CO₂ + ATP + H₂O → Oxaloacetate + ADP + P_i + 2 H⁺

3. Citric Acid Cycle (TCA Cycle)

Conceptual Overview

Citric Acid Cycle (Krebs Cycle / Tricarboxylic Acid Cycle): A cyclic, 8-step sequence of mitochondrial reactions that oxidizes the acetyl group of Acetyl-CoA into two molecules of CO₂, producing high-energy electron carriers (NADH, FADH₂) and GTP/ATP.

Subcellular Location: Mitochondrial Matrix (except Succinate Dehydrogenase, which is integrated into the inner mitochondrial membrane as Complex II).

Sequence of Reactions in the Citric Acid Cycle

1. Citrate Formation (Condensation):

   Acetyl-CoA (2C) condenses with Oxaloacetate (4C) and H₂O to yield Citrate (6C) and free CoA-SH, catalyzed by Citrate Synthase. Irreversible entry step.

   Acetyl-CoA + Oxaloacetate + H₂O → Citrate + CoA-SH

2. Isomerization of Citrate to Isocitrate:

   Citrate is isomerized to Isocitrate via an intermediate, cis-aconitate, by Aconitase (containing an Fe-S iron-sulfur center).

   Citrate ⇄ cis-Aconitate ⇄ Isocitrate

3. First Oxidative Decarboxylation:

   Isocitrate undergoes oxidative decarboxylation to form α-Ketoglutarate (5C) and CO₂, catalyzed by Isocitrate Dehydrogenase. Generates 1 NADH. Primary rate-limiting enzyme of TCA cycle.

   Isocitrate + NAD⁺ → α-Ketoglutarate + CO₂ + NADH + H⁺

4. Second Oxidative Decarboxylation:

   α-Ketoglutarate undergoes oxidative decarboxylation to form Succinyl-CoA (4C) and CO₂ via α-Ketoglutarate Dehydrogenase Complex. Generates 1 NADH. Requires the same 5 coenzymes as PDH.

   α-Ketoglutarate + NAD⁺ + CoA-SH → Succinyl-CoA + CO₂ + NADH + H⁺

5. Substrate-Level Phosphorylation:

   Cleavage of high-energy thioester bond in Succinyl-CoA yields Succinate (4C) and generates GTP (or ATP) from GDP (or ADP) and P_i, catalyzed by Succinyl-CoA Synthetase (Succinate Thiokinase).

   Succinyl-CoA + GDP + P_i ⇄ Succinate + GTP + CoA-SH

6. Oxidation of Succinate:

   Succinate is oxidized to Fumarate (4C) by Succinate Dehydrogenase, transferring electrons to FAD to form FADH₂. Inhibited competitively by Malonate.

   Succinate + FAD ⇄ Fumarate + FADH₂

7. Hydration of Fumarate:

   Water is added across the double bond of Fumarate to form L-Malate, catalyzed by Fumarase (Fumarate Hydratase).

   Fumarate + H₂O ⇄ L-Malate

8. Oxidation of Malate (Regeneration of Oxaloacetate):

   L-Malate is oxidized to Oxaloacetate by Malate Dehydrogenase, generating 1 NADH. Completes the cycle.

   L-Malate + NAD⁺ ⇄ Oxaloacetate + NADH + H⁺

Stoichiometry and Energy Yield

Per one Acetyl-CoA molecule oxidized in the TCA cycle:

• CO₂ released: 2 molecules
• NADH generated: 3 molecules (Steps 3, 4, 8) → ~7.5 ATP via oxidative phosphorylation
• FADH₂ generated: 1 molecule (Step 6) → ~1.5 ATP via oxidative phosphorylation
• GTP / ATP generated: 1 molecule (Step 5)
• Total ATP Yield per Acetyl-CoA: 10 ATP (traditional calculation: 12 ATP)
• Total ATP Yield per Glucose (2 Acetyl-CoA): 20 ATP from TCA cycle alone

Amphibolic Nature of the Citric Acid Cycle

The TCA cycle is amphibolic (functions in both catabolism and anabolism):

• Catabolic Role: Complete oxidation of acetyl groups derived from carbohydrates, fatty acids, and amino acids.
• Anabolic Functions:
  • Citrate: Exported to cytosol for fatty acid and cholesterol synthesis.
  • α-Ketoglutarate: Transaminated to Glutamate (amino acid and purine synthesis).
  • Succinyl-CoA: Utilized for Heme biosynthesis.
  • Oxaloacetate: Transaminated to Aspartate or directed to Gluconeogenesis.

