Biochemistry and Molecular Biology - Belyasova N.A. 2002

Metabolism. Energy-Yielding Processes
Tricarboxylic Acid Cycle
Oxidative Decarboxylation of Pyruvate

The catabolic products of CARBOHYDRATES, Fatty acids, and Amino Acids in most aerobic organisms (Cells) undergo complete oxidation in The Tricarboxylic Acid Cycle (also known as The Citric Acid Cycle or Krebs cycle). In this system of coupled reactions, $ ext{CO}_2$ and $ ext{H}_2 ext{O}$ are produced, a small amount of energy is conserved at the substrate-level phosphorylation stage, and A large number of reducing equivalents are formed. The latter are transported via carriers into the Respiratory Chain, where they drive ATP synthesis (Oxidative Phosphorylation). Thus, The primary function of the tricarboxylic acid cycle (TCA cycle) is the terminal oxidation of fuel molecules and the capture of reducing equivalents. In addition, the TCA cycle yields A number of intermediates that serve as substrates for the Biosynthesis of Essential compounds (such as amino acids, Porphyrins, and glucose). Therefore, the TCA cycle is considered an intermediary or central metabolic pathway—amphibolic (from the Greek "amphi," meaning both)—linking Catabolic and anabolic reactions.

Fuel molecules enter the TCA cycle as an activated two-carbon compound, acetyl-CoA. In turn, acetyl-CoA is generated through the Oxidation of Fatty acids, The breakdown of Certain amino acids, and The oxidative decarboxylation of Pyruvate.

As shown previously (Chapter 9), the primary product of Carbohydrate Catabolism is pyruvate. This three-carbon compound is also produced during The oxidation of many amino acids. For pyruvate to undergo further oxidation in the TCA cycle, it must first be converted into acetyl-CoA. This highly complex process is catalyzed by the pyruvate dehydrogenase complex, which comprises three Enzymes and five Cofactors. The overall equation for the Oxidative decarboxylation of pyruvate appears relatively simple (Fig. 11.1), yet the MECHANISM OF ACTION of all Components of the pyruvate dehydrogenase complex involves multiple steps.

In the first step, pyruvate decarboxylase (also referred to as dehydrogenase), utilizing its cofactor Thiamine diphosphate, catalyzes the decarboxylation of pyruvate to form a hydroxyethyl derivative ($ ext{— CHOH—CH}_3$), which remains bound to both the cofactor and the enzyme. Next, the hydroxyethyl residue is transferred to a molecule of Lipoic Acid, which serves as the cofactor for the second enzyme of the complex, dihydrolipoate acetyltransferase. This transfer culminates in the oxidation of the hydroxyethyl group to an acetyl group and the partial reduction of lipoic acid. In the third step, the acetyl group is transferred from lipoic acid to the thiol group of coenzyme A, releasing acetyl-CoA from the complex and yielding the fully reduced form of lipoic acid, dihydrolipoic acid. The final step of the process is catalyzed by the third enzyme, dihydrolipoamide dehydrogenase, which contains FAD as a prosthetic group. This enzyme catalyzes the oxidation of dihydrolipoic acid back to lipoic acid while reducing FAD to $ ext{FADH}_2$. Finally, the reducing equivalents from $ ext{FADH}_2$ are transferred to $ ext{NAD}^+$, which transports them into the respiratory chain.

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Fig. 11.1. Overall equation for the oxidative decarboxylation of pyruvate

It should be noted that a high-energy bond is formed within the acetyl-CoA molecule produced during the oxidative decarboxylation of pyruvate (indicated by a wavy line in Fig. 11.1).

The activity of the pyruvate dehydrogenase complex depends on the cellular ATP level and is regulated via enzymatic protein modification: an ATP-dependent protein kinase phosphorylates specific amino acid residues on the pyruvate decarboxylase subunit, causing the enzyme to lose activity. The reverse action—restoring pyruvate decarboxylase activity—is performed by a specific phosphatase that dephosphorylates the protein molecule.