Textbook - BIOLOGICAL CHEMISTRY - Gubsky Yu.I. - 2000

Chapter II. GENERAL PRINCIPLES OF METABOLISM

CHAPTER 10. THE TRICARBOXYLIC ACID CYCLE

10.2. ENZYMATIC REACTIONS OF THE TRICARBOXYLIC ACID CYCLE

1. Formation of citric acid (citrate) via Condensation of acetyl-CoA with oxaloacetic acid (oxaloacetate):

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The reaction is catalyzed by the enzyme citrate synthase. It is a regulatory enzyme whose activity is inhibited by ATP, NADH, succinyl-CoA, and long-chain acyl-CoAs.

2. Conversion (isomerization) of citrate to isocitrate. The reaction is catalyzed by the enzyme aconitase and consists of two stages:

2.1. Dehydration of citric acid to yield cis-aconitic acid (cis-aconitate):

2.2. Addition of a Water molecule to cis-aconitate. When H⁺ and OH^- add to the double bond of cis-aconitate in a trans-configuration, the resulting product is isocitric acid (isocitrate):

3. Dehydrogenation and decarboxylation of isocitrate. The reaction is catalyzed by NAD-dependent isocitrate dehydrogenase and leads to The formation of α-ketoglutaric acid (α-ketoglutarate).

Isocitrate dehydrogenase is a regulatory enzyme whose positive modulator is ADP and negative modulator is NADH.

The enzyme exists in two molecular forms: monomeric (the molecular mass of isocitrate dehydrogenase from Heart Cell/35.html">Mitochondria is 330 kD) and dimeric. In the presence of the positive modulator ADP, monomers aggregate to form a dimer. The negative modulator NADH counteracts the ADP-induced aggregation of the monomeric enzyme forms. Both molecular forms of isocitrate dehydrogenase possess catalytic properties, but under low ADP concentrations, the dimer is significantly more active.

4. Oxidation of α-ketoglutarate to succinate.

This process proceeds in two stages:

4.1. Oxidative Decarboxylation of α-ketoglutarate to yield succinyl-CoA, a stage catalyzed by the multi-enzyme α-ketoglutarate dehydrogenase complex. The end product is the high-energy thioester succinyl-CoA, whose high-energy bond stores the chemical energy released by the preceding oxidation-reduction reaction:

The NADH produced in this reaction is oxidized in the mitochondrial Respiratory Chain with the generation of 3 ATP molecules.

Mechanistically, this process resembles The oxidative decarboxylation of Pyruvate to acetyl-CoA (see Chapter 11); similarly to the pyruvate dehydrogenase complex, the α-ketoglutarate dehydrogenase complex contains the Coenzymes Thiamine diphosphate (TDP), Lipoic Acid (LA), CoA, NAD⁺, and FAD. The molecular mass of this complex from E. coli Cells is 2.1 · 10⁶.

4.2. Deacylation of succinyl-CoA (conversion to succinic acid (succinate)).

The reaction is catalyzed by the enzyme succinyl-CoA synthetase (succinic thiokinase). As a result, the high-energy bond in the succinyl-CoA molecule is cleaved, and this energy is utilized to synthesize a new high-energy compound, the nucleoside triphosphate GTP:

Then GTP transfers its terminal phosphate group to ADP in a nucleoside phosphokinase reaction, yielding ATP:

5. Oxidation of succinic acid to fumaric acid (fumarate). This reaction is catalyzed by the FAD-dependent enzyme succinate dehydrogenase:

The oxidation of the reduced coenzyme (FADH₂) by coenzyme Q of the mitochondrial respiratory chain drives the synthesis of 2 ATP molecules via Oxidative Phosphorylation.

6. Conversion of fumaric acid to malic acid (malate) through The addition of a water molecule to fumarate.

The reaction is catalyzed by the enzyme fumarate hydratase (fumarase):

7. Oxidation of malate to oxaloacetate.

The reaction is catalyzed by the NAD-dependent mitochondrial enzyme malate dehydrogenase:

The subsequent oxidation of the generated NADH in the mitochondrial respiratory chain leads to the generation of 3 ATP molecules.

The malate dehydrogenase reaction completes The Tricarboxylic Acid Cycle. Oxaloacetate, the product of this reaction, can then react with new acetyl-CoA molecules.

The general metabolic map of the tricarboxylic acid cycle is presented in Fig. 10.2

Fig. 10.2. Metabolic map of the tricarboxylic acid cycle and its connection with the mitochondrial respiratory chain.