Glycolysis, Gluconeogenesis, and Glycogen Metabolism

Glycolysis: An Overview

Phases of Glycolysis

Glycolysis is a central metabolic pathway that converts glucose into pyruvate, generating energy in the form of ATP and NADH. It consists of 10 enzyme-catalyzed reactions, which are divided into two main phases:

• Energy-Investment Phase: Two ATP molecules are consumed to phosphorylate glucose and convert it into two triose phosphates.

• Energy-Generation Phase: The two triose phosphates are oxidized to pyruvate, producing four ATP and two NADH molecules.

Net Reaction of Glycolysis:

$\text{Glucose} + 2\,\text{NAD}^+ + 2\,\text{ADP} + 2\,\text{P}_i \rightarrow 2\,\text{Pyruvate} + 2\,\text{NADH} + 2\,\text{ATP} + 2\,\text{H}_2\text{O} + 2\,\text{H}^+$

Reactions of Glycolysis

Stepwise Enzymatic Reactions

1. Hexokinase: Phosphorylates glucose to glucose-6-phosphate (G6P) using ATP.

   • $\alpha$-D-Glucose + ATP $\rightarrow$ $\alpha$-D-Glucose-6-phosphate + ADP

   • $\Delta G^\circ' = -18.4$ kJ/mol

2. Glucose-6-Phosphate Isomerase: Converts G6P to fructose-6-phosphate (F6P).

   • $\alpha$-D-Glucose-6-phosphate $\rightleftharpoons$ D-Fructose-6-phosphate

   • $\Delta G^\circ' = +1.7$ kJ/mol

3. Phosphofructokinase (PFK): Phosphorylates F6P to fructose-1,6-bisphosphate (F1,6BP) using ATP. This is a major regulatory step.

   • PFK is an allosteric enzyme and a major control point for glycolysis.

   • $\Delta G^\circ' = -15.9$ kJ/mol

4. Aldolase: Cleaves F1,6BP into two triose phosphates: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (GAP).

   • Although $\Delta G^\circ'$ is positive, the reaction proceeds in vivo due to cellular conditions.

5. Triose Phosphate Isomerase: Interconverts DHAP and GAP.

   • Only GAP continues directly in glycolysis.

   • $\Delta G^\circ' = +7.6$ kJ/mol

6. Glyceraldehyde-3-Phosphate Dehydrogenase (GAPDH): Oxidizes GAP to 1,3-bisphosphoglycerate (BPG), reducing NAD+ to NADH.

   • $\Delta G^\circ' = +6.3$ kJ/mol

7. Phosphoglycerate Kinase: Transfers a phosphate from BPG to ADP, forming ATP and 3-phosphoglycerate (3PG).

   • This is a substrate-level phosphorylation.

   • $\Delta G^\circ' = -17.2$ kJ/mol

8. Phosphoglycerate Mutase: Converts 3PG to 2-phosphoglycerate (2PG).

   • $\Delta G^\circ' = +4.4$ kJ/mol

9. Enolase: Dehydrates 2PG to phosphoenolpyruvate (PEP).

   • This is an α,β-elimination reaction; water is eliminated.

   • $\Delta G^\circ' = -3.2$ kJ/mol

10. Pyruvate Kinase (PK): Transfers a phosphate from PEP to ADP, forming ATP and pyruvate.

    • This is the second substrate-level phosphorylation.

    • $\Delta G^\circ' = -29.7$ kJ/mol

    • The reaction is overall exergonic due to spontaneous tautomerization of enolpyruvate to the more stable keto form (pyruvate).

Anaerobic Fates of Pyruvate

Regeneration of NAD+

• For glycolysis to continue, NADH must be reoxidized to NAD+.

• Under anaerobic conditions, pyruvate is reduced to lactate (in animals) or ethanol (in yeast), regenerating NAD+.

Examples:

• Lactate fermentation: Occurs in animal cells and lactic acid bacteria.

