Citric Acid Cycle and Glyoxylate Cycle

Overview of the Citric Acid Cycle

  • The citric acid cycle, also known as the Krebs cycle, is crucial for energy conversion in cells.
  • It involves the oxidation of carbon fuels (primarily acetyl CoA) to produce ATP and high-energy electron carriers (NADH and FADH2).
  • Functions as a biochemical hub for energy production and biosynthesis.

19.1 The Structure of the Citric Acid Cycle

  • The cycle consists of two main stages:
    • Stage One: Oxidizes acetyl CoA to gather high-energy electrons.
    • Stage Two: Regenerates oxaloacetate and continues the cycle.
  • Illustrated by the overall pathway where two-carbon atoms enter and two are expelled as carbon dioxide (CO2).

19.2 Stage One: Oxidation of Two Carbon Atoms

  • Citrate Formation:
    • Acetyl CoA (2 carbons) condenses with oxaloacetate (4 carbons) to form citrate (6 carbons) via citrate synthase.
  • Oxidative Decarboxylation:
    • Citrate loses CO2 to form isocitrate, then is further processed to alpha-ketoglutarate,
    • This yields NADH, a key high-energy electron carrier.
  • Key Reactions:
    • Isocitrate dehydrogenase catalyzes the conversion of isocitrate to alpha-ketoglutarate, generating NADH.
    • Alpha-ketoglutarate dehydrogenase transforms alpha-ketoglutarate into succinyl CoA, producing a second NADH.

19.3 Stage Two: Regeneration of Oxaloacetate

  • Succinyl CoA Hydrolysis:
    • Succinyl CoA is hydrolyzed to succinate and concurrently generates ATP through substrate-level phosphorylation by succinyl CoA synthetase.
  • Oxidation of Succinate:
    • Succinate is oxidized to fumarate with the enzyme succinate dehydrogenase, reducing FAD to FADH2.
  • Fumarate to Malate:
    • Fumarate is hydrated to malate by fumarase.
  • Final Oxidation:
    • Malate is oxidized back to oxaloacetate by malate dehydrogenase, producing another NADH.

19.4 Regulation of the Citric Acid Cycle

  • Regulation is tightly controlled by energy needs and substrate availability:
    • Key Control Points:
    • Isocitrate Dehydrogenase: Activated by ADP and inhibited by ATP and NADH.
    • Alpha-Ketoglutarate Dehydrogenase: Similar regulation, inhibited by succinyl CoA and NADH.
  • The cycle responds to cellular energy status, with a high ATP concentration reducing the cycle rate.

19.5 The Glyoxylate Cycle

  • Unique to plants and some bacteria, allowing conversion of fats to carbohydrates.
  • Bypasses two decarboxylation steps of the citric acid cycle, utilizing isocitrate lyase and malate synthase.
  • Enables production of glucose from acetyl CoA derived from fat stores, vital for seedling growth.

Energy Yield of the Citric Acid Cycle

  • Each turn of the citric acid cycle generates the following:
    • 3 NADH
    • 1 FADH2
    • 1 ATP
    • Total ATP generation per cycle: approximately 10 ATP (via oxidative phosphorylation)
  • Comparison to anaerobic glycolysis which generates only 2 ATP per glucose molecule.

Biosynthetic Roles

  • Provides essential intermediates for biosynthesis:
    • Important for synthesizing amino acids, nucleotide bases, and heme groups.
  • Intermediates can be replenished through anaplerotic reactions when drawn off for biosynthesis.

Clinical Insight: Citric Acid Cycle and Cancer

  • Defects in enzymes (e.g., succinate dehydrogenase) linked to cancer growth and metabolism alterations.
  • Strain on regulatory mechanisms can lead to enhanced glycolysis even in aerobic conditions.
  • Insights into metabolic pathways open new potential treatments for metabolic diseases, especially in cancer research.

Summary

  • The citric acid cycle consists of two stages that oxidize acetyl CoA, regenerate oxaloacetate, and produce energy-rich electron carriers and ATP.
  • The cycle operates only under aerobic conditions, linking tightly with the electron transport chain for ATP production.