CAC_REGULATION

Citric Acid Cycle (CAC) Regulation

• CAC Regulation: Involves various steps and enzymes that control the rate of the cycle based on the energy needs of the cell.

• Friday Meeting (Week 1): Discussion of the reactions related to the CAC and its regulation strategies.

Products of the Citric Acid Cycle

• Key Intermediates:

  • Acetyl-CoA

  • Oxaloacetate

  • NADH

  • Malate

  • Fumarate

  • Citrate

  • Isocitrate

  • CO2

  • α-Ketoglutarate

  • FADH2

  • Succinate

  • Succinyl-CoA

  • GTP

Electron Funnel into ATP Synthesis

• ATP Generation per Turn:

  • 1 Turn of CAC:

    • 2 Pyruvate ➞ 2 Acetyl-CoA ➞ 2 NADH ➞ 5 ATP

    • 2 ATP produced via substrate-level phosphorylation.

    • Total of 30 ATP molecules produced per glucose molecule in aerobic conditions:

      • 6 NADH → 15 ATP

      • 2 FADH2 → 3 ATP

      • 2 GTP → 2 ATP

Regulation of the Citric Acid Cycle

• Key Concepts:

  • Energy demand regulates the CAC capacity at the pyruvate dehydrogenase step.

  • The CAC continues but can adjust its activity levels according to cellular needs.

  • Major influence points include the conversion of Acetyl-CoA to Oxaloacetate and key intermediates.

Regulatory Enzymes of the PDH Complex

• PDH Kinase and PDH Phosphatase:

  • PDH Active Form: Involved in producing Acetyl-CoA from Pyruvate.

  • Inhibition Factors for PDH Kinase:

    • High levels of ATP, NADH, and Acetyl-CoA.

  • Activators of PDH Phosphatase:

    • ADP and Pyruvate promote conversion to active PDH form.

Molecules Affecting PDH and PDH Phosphatase

• PDH Kinase regulated by:

  • ATP, NADH (inhibitors)

  • ADP, Pyruvate (activators)

• PDH Phosphatase regulated by:

  • Insulin, Ca2+, and ADP (activators)

  • Inhibition by ATP and NADH.

Regulation of the Citric Acid Cycle Components

• Key Intermediates:

  • Pyruvate, Ca2+, Oxaloacetate, Malate, Acetyl-CoA, Citrate, Isocitrate, Fumarate, NADH, GTP, and Succinate all contribute to regulatory mechanisms.

Enzymes Operating Far From Equilibrium

• Notable Enzymes:

  • Citrate synthase: AG (Standard Free Energy Change) -31.5 kJ/mol (negative)

  • Isocitrate dehydrogenase: -21 kJ/mol (negative)

  • α-Ketoglutarate dehydrogenase: -33 kJ/mol (negative)

Checkpoints in the Cycle

• Assessment of ATP yield from glucose metabolism.

• Identification of regulatory steps influencing cycle flux.

• Roles of ATP/ADP, Ca2+, insulin, and pyruvate in regulating the PDH complex and CAC.

Reactions Related to the Citric Acid Cycle

• Key Concepts:

  • Provides metabolites for gluconeogenesis, fatty acid synthesis, and amino acid synthesis.

  • Intermediates of the CAC can be replenished by other metabolic pathways.

  • Glyoxylate cycle variation allows organisms to convert Acetyl-CoA to Oxaloacetate.

Anaplerotic Reactions of the CAC

• Replenishment Pathways:

  • Pyruvate ➞ Oxaloacetate (via pyruvate carboxylase)

  • α-Ketoglutarate generated from glutamate (converts via transaminase).

Amino Acid Biosynthesis

• Glutamate Biosynthesis:

  • Conversion of α-Ketoglutarate into Glutamate by reductive amination.

• Aspartate Biosynthesis:

  • Transaminase reaction adding amine groups to α-Ketoglutarate.

Lipid Synthesis from Citrate

• Excess Citrate Utilization:

  • Conversion of citrate to Acetyl-CoA for fatty acid synthesis in cytosol through the action of Acetyl-CoA carboxylase and Fatty Acid Synthase.

Glyoxylate Cycle Overview

• Found in seeds/bacteria converting triglycerides to glucose via Acetyl-CoA.

• Unique Steps:

  • Skips CO2 generating steps of CAC, facilitating net conversion of Acetyl-CoA to Oxaloacetate.

Mitochondrial Structure and Function

• Mitochondria Composed of:

  • Outer Membrane (OMM) with beta-barrel proteins for transport.

  • Inner Membrane (IMM) rich in proteins, forming cristae, crucial for ATP synthesis.

  • Different permeabilities and composition profiles for metabolite transport.

Regulatory and Structural Aspects of Mitochondria

• Transport Proteins: Required for importing reducing equivalents.

• Proton Gradient Established: Through the inner mitochondrial membrane, important for ATP production.