Glycolysis, Gluconeogenesis, and Glycogen Metabolism: Pathways and Regulation

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 split it into two triose phosphates.

• Energy-Generation Phase: The 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 using ATP. $\alpha$-D-Glucose + ATP $\xrightarrow{\text{Hexokinase}}$ $\alpha$-D-Glucose-6-phosphate + ADP $\Delta G'^{\circ} = -18.4$ kJ/mol

2. Glucose-6-Phosphate Isomerase: Converts glucose-6-phosphate to fructose-6-phosphate. $\alpha$-D-Glucose-6-phosphate $\rightleftharpoons$ D-Fructose-6-phosphate $\Delta G'^{\circ} = +1.7$ kJ/mol

3. Phosphofructokinase (PFK): Phosphorylates fructose-6-phosphate to fructose-1,6-bisphosphate using ATP. PFK is an allosteric enzyme and a major control point for glycolysis. $\text{F6P} + \text{ATP} \xrightarrow{\text{PFK}} \text{F1,6BP} + \text{ADP}$ $\Delta G'^{\circ} = -15.9$ kJ/mol

4. Aldolase: Cleaves fructose-1,6-bisphosphate into two triose phosphates (dihydroxyacetone phosphate and glyceraldehyde-3-phosphate). $\Delta G'^{\circ}$ is positive, but under cellular conditions, $\Delta G$ is negative, so the reaction proceeds forward in vivo.

5. Triose Phosphate Isomerase: Interconverts dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (GAP). $\Delta G'^{\circ} = +7.6$ 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 (PG). This is a substrate-level phosphorylation. $\Delta G'^{\circ} = -17.2$ kJ/mol

8. Phosphoglycerate Mutase: Converts 3-phosphoglycerate to 2-phosphoglycerate. $\Delta G'^{\circ} = +4.4$ kJ/mol

9. Enolase: Dehydrates 2-phosphoglycerate to phosphoenolpyruvate (PEP). This is an $\alpha,\beta$-elimination; the elimination product is $\text{H}_2\text{O}$. $\Delta G'^{\circ} = -3.2$ kJ/mol

10. Pyruvate Kinase: 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

Mechanistic Note:

• Although ATP synthesis in the pyruvate kinase step is endergonic, the overall reaction is exergonic due to the 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+.

Gluconeogenesis

Glucose Synthesis and Use

• The human brain requires about 120 g/day of glucose; the body contains about one day's supply in glycogen and free glucose.

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

• Gluconeogenesis is essentially glycolysis in reverse, but three irreversible steps must be bypassed by specific enzymes.

Irreversible Steps and Bypass Enzymes

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.

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

Major Control Points

• Key allosteric enzymes (e.g., PFK, fructose-1,6-bisphosphatase) are regulated by metabolites such as ATP, AMP, citrate, and fructose-2,6-bisphosphate.

• Hormonal regulation (insulin, glucagon) adjusts enzyme activity to meet physiological needs.

Glycogen Metabolism

Glycogen Utilization in Cells

• Glycogen phosphorylase cleaves $\alpha(1 \rightarrow 4)$ glycosidic bonds in glycogen via phosphorolysis, yielding $\alpha$-D-glucose-1-phosphate.

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

Debranching Process in Glycogen Catabolism

• The bifunctional glucantransferase (debranching enzyme) catalyzes:

  • Transferase activity: Moves three glucose residues from a branch to a nonreducing end.

  • Glucosidase activity: Removes the remaining $\alpha(1 \rightarrow 6)$-linked glucose.

Synthesis of Glycogen from UDP-Glucose

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

• Glycogen synthase adds glucose units from UDP-glucose to the growing glycogen chain via $\alpha(1 \rightarrow 4)$ linkages.

• Branching enzyme introduces $\alpha(1 \rightarrow 6)$ branches.

Glycosidic Bond Cleavage of Disaccharides

• Dietary polysaccharides are metabolized by hydrolysis to monosaccharides.

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

Digestion of Amylopectin or Glycogen

• $\alpha$-amylase in saliva cleaves $\alpha(1 \rightarrow 4)$ linkages but cannot cleave $\alpha(1 \rightarrow 6)$ branch points.

• $\alpha(1 \rightarrow 6)$-glucosidase (debranching enzyme) is required to remove limit dextrins and expose additional linkages for further digestion.

Additional info:

• Allosteric regulation and hormonal control are essential for maintaining metabolic homeostasis.

• Substrate-level phosphorylation refers to the direct synthesis of ATP from ADP by transfer of a phosphate group from a high-energy intermediate.