Regulation of the Citric Acid Cycle

The cycle is regulated primarily by substrate availability and product inhibition at three exergonic key steps:

• Citrate Synthase: Inhibited by ATP, NADH, Succinyl-CoA, and Citrate.
• Isocitrate Dehydrogenase: Activated by ADP and Ca²⁺; Inhibited by ATP and NADH.
• α-Ketoglutarate Dehydrogenase: Activated by Ca²⁺; Inhibited by Succinyl-CoA, NADH, and ATP.

4. Pentose Phosphate Pathway (PPP)

Conceptual Overview

Pentose Phosphate Pathway (Hexose Monophosphate Shunt / HMP Pathway): An alternative cytosolic pathway of glucose catabolism that does not directly produce ATP, but generates NADPH for reductive biosynthesis and ribose-5-phosphate for nucleotide synthesis.

Subcellular Location: Cytosol

Tissue Distribution: Active in tissues synthesizing lipids/steroids or exposed to oxygen radicals (Liver, Adipose tissue, Adrenal cortex, Lactating mammary glands, Erythrocytes, Testes).

Phases of the Pentose Phosphate Pathway

Phase 1: Irreversible Oxidative Phase

Generates 2 NADPH and 1 Pentose Phosphate per Glucose-6-phosphate oxidized with loss of 1 CO₂.

1. Oxidation of Glucose-6-Phosphate:

   Glucose-6-phosphate is oxidized to 6-Phosphoglucono-δ-lactone by Glucose-6-Phosphate Dehydrogenase (G6PD), generating 1 NADPH. Key rate-limiting step.

   Glucose-6-phosphate + NADP⁺ → 6-Phosphoglucono-δ-lactone + NADPH + H⁺

2. Hydrolysis:

   6-Phosphoglucono-δ-lactone is hydrolyzed to 6-Phosphogluconate by 6-Phosphogluconolactonase.

   6-Phosphoglucono-δ-lactone + H₂O → 6-Phosphogluconate + H⁺

3. Oxidative Decarboxylation:

   6-Phosphogluconate undergoes oxidative decarboxylation by 6-Phosphogluconate Dehydrogenase to yield Ribulose-5-phosphate, CO₂, and 1 NADPH.

   6-Phosphogluconate + NADP⁺ → Ribulose-5-phosphate + CO₂ + NADPH

Phase 2: Reversible Non-Oxidative Phase

Interconverts 3C, 4C, 5C, 6C, and 7C sugar phosphates to recycle excess pentose phosphates back into glycolytic intermediates (Fructose-6-phosphate and Glyceraldehyde-3-phosphate).

• Isomerization/Epimerization: Ribulose-5-phosphate is converted into Ribose-5-phosphate (by Ribose-5-Phosphate Isomerase) or Xylulose-5-phosphate (by Ribulose-5-Phosphate Epimerase).
• Transketolase Reactions: Transfer 2-carbon units between sugars. Requires Thiamine pyrophosphate (TPP, Vitamin B₁) as cofactor.
• Transaldolase Reaction: Transfers 3-carbon units between sugars.

Overall Non-Oxidative Stoichiometry:
3 Ribose-5-phosphate ⇄ 2 Fructose-6-phosphate + 1 Glyceraldehyde-3-phosphate

Functions of the Pathway Products

• NADPH: Provides reducing power for lipid/steroid biosynthesis, maintenance of reduced Glutathione (GSH) in erythrocytes to clear ROS and H₂O₂, and cytochrome P450 hydroxylation reactions.
• Ribose-5-Phosphate: Required for de novo synthesis of RNA, DNA, ATP, NADH, FAD, and Coenzyme A.

Clinical Significance: G6PD Deficiency

Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency: An X-linked recessive genetic disorder. Deficiency in G6PD impairs NADPH generation in erythrocytes. Without adequate NADPH, cells cannot regenerate reduced glutathione (GSH). Exposure to oxidative stress (fava beans, antimalarials like primaquine, infections, sulfonamides) causes hemoglobin denaturing (Heinz bodies) and acute hemolytic anemia.

5. Gluconeogenesis

Conceptual Overview

Gluconeogenesis: The metabolic pathway through which glucose is synthesized from non-carbohydrate precursors during periods of fasting, starvation, or low carbohydrate intake.

Primary Sites: Liver (80-90%) and Kidney Cortex (10-20%).

Subcellular Location: Shared between Mitochondria, Cytosol, and Endoplasmic Reticulum.

Precursors for Gluconeogenesis

• Lactate: Derived from anaerobic glycolysis in RBCs and skeletal muscle (converted to pyruvate via Lactate Dehydrogenase in liver).
• Glucogenic Amino Acids: Mainly Alanine from muscle protein degradation (transaminated to pyruvate).
• Glycerol: Released during triacylglycerol breakdown in adipose tissue (phosphorylated by Glycerol Kinase to glycerol-3-phosphate, then oxidized to DHAP).
• Propionate: From odd-chain fatty acid oxidation (converted to Succinyl-CoA).

