Biochem Lecture 12_Krebs Cycle

Krebs Cycle Overview

  • Other Names: Citric Acid Cycle, Tricarboxylic Acid Cycle

  • Location: Mitochondrial matrix

  • Function:

    • Common pathway for the oxidation of fuel molecules: amino acids, fatty acids, carbohydrates.

    • Central to the catabolism of carbohydrates, fatty acids, and amino acids.

    • Amphibolic Pathway: Serves both catabolic and anabolic processes and provides precursors for biosynthetic pathways.

Oxidative Decarboxylation

  1. Pyruvate from glycolysis enters mitochondria.

  2. Pyruvate Dehydrogenase Complex converts Pyruvate to Acetyl CoA.

    • Enzyme components:

      • E1: Pyruvate oxidative decarboxylase (TPP-dependent).

      • E2: Dihydrolipoyl transacetylase (transfers acetyl to CoA).

      • E3: Dihydrolipoyl dehydrogenase (uses FAD).

Main Steps of Krebs Cycle

  1. Citrate Synthase: Condensation of Oxaloacetate (4C) and Acetyl CoA (2C) to form Citrate (6C).

  2. Aconitase: Dehydration and isomerization of Citrate.

  3. Isocitrate Dehydrogenase: First oxidative decarboxylation producing NADH and CO2.

  4. Alpha-Ketoglutarate Dehydrogenase Complex: Second oxidative decarboxylation producing NADH and CO2.

  5. Succinyl-CoA Synthetase: Forms GTP via hydrolysis of thioester bond.

  6. Succinate Dehydrogenase: Converts Succinate to Fumarate with FADH2 production.

  7. Fumarase: Regenerates Oxaloacetate via hydration of Fumarate.

  8. Malate Dehydrogenase: Completes cycle with NADH production.

Summary of Krebs Cycle Reaction

  • Overall Reaction: Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi → 2 CO2 + 3 NADH + H+ + FADH2 + GTP + CoA-SH

Anaplerotic Reactions

  • Necessary for replenishing TCA cycle intermediates like Oxaloacetate and Malate for biosynthesis.

  • Reactions:

    • Pyruvate carboxylase converts Pyruvate + HCO3 + ATP to Oxaloacetate.

    • PEP carboxykinase and other enzymes contribute to replenishment.

Regulatory Points

  • Key regulatory sites in the cycle:

    • Pyruvate Dehydrogenase Complex: Inhibited by NADH, Acetyl CoA, and ATP.

    • Citrate Synthase: Inhibited by NADH, Succinyl CoA, and citrate.

    • Isocitrate Dehydrogenase: Inhibited by NADH and ATP.

    • Alpha-Ketoglutarate Dehydrogenase Complex: Inhibited by NADH, Succinyl CoA, and ATP.

Intracellular Locations and NADH Transport

  • NADH from the Krebs cycle is produced in mitochondria; however, NADH from glycolysis is cytosolic.

  • Shuttle Systems: Transport NADH across membranes:

    • Malate-Aspartate Shuttle: Efficient; results in 2.5 ATPs from NADH.

    • Glycerol-3-Phosphate Shuttle: Less efficient; results in 1.5 ATPs from NADH.

ATP Yield Summary

  • Net ATP Yield:

    • From Malate-Aspartate Shuttle: 2.5 ATPs per NADH.

    • From Glycerol-3-Phosphate Shuttle: 1.5 ATPs per NADH.

    • Final ATP yield depends on shuttle systems and substrates used in metabolism.