ABI 102 Midterm 4

Glycolysis and TCA Cycle Overview

  • Glycolysis is the first step in glucose metabolism leading to pyruvate production.

  • The TCA (Tricarboxylic Acid) Cycle involves the complete oxidation of pyruvate to carbon dioxide (CO2).

TCA Cycle Functionality

  • Complete Oxidation of Pyruvate:

    • Converts pyruvate into CO2.

    • Produces NADH and FADH2, which are essential for ATP production via oxidative phosphorylation.

  • Presence in Cells:

    • Found in all aerobic cells during catabolic processes.

    • Pyruvate metabolism links glycolysis with the TCA cycle, undergoing conversion to acetyl-CoA.

Key Processes in TCA Cycle

  • Acetyl-CoA:

    • Central metabolite formed from pyruvate oxidation, generating one NADH in the process.

  • Goals and Products of TCA Cycle:

    • Main objective: Oxidize one acetyl-CoA to yield two CO2.

    • End products: 3 NADH, 1 FADH2, and 1 GTP (which can convert to ATP).

Steps of TCA Cycle

  • Step 1: Formation of Citrate

    • Enzyme: Citrate synthase

    • Reactants: Acetyl-CoA + Oxaloacetate (4C) + H2O

    • Product: Citrate (6C)

  • Step 2: Isomerization of Citrate

    • Enzyme: Aconitase

    • Intermediate: Cis-aconitate, leading to isocitrate.

TCA Cycle and Reduced Coenzymes

  • The TCA cycle generates more reduced coenzymes (6 NADH and 2 FADH2) compared to glycolysis.

  • Reduced coenzymes are crucial for oxidative phosphorylation or fermentation (anaerobic respiration).

Catabolism of Other Nutrients

  • The TCA cycle can catabolize various nutrients, including:

    • Fatty acids

    • Amino acids

    • Glucose

    • Any substrate that produces acetyl-CoA.

Anabolic Functions of TCA Cycle

  • Beyond catabolism, the TCA cycle plays a role in anabolism.

  • Intermediates produced can be utilized for synthetic pathways, such as amino acid synthesis.

Summary of TCA Steps (Continued)

  • Step 3: Isocitrate Decarboxylation/Oxidation

    • Enzyme: Isocitrate dehydrogenase

    • Product: Ɑ-ketoglutarate, NADH, and CO2.

  • Step 4: Ɑ-ketoglutarate Decarboxylation/Oxidation

    • Enzyme: Ɑ-ketoglutarate dehydrogenase complex

    • Product: Succinyl-CoA, NADH, and CO2.

  • Step 5: Succinyl-CoA to Succinate

    • Enzyme: Succinyl-CoA synthetase

    • Product: Succinate and GTP (ATP converter depending on isoenzyme).

  • Step 6: Succinate Oxidation

    • Enzyme: Succinate Dehydrogenase (part of respiratory chain in oxidative phosphorylation)

    • Product: Fumarate and FADH2 (remains enzyme-bound).

  • Step 7: Fumarate Hydration

    • Enzyme: Fumarase

    • Product: L-malate.

  • Step 8: Oxidation of L-malate

    • Enzyme: L-malate dehydrogenase

    • Product: Oxaloacetate and NADH.