Bypass Reactions of Gluconeogenesis

Glycolysis has 3 irreversible steps (catalyzed by Hexokinase, PFK-1, and Pyruvate Kinase). Gluconeogenesis uses 4 unique enzymes to bypass these thermodynamic barriers:

Bypass 1: Conversion of Pyruvate to Phosphoenolpyruvate (Bypassing Pyruvate Kinase)

Requires two sequential enzymatic steps:

1. Pyruvate Carboxylation (Mitochondrial Matrix):

   Pyruvate is converted to Oxaloacetate by Pyruvate Carboxylase. Consumes 1 ATP. Requires Biotin as cofactor. Acetyl-CoA acts as an obligate allosteric activator.

   Pyruvate + CO₂ + ATP + H₂O → Oxaloacetate + ADP + P_i + 2 H⁺

2. Transport of Oxaloacetate to Cytosol:

   Oxaloacetate cannot directly cross the inner mitochondrial membrane; it is reduced to Malate by mitochondrial Malate Dehydrogenase, transported to cytosol, and re-oxidized back to Oxaloacetate by cytosolic Malate Dehydrogenase.

3. Decarboxylation and Phosphorylation (Cytosol):

   Oxaloacetate is converted to Phosphoenolpyruvate (PEP) by Phosphoenolpyruvate Carboxykinase (PEPCK). Consumes 1 GTP.

   Oxaloacetate + GTP → Phosphoenolpyruvate + GDP + CO₂

Bypass 2: Dephosphorylation of Fructose-1,6-Bisphosphate (Bypassing PFK-1)

Fructose-1,6-bisphosphate is hydrolyzed to Fructose-6-phosphate by Fructose-1,6-Bisphosphatase (FBPase-1). Releases P_i without generating ATP. Key regulatory rate-limiting step of gluconeogenesis.

Fructose-1,6-bisphosphate + H₂O → Fructose-6-phosphate + P_i

Bypass 3: Dephosphorylation of Glucose-6-Phosphate (Bypassing Hexokinase/Glucokinase)

Glucose-6-phosphate is transported into the lumen of the Endoplasmic Reticulum and hydrolyzed to free Glucose by Glucose-6-Phosphatase. Free glucose is exported into blood stream via GLUT transporters.

Glucose-6-phosphate + H₂O → Glucose + P_i

Exam Note: Muscle tissue lacks Glucose-6-Phosphatase, meaning muscle glycogen cannot directly supply glucose to the bloodstream.

Net Energetics of Gluconeogenesis

Synthesis of 1 molecule of glucose from 2 molecules of pyruvate requires substantial energy investment:

2 Pyruvate + 4 ATP + 2 GTP + 2 NADH + 2 H⁺ + 6 H₂O → Glucose + 4 ADP + 2 GDP + 6 P_i + 2 NAD⁺

Reciprocal Regulation of Glycolysis and Gluconeogenesis

To prevent futile cycles, glycolysis and gluconeogenesis are reciprocally regulated by metabolic signals and hormones:

• Fructose-2,6-Bisphosphate (F-2,6-BP):
  • Activates PFK-1 (Stimulates Glycolysis).
  • Inhibits FBPase-1 (Inhibits Gluconeogenesis).
• Glucagon: Decreases F-2,6-BP levels via PKA activation, stimulating gluconeogenesis during fasting.
• Insulin: Increases F-2,6-BP levels, stimulating glycolysis and repressing expression of PEPCK and Glucose-6-phosphatase.
• Acetyl-CoA: Activates Pyruvate Carboxylase (stimulates gluconeogenesis) and inhibits Pyruvate Dehydrogenase.

6. Glycogenesis and Glycogenolysis

Glycogenesis (Glycogen Synthesis)

Glycogenesis: The pathway by which excess glucose is polymerized into glycogen for storage in liver and skeletal muscle cytosol.

Subcellular Location: Cytosol.

Sequence of Reactions in Glycogenesis

1. Glucose Activation to Glucose-1-Phosphate:

   Glucose is phosphorylated to Glucose-6-P (Hexokinase/Glucokinase) and converted to Glucose-1-phosphate (G-1-P) by Phosphoglucomutase.

   Glucose-6-phosphate ⇄ Glucose-1-phosphate

2. UDP-Glucose Synthesis:

   G-1-P reacts with Uridine Triphosphate (UTP) to yield UDP-Glucose, catalyzed by UDP-Glucose Pyrophosphorylase. Irreversible due to rapid pyrophosphate (PP_i) hydrolysis by Inorganic Pyrophosphatase.