• Alcoholic fermentation: Occurs in yeast, producing ethanol and CO2.

Gluconeogenesis

Glucose Synthesis and Use

• The human brain requires about 120 g/day of glucose out of 160 g needed by the body.

• Glycogen reserves and glucose in body fluids provide about one day's supply.

• When glucose is depleted (e.g., fasting, prolonged exercise), it must be synthesized from non-carbohydrate precursors via gluconeogenesis.

Definition: Gluconeogenesis is the synthesis of new glucose from non-carbohydrate sources (e.g., lactate, amino acids, glycerol).

Relationship to Glycolysis: Gluconeogenesis is not a simple reversal of glycolysis; three irreversible steps in glycolysis are bypassed by distinct enzymes in gluconeogenesis.

Irreversible Steps and Bypass Reactions

Irreversible Reaction in Glycolysis	Bypass in Gluconeogenesis
Hexokinase	Glucose-6-phosphatase
Phosphofructokinase	Fructose-1,6-bisphosphatase
Pyruvate kinase	Pyruvate carboxylase and phosphoenolpyruvate carboxykinase

Cori Cycle

• The liver is the most active gluconeogenic organ.

• The Cori cycle describes the recycling of lactate (produced by anaerobic glycolysis in muscle) back to glucose in the liver.

Coordinated Regulation of Glycolysis and Gluconeogenesis

Control of Glucose Breakdown and Synthesis

• Glycolysis and gluconeogenesis are reciprocally regulated to prevent futile cycles (simultaneous operation of both pathways).

• Regulation also maintains pools of intermediates for other biosynthetic pathways.

Major Control Points

• Key enzymes are regulated allosterically and by hormones (e.g., insulin, glucagon).

• Allosteric activators and inhibitors modulate the activity of hexokinase/glucose-6-phosphatase, phosphofructokinase/fructose-1,6-bisphosphatase, and pyruvate kinase/pyruvate carboxylase-PEP carboxykinase.

Example: High ATP and citrate inhibit PFK (glycolysis), while AMP activates it. Conversely, fructose-1,6-bisphosphatase is inhibited by AMP.

Glycogen Metabolism in Muscle and Liver

Glycogen Utilization

• Glycogen phosphorylase cleaves α(1→4) glycosidic bonds in glycogen via phosphorolysis, producing α-D-glucose-1-phosphate.

• Glucose-1-phosphate is converted to glucose-6-phosphate by phosphoglucomutase for entry into glycolysis or other pathways.

Debranching Process

• The bifunctional glucantransferase (debranching enzyme) catalyzes two reactions:

  • Transfers three glucose residues from a branch to another nonreducing end (α1→4 linkage).

  • Removes the remaining glucose at the branch point (α1→6 linkage) via glucosidase activity.

Synthesis of Glycogen

• UDP-glucose is the activated form of glucose used for glycogen synthesis.

• Glycogen synthase adds glucose units from UDP-glucose to the growing glycogen chain (α1→4 linkages).

• Branching enzyme (amylo-(1,4→1,6)-transglycosylase) creates α1→6 branches by transferring 6–7 residues to a branch point.

Glycosidic Bond Cleavage of Disaccharides

Hydrolysis and Phosphorolysis

• Dietary polysaccharides are metabolized by hydrolysis to monosaccharides.

• Intracellular carbohydrate stores (e.g., glycogen) are mobilized as phosphorylated monosaccharides by phosphorolysis.

Digestion of Amylopectin or Glycogen

• α-amylase in saliva cleaves α(1→4) linkages from the nonreducing ends but cannot cleave α(1→6) linkages at branch points.

• α(1→6)-glucosidase (debranching enzyme) is required to remove the limit dextrin and expose additional α(1→4) linkages for further digestion.

Additional info:

• Substrate-level phosphorylation refers to the direct transfer of a phosphate group to ADP to form ATP, as seen in glycolysis.

• Allosteric regulation allows rapid response to cellular energy needs.