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Krebs Cycle Overview

Aug 21, 2025

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Overview

This lecture explains the Krebs cycle (citric acid cycle), its stages, key enzymes, and main products in aerobic respiration.

Introduction to the Krebs Cycle

  • The Krebs cycle is the third stage of aerobic respiration, following oxidative decarboxylation.
  • Also known as the citric acid cycle, named after Hans Krebs.
  • Occurs in the mitochondrial matrix.

Eight Stages of the Krebs Cycle

  • Stage 1: Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C); catalyzed by citrate synthase; coenzyme A is released.
  • Stage 2: Citrate is rearranged to isocitrate (6C) via dehydration and hydration; catalyzed by aconitase.
  • Stage 3: Isocitrate (6C) is converted to alpha-ketoglutarate (5C); CO₂ is released, NAD⁺ is reduced to NADH; catalyzed by isocitrate dehydrogenase.
  • Stage 4: Alpha-ketoglutarate (5C) becomes succinyl CoA (4C); CO₂ is released, NAD⁺ is reduced to NADH, CoA enters; catalyzed by alpha-ketoglutarate dehydrogenase.
  • Stage 5: Succinyl CoA (4C) transforms into succinate (4C); GDP is phosphorylated to GTP (then to ATP), CoA is released; catalyzed by succinyl CoA synthase.
  • Stage 6: Succinate (4C) is converted to fumarate (4C); FAD is reduced to FADH₂; catalyzed by succinate dehydrogenase.
  • Stage 7: Fumarate becomes malate; hydration (addition of water) occurs; catalyzed by fumarase.
  • Stage 8: Malate is converted back to oxaloacetate; NAD⁺ is reduced to NADH; catalyzed by malate dehydrogenase.

Cycle Output Per Glucose

  • Per two acetyl-CoA: 6 NADH, 2 FADH₂, 2 ATP (or GTP), and 4 CO₂ are produced.

Key Terms & Definitions

  • Krebs cycle / Citric acid cycle — A series of chemical reactions in the mitochondrial matrix converting acetyl-CoA into CO₂, NADH, FADH₂, and ATP.
  • Acetyl-CoA — A two-carbon molecule entering the Krebs cycle, produced from pyruvate.
  • Oxaloacetate — Four-carbon molecule that combines with acetyl-CoA to start the cycle.
  • NADH & FADH₂ — Reduced coenzymes that store electrons for the electron transport chain.
  • ATP (adenosine triphosphate) — Main energy currency of the cell.

Action Items / Next Steps

  • Review the eight enzymatic steps and their respective enzymes.
  • Study the molecular changes (carbon count, energy carriers) at each stage.
  • Prepare for questions about the inputs, outputs, and location of the Krebs cycle.

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