BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 16. ENERGY METABOLISM

16.3. The Final Stage of Catabolism — The Main Source of Hydrogen Donors for the ETC

CARBOHYDRATES, Fatty acids, and Most Amino Acids are eventually oxidized via The Tricarboxylic Acid Cycle (Citric Acid Cycle) to CO₂ and H₂O. Before these molecules enter the final stage of Catabolism, their carbon skeletons are converted into a two-carbon fragment in the form of acetyl-CoA (Fig. 16.13). It is in this form that the bulk of "fuel" molecules enter the tricarboxylic acid cycle (TCA cycle).

Acetyl-CoA is produced through specific Catabolic pathways of Fatty Acids and Certain amino acids. However, its primary source is pyruvic acid, which is generated during the catabolism of glucose and specific amino acids.

The conversion of Pyruvate into acetyl-CoA is catalyzed by Enzymes structurally organized into the pyruvate dehydrogenase complex (PDC). The acetyl residue of acetyl-CoA is subsequently oxidized in the TCA cycle to CO₂ and H₂O. These oxidation reactions involve NAD^- and FAD-dependent dehydrogenases, which supply electrons and protons to the Electron Transport Chain (ETC), through which they are ultimately transferred to O₂.

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Fig. 16.13. Catabolism of major nutrients:

1-3 - Digestion; 4-8 - specific catabolic pathways; 9-10 - final (common) pathway of catabolism;

11 - ETC; 12 - Oxidative Phosphorylation