2nd Phase of Glycolysis

Overview of the Preparatory and Payoff Phases

The second phase of glycolysis, also known as the payoff phase, involves the conversion of glyceraldehyde-3-phosphate into pyruvate, generating ATP and NADH. This phase is crucial for cellular energy production and involves substrate-level phosphorylation and redox reactions.

• ATP Generation: Two molecules of ATP are produced per molecule of glucose.

• NADH Production: Two molecules of NADH are generated, which can be used in oxidative phosphorylation.

• Substrate-level Phosphorylation: Direct transfer of phosphate groups to ADP to form ATP.

Reaction 6: Oxidation and Phosphorylation of Glyceraldehyde-3-phosphate

This reaction involves the oxidation of glyceraldehyde-3-phosphate (G3P) and its phosphorylation to form 1,3-bisphosphoglycerate (1,3-BPG). The enzyme responsible is glyceraldehyde-3-phosphate dehydrogenase (GAPDH).

• Reaction:

• Enzyme: GAPDH catalyzes the oxidation and phosphorylation, forming a high-energy acyl phosphate intermediate.

• Mechanism: Involves nucleophilic attack, formation of a thiohemiacetal intermediate, and transfer of electrons to NAD+.

• Energetics: The reaction is coupled to NAD+ reduction, making it energetically favorable.

Reaction 7: Phosphoryl Transfer from 1,3-bisphosphoglycerate

This step is catalyzed by phosphoglycerate kinase and involves the transfer of a phosphate group from 1,3-BPG to ADP, forming ATP and 3-phosphoglycerate.

• Reaction:

• Substrate-level Phosphorylation: Direct formation of ATP from ADP.

• Energetics: The reaction is highly exergonic, driving ATP synthesis.

Reaction 8: Conversion of 3-phosphoglycerate into 2-phosphoglycerate

The enzyme phosphoglycerate mutase catalyzes the reversible shift of the phosphate group from the C3 to the C2 position of glycerate.

• Reaction:

• Enzyme: Phosphoglycerate mutase requires Mg2+ for activity and a phosphorylated histidine intermediate.

• Mechanism: Involves a phosphohistidine intermediate and transfer of phosphate groups.

Reaction 9: Dehydration of 2-phosphoglycerate

Enolase catalyzes the reversible elimination of water from 2-phosphoglycerate to form phosphoenolpyruvate (PEP), a high-energy intermediate.

• Reaction:

• Enzyme: Enolase requires Mg2+ for activity.

• Energetics: Formation of PEP is highly endergonic, storing energy for subsequent ATP generation.

Reaction 10: Transfer of the Phosphoryl Group of Phosphoenolpyruvate

The final step of glycolysis is catalyzed by pyruvate kinase, transferring the phosphate group from PEP to ADP, forming ATP and pyruvate.

• Reaction:

• Regulation: Pyruvate kinase is regulated allosterically and by covalent modification.

• Energetics: Highly exergonic, driving ATP synthesis.

Fate of Pyruvate

Overview

Pyruvate, the end product of glycolysis, can be metabolized in several ways depending on cellular conditions. The main fates include aerobic oxidation, lactic acid fermentation, and alcoholic fermentation.

• Aerobic Conditions: Pyruvate is converted to acetyl-CoA and enters the citric acid cycle.

• Anaerobic Conditions: Pyruvate is reduced to lactate (in animals) or ethanol (in yeast/plants) to regenerate NAD+.

1.3 Homolactic Acid Fermentation

Under anaerobic conditions, pyruvate is reduced to lactate by lactate dehydrogenase (LDH), regenerating NAD+ for glycolysis.

• Reaction:

• Enzyme: LDH catalyzes the transfer of electrons from NADH to pyruvate.

• Energetics:

2.3 Alcoholic Fermentation

In yeast and some plants, pyruvate is converted to ethanol and CO2 via two steps: decarboxylation by pyruvate decarboxylase and reduction by alcohol dehydrogenase.

• Step 1: Pyruvate decarboxylase converts pyruvate to acetaldehyde and CO2.

• Step 2: Alcohol dehydrogenase reduces acetaldehyde to ethanol, regenerating NAD+.

• Cofactor: Thiamine pyrophosphate (TPP) is required for pyruvate decarboxylase activity.

Energetics of Glycolysis and Fermentation

Standard Free Energy Changes

The efficiency of glycolysis and fermentation can be evaluated by comparing the standard free energy changes () and ATP yield.

Muscle Fiber Types and Metabolism

Fast-twitch vs. Slow-twitch Muscle Fibers

Muscle fibers differ in their metabolic properties and efficiency in ATP production.

• Fast-twitch fibers: Rely mainly on glycolysis for rapid ATP production; fatigue quickly.

• Slow-twitch fibers: Rely on oxidative phosphorylation; more resistant to fatigue and suited for endurance.

• Examples: Sprinters have more fast-twitch fibers; marathon runners have more slow-twitch fibers.

Key Enzymes and Cofactors

Thiamine Pyrophosphate (TPP)

TPP is a cofactor for pyruvate decarboxylase, stabilizing the carbanion intermediate during decarboxylation.

• Function: Electron sink, stabilizes negative charge on carboxyl carbon.

• Structure: Contains thiazolium and pyrimidine rings.

Summary Table: Glycolytic Reactions and Enzymes

Additional info:

• Regulation of glycolysis involves allosteric effectors and covalent modification, especially for pyruvate kinase.

• Glycolysis is central to both aerobic and anaerobic metabolism, providing precursors for other pathways.

• Fermentation pathways are essential for regenerating NAD+ under anaerobic conditions.