12.1 An Overview of Glycolysis

Phases of Glycolysis

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

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

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

Net Reaction of Glycolysis:

Example: Glycolysis is the primary source of ATP in anaerobic conditions, such as in muscle cells during intense exercise.

12.2 Reactions of Glycolysis

Stepwise Enzymatic Reactions

Each step of glycolysis is catalyzed by a specific enzyme, with key regulatory and energy-yielding steps highlighted below.

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

2. Glucose-6-Phosphate Isomerase: Converts glucose-6-phosphate to fructose-6-phosphate.

3. Phosphofructokinase (PFK): Phosphorylates fructose-6-phosphate to fructose-1,6-bisphosphate. This is a major control point and allosteric enzyme.

4. Aldolase: Cleaves fructose-1,6-bisphosphate into two triose phosphates (dihydroxyacetone phosphate and glyceraldehyde-3-phosphate). , but in vivo is negative.

5. Triose Phosphate Isomerase: Interconverts DHAP and GAP.

6. Glyceraldehyde 3-Phosphate Dehydrogenase (GAPDH): Oxidizes GAP to 1,3-bisphosphoglycerate, producing NADH.

7. Phosphoglycerate Kinase: Transfers a phosphate from 1,3-BPG to ADP, forming ATP (substrate-level phosphorylation).

8. Phosphoglycerate Mutase: Converts 3-phosphoglycerate to 2-phosphoglycerate.

9. Enolase: Dehydrates 2-phosphoglycerate to phosphoenolpyruvate (PEP) via an α,β-elimination.

10. Pyruvate Kinase: Transfers phosphate from PEP to ADP, forming ATP and pyruvate (second substrate-level phosphorylation).

Mechanistic Note: The pyruvate kinase reaction is exergonic due to spontaneous tautomerization of enolpyruvate to the more stable keto form (pyruvate).

Anaerobic Fates of Pyruvate

Under anaerobic conditions, pyruvate can be reduced to lactate (in animals) or ethanol (in yeast), regenerating NAD+ for glycolysis to continue.

• Lactate fermentation:

• Alcoholic fermentation: ;

12.5 Gluconeogenesis

Glucose Synthesis and Use

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

• Brain glucose requirement: ~120 g/day out of 160 g/day for the whole body.

• Glycogen reserves: Provide about one day's supply of glucose.

• Precursors: Lactate, glycerol, and amino acids can be used for gluconeogenesis.

Key Point: Gluconeogenesis is not a simple reversal of glycolysis; three irreversible glycolytic reactions must be bypassed.

Irreversible Reaction Bypass in Gluconeogenesis

Example: During fasting, the liver synthesizes glucose via gluconeogenesis to maintain blood glucose levels.

Cori Cycle

The Cori cycle describes the metabolic cooperation between muscle and liver during anaerobic exercise. Lactate produced in muscle is transported to the liver, converted to glucose via gluconeogenesis, and returned to muscle.

• Liver: Most active gluconeogenic organ.

12.6 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 pools of metabolic intermediates for biosynthesis.

• Substrate cycles: Opposing pathways are regulated so only one is active at a time.

• Regulation: Allosteric effectors, hormones, and energy status influence pathway activity.

Control Points

Example: Insulin promotes glycolysis, while glucagon stimulates gluconeogenesis in the liver.

Glycosidic Bond Cleavage of Disaccharides

Hydrolysis and Phosphorolysis

Disaccharides and polysaccharides are broken down to monosaccharides by hydrolysis (dietary) or phosphorolysis (intracellular stores).

• Hydrolysis: Addition of water to cleave glycosidic bonds.

• Phosphorolysis: Addition of phosphate to cleave glycosidic bonds, yielding phosphorylated sugars.

Digestion of Amylopectin or Glycogen

Enzymatic Breakdown

Glycogen and amylopectin are digested by specific enzymes:

• α-amylase: Cleaves α(1→4) linkages from nonreducing ends but cannot cleave α(1→6) branch points.

• α(1→6)-glucosidase: Debranching enzyme required to remove limit dextrin and expose additional linkages.

12.8 Glycogen Metabolism in Muscle and Liver

Glycogen Utilization in Cells

Glycogen is a major storage form of glucose in animals. Its breakdown and synthesis are tightly regulated.

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

• Phosphoglucomutase: Converts glucose-1-phosphate to glucose-6-phosphate for entry into glycolysis.

The Debranching Process in Glycogen Catabolism

• Glucantransferase (transferase activity): Transfers three glucose residues from a branch to another nonreducing end.

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

Synthesis of Glycogen from UDP-Glucose

• UDP-glucose: Activated form of glucose used for glycogen synthesis.

• Glycogen synthase: Catalyzes the addition of glucose units to glycogen via α(1→4) linkages.

• Branching enzyme: Creates α(1→6) branches by transferring residues from a long chain to a new branch point.

Example: Muscle glycogen is mobilized during exercise to provide glucose for ATP production.

12.9 Coordinated Regulation of Glycogen Metabolism

Controlled Synthesis and Breakdown of Glycogen

Glycogen metabolism is regulated by hormonal signals and phosphorylation cascades.

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

2. G-protein activates adenylate cyclase, increasing cAMP levels.

3. cAMP activates protein kinase A (PKA), which phosphorylates phosphorylase b kinase.

4. Phosphorylase b kinase activates glycogen phosphorylase, promoting glycogen breakdown.

Example: During fight-or-flight response, epinephrine stimulates rapid glycogen breakdown in muscle.

12.10 The Pentose Phosphate Pathway (PPP)

Metabolic Requirement and Modes

The PPP is an alternative pathway for glucose oxidation, providing NADPH for reductive biosynthesis and ribose-5-phosphate for nucleotide synthesis.

• NADPH: Used in fatty acid and nucleotide synthesis, and for maintaining reduced glutathione.

• Ribose-5-phosphate: Precursor for RNA, DNA, and nucleotide synthesis.

Example: PPP is highly active in tissues involved in biosynthesis, such as liver, adipose tissue, and rapidly dividing cells.