Overview of Glycolysis Chapter

Overview of Glycolysis

• Glycolysis is a universal metabolic pathway occurring in nearly all cells.
• Consists of ten reactions categorized into two phases: conversion of glucose to two G-3-P and processing those to two pyruvates.
• Key products include pyruvate, ATP, and NADH.
• Pyruvate can undergo three potential fates depending on aerobic or anaerobic conditions.

First Phase of Glycolysis

• First reaction involves phosphorylation of glucose by hexokinase or glucokinase; this is a priming step consuming ATP for energy gain later.
• Hexokinase: has a low Km (0.1 mM), indicating high affinity for glucose, making it active under normal glucose levels (4 mM).
• Glucokinase: present in liver and pancreas, has a higher Km (10 mM) and activates only when glucose is abundant, slowing down glycolysis for glycogen synthesis.

Key Reactions in First Phase

• Rx 2: Phosphoglucoisomerase

  • Converts glucose-6-phosphate to fructose-6-phosphate, facilitating activation for subsequent reactions.

• Rx 3: Phosphofructokinase (PFK)

  • Serves as the committed step in glycolysis, highly regulated by ATP (inhibitory), AMP (activatory), and citrate (inhibitory).
  • Fructose-2,6-bisphosphate acts as an allosteric activator, enhancing PFK activity when energy is low.

Second Phase of Glycolysis

• Results in a net yield of two ATP from substrate-level phosphorylation, generating four ATP overall.
• High-energy intermediates, 1,3-BPG and phosphoenolpyruvate (PEP), play crucial roles in the energy payoff phase.

Key Reactions in Second Phase

• Rx 6: Glyceraldehyde-3-Phosphate Dehydrogenase

  • Converts G-3-P into 1,3-BPG, generating NADH in the process and utilizing energy from transforming aldehyde to carboxylic acid.

• Rx 7: Phosphoglycerate Kinase

  • Enables ATP synthesis from high-energy phosphate substrates through substrate-level phosphorylation.

• Rx 9: Enolase

  • Converts 2-phosphoglycerate to PEP, rearranging to release more energy in subsequent hydrolysis.

• Rx 10: Pyruvate Kinase

  • Converts PEP to pyruvate, producing ATP; regulated by AMP (activator) and ATP/acetyl-CoA (inhibitors).
  • Conversion involves a spontaneous enol-keto tautomerization that releases free energy.

The Fate of NADH and Pyruvate

• NADH can be oxidized via aerobic respiration in the electron transport chain to generate ATP during aerobic conditions.
• In anaerobic conditions, lactate dehydrogenase re-oxidizes NADH to NAD+, allowing glycolysis to continue despite lack of oxygen.

Test your knowledge of glycolysis, a vital metabolic pathway present in nearly all cells. This quiz covers the first phase of glycolysis, including key reactions, enzymes, and products like ATP and pyruvate. Understand the roles of hexokinase and glucokinase in glucose metabolism.