   Glucose-1-phosphate + UTP → UDP-Glucose + PP_i

3. Glycogen Chain Elongation:

   Glycogen Synthase transfers glucosyl residues from UDP-Glucose to non-reducing ends of a pre-existing glycogen chain or Glycogenin protein primer via α(1→4) glycosidic bonds. Key rate-limiting step.

   UDP-Glucose + Glycogen_{(n residues)} → UDP + Glycogen_{(n+1 residues)}

4. Branch Formation:

   When a linear chain reaches ~11 residues, the Branching Enzyme (Amylo-α(1,4)→α(1,6)-transglucosidase) cleaves a block of 6–7 glucose residues and transfers it to create an α(1→6) glycosidic branch point at least 4 residues away from existing branches.

Glycogenolysis (Glycogen Breakdown)

Glycogenolysis: The enzymatic breakdown of stored glycogen into glucose-1-phosphate (and free glucose) to meet systemic or local energy demands.

Subcellular Location: Cytosol.

Sequence of Reactions in Glycogenolysis

1. Phosphorolytic Cleavage of α(1→4) Bonds:

   Glycogen Phosphorylase cleaves terminal glucose residues from non-reducing ends by inorganic phosphate (phosphorolysis) to release Glucose-1-phosphate. Requires Pyridoxal Phosphate (PLP, Vitamin B₆). Key rate-limiting step. Action stops when 4 glucose units remain from a branch point (Limit Dextrin).

   Glycogen_{(n residues)} + P_i → Glycogen_{(n-1 residues)} + Glucose-1-phosphate

2. Debranching Process:

   The Debranching Enzyme (Bifunctional Enzyme) performs two catalytic activities:

   • Oligo-α(1,4)→α(1,4)-Glucan Transferase activity: Transfers 3 glucose residues from outer branch to non-reducing end of an adjacent chain.
   • Amylo-α(1,6)-Glucosidase activity: Hydrolyzes the remaining single α(1→6) branch point glucose residue, releasing 1 molecule of FREE Glucose.
3. Conversion of G-1-P to G-6-P:

   Phosphoglucomutase converts G-1-P into G-6-P.

4. Tissue-Specific Fate of Glucose-6-Phosphate:
   • In Liver: Glucose-6-Phosphatase removes phosphate to release free glucose into blood, maintaining blood glucose homeostasis during fasting.
   • In Skeletal Muscle: Enters glycolysis directly for ATP synthesis during muscle contraction (because muscle lacks Glucose-6-Phosphatase).

Hormonal Regulation of Glycogen Metabolism

Glycogen synthesis and breakdown are reciprocally controlled via reversible phosphorylation/dephosphorylation regulated by hormones:

• Glucagon (Fasting - Liver) & Epinephrine (Stress/Exercise - Liver and Muscle):
  • Bind cell surface GPCRs → Increases cyclic AMP (cAMP) → Activates Protein Kinase A (PKA).
  • PKA phosphorylates Glycogen Synthase → Inactivates Glycogenesis.
  • PKA phosphorylates Phosphorylase Kinase, which phosphorylates Glycogen Phosphorylase → Activates Glycogenolysis.
• Insulin (Fed State):
  • Activates Protein Phosphatase-1 (PP1) and Phosphodiesterase (lowers cAMP).
  • PP1 dephosphorylates Glycogen Synthase → Activates Glycogenesis.
  • PP1 dephosphorylates Glycogen Phosphorylase and Phosphorylase Kinase → Inactivates Glycogenolysis.

7. Summary and Comparison Tables

Comparison 1: Glycolysis vs Gluconeogenesis

Comparison 2: Glycogenesis vs Glycogenolysis

Summary of Key Rate-Limiting Enzymes

Common Pitfalls & Exam Notes

• Muscle Glycogen Failure to Regulate Blood Glucose: Muscle cells lack Glucose-6-Phosphatase. Therefore, G-6-P generated from muscle glycogenolysis enters glycolysis locally and cannot release free glucose into circulation.
• PFK-1 vs PFK-2: PFK-1 is the glycolytic enzyme converting F-6-P to F-1,6-BP. PFK-2 is a bifunctional regulatory enzyme that produces F-2,6-BP, the potent allosteric regulator.
• Fluoride Inhibition: Enolase (Step 9 of glycolysis) is inhibited by fluoride ions. Blood collection tubes for blood glucose estimation contain sodium fluoride to stop glycolysis in erythrocytes.
• Substrate-Level Phosphorylation Steps: Occurs at Steps 7 and 10 of glycolysis and Step 5 of the Citric Acid Cycle. Does not rely on electron transport chain or oxygen.