Glycolysis, Gluconeogenesis, and Glycogen Metabolism

Glycolysis and Its Regulation

Overview of Glycolysis

Glycolysis is a central metabolic pathway that converts glucose into pyruvate, generating ATP and NADH. It occurs in the cytoplasm of cells and is divided into two main phases: the energy-investment phase and the energy-generation phase.

• 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:

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

Phases of Glycolysis

• Energy-Investment Phase (Steps 1–5): Glucose is phosphorylated and cleaved into two three-carbon sugars. Two ATP are used.

• Energy-Generation Phase (Steps 6–10): The three-carbon sugars are converted to pyruvate, generating four ATP and two NADH.

Reactions of Glycolysis

Stepwise Reactions and Key Enzymes

1. Hexokinase: Phosphorylates glucose to glucose-6-phosphate (G6P) using ATP. $\alpha$-D-Glucose + ATP $\rightarrow$ $\alpha$-D-Glucose-6-phosphate + ADP + H$^+$ $\Delta G'^{\circ} = -18.4$ 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. D-Fructose-6-phosphate + ATP $\rightarrow$ D-Fructose-1,6-bisphosphate + ADP + H$^+$ $\Delta G'^{\circ} = -15.9$ kJ/mol PFK is an allosteric enzyme and a major control point for glycolysis.

4. Aldolase: Cleaves F1,6BP into two triose phosphates: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (GAP). $\Delta G'^{\circ} > 0$, but $\Delta G$ is negative in vivo.

5. Triose Phosphate Isomerase: Interconverts DHAP and 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. D-GAP + NAD$^+$ + P$_i$ $\rightarrow$ 1,3-BPG + NADH + H$^+$ $\Delta G'^{\circ} = +6.3$ kJ/mol

7. Phosphoglycerate Kinase: Transfers a phosphate from BPG to ADP, forming ATP and 3-phosphoglycerate (3PG). 1,3-BPG + ADP $\rightarrow$ 3PG + ATP $\Delta G'^{\circ} = -17.2$ kJ/mol This is a substrate-level phosphorylation.

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

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

10. Pyruvate Kinase: Transfers a phosphate from PEP to ADP, forming ATP and pyruvate. PEP + ADP + H$^+$ $\rightarrow$ Pyruvate + ATP $\Delta G'^{\circ} = -29.7$ kJ/mol This is the second substrate-level phosphorylation.

Additional info: The overall exergonic nature of the pyruvate kinase reaction is due to the spontaneous tautomerization of enolpyruvate to the more stable keto form (pyruvate).

Anaerobic Fates of Pyruvate

Under anaerobic conditions, pyruvate is metabolized to regenerate NAD$^+$, allowing glycolysis to continue.

• Lactate Fermentation: In animal cells and lactic acid bacteria, pyruvate is reduced to lactate.

• Alcoholic Fermentation: In yeast, pyruvate is converted to ethanol and CO$_2$.

Gluconeogenesis

Glucose Synthesis and Use

Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors, essential during fasting or prolonged exercise when glucose reserves are depleted.

• The human brain requires about 120 g/day of glucose.

• Glycogen reserves and blood glucose provide about one day's supply.

• When depleted, glucose is synthesized from sources such as amino acids, lactate, and glycerol.

Bypass Reactions in Gluconeogenesis

Gluconeogenesis largely reverses glycolysis, but three irreversible glycolytic steps are bypassed by unique 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 Cori cycle describes the metabolic cooperation between muscle and liver during anaerobic glycolysis. Lactate produced in muscle is transported to the liver, converted to glucose via gluconeogenesis, and returned to muscle.

• The liver is the most active gluconeogenic organ.

Coordinated Regulation of Glycolysis and Gluconeogenesis

Control of Glucose Breakdown and Synthesis

Glycolysis and gluconeogenesis are reciprocally regulated to prevent futile cycles and to maintain metabolic balance.

• Regulation ensures that when glycolysis is active, gluconeogenesis is inhibited, and vice versa.

• Allosteric effectors, hormones, and energy status modulate key enzymes.

Major Control Points

• Hexokinase/Glucose-6-phosphatase

• Phosphofructokinase/Fructose-1,6-bisphosphatase

• Pyruvate kinase/Pyruvate carboxylase & PEP carboxykinase

Allosteric activators and inhibitors include ATP, AMP, citrate, fructose-2,6-bisphosphate, and hormonal signals (insulin, glucagon).

Glycogen Metabolism

Glycosidic Bond Cleavage of Disaccharides

• Dietary polysaccharides are broken down by hydrolysis to monosaccharides.

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

Digestion of Amylopectin or Glycogen

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

• α(1→6)-glucosidase (a debranching enzyme) removes the limit dextrin, exposing additional α(1→4) linkages for further digestion.

Glycogen Utilization in Cells

• Glycogen phosphorylase cleaves α(1→4) bonds via phosphorolysis, yielding α-D-glucose-1-phosphate.

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

The Debranching Process in Glycogen Catabolism

• Glucantransferase activity transfers three glucose residues from a branch to another nonreducing end via a new α(1→4) linkage.

• α(1→6)-glucosidase activity removes the remaining glucose at the branch point.

Synthesis of Glycogen from UDP-Glucose

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

• Glycogen synthase catalyzes the addition of glucose units from UDP-glucose to the growing glycogen chain via α(1→4) linkages.

Glycogen Branching

• Amylo-(1,4→1,6)-transglycosylase (branching enzyme) transfers 6–7 glucose residues from a branch terminus at least 11 residues long to create a new α(1→6) branch point.

Coordinated Regulation of Glycogen Metabolism

Hormonal Regulation

1. Hormone (e.g., epinephrine or glucagon) binds to a cell surface receptor, activating a G-protein.

2. G-protein activates adenylate cyclase, which synthesizes cAMP.

3. cAMP activates protein kinase A (PKA) by releasing its catalytic subunit.

4. PKA phosphorylates phosphorylase b kinase, activating it.

5. Active kinase converts inactive phosphorylase b to active phosphorylase a, catalyzing glycogen breakdown.

Pentose Phosphate Pathway (PPP)

Metabolic Requirement of PPP

• Provides NADPH for reductive biosynthesis (e.g., fatty acid synthesis, antioxidant defense).

• Generates ribose-5-phosphate for nucleotide and nucleic acid synthesis.

Modes of PPP

• The pathway can operate in different modes depending on cellular needs for NADPH, ribose-5-phosphate, or ATP.