BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 16. ENERGY METABOLISM

16.4. The Tricarboxylic Acid Cycle

The Tricarboxylic Acid Cycle (TCA cycle, Citric Acid Cycle, or Krebs cycle) is the common pathway for The oxidation of CARBOHYDRATES, Proteins, and Lipids, as the METABOLISM of glucose, Amino Acids, and Fatty acids yields acetyl-CoA (Fig. 16.17). In this cycle, which occurs in the Cell/35.html">Mitochondria of Eukaryotic Cells, acetate is oxidized to CO₂, and O₂ itself serves as the final electron acceptor. The TCA cycle also participates in Gluconeogenesis, Transamination, deamination, and Lipogenesis.

The TCA cycle begins with the interaction of acetyl-CoA and oxaloacetate catalyzed by the enzyme citrate synthase, leading to The formation of citrate. This reaction is triphasic. First, acetyl-CoA is converted into its enol form:

Class="center">

Then, this form of acetyl-CoA reacts with oxaloacetate to form citryl-CoA:

Fig. 16.17. General scheme of the tricarboxylic acid cycle:

1 - formation of citrate; CS - citrate synthase; 2 - conversion of citrate to isocitrate; A - aconitase; 3 - Oxidative Decarboxylation of isocitrate; IDH - isocitrate dehydrogenase; 4 - oxidative decarboxylation of α-ketoglutarate; α-KGDH - α-ketoglutarate dehydrogenase; 5 - conversion of succinyl-CoA to succinate; STK - succinate thiokinase; 6 - dehydrogenation of succinate; SDH - succinate dehydrogenase; 7 - formation of malate from fumarate;

F - fumarase; 8 - dehydrogenation of malate; MDH — malate dehydrogenase

Next, citrate is formed through the Hydrolysis of the thioester bond with the release of coenzyme A, mediated by the enzyme citrate synthase:

This reaction is characterized by a relatively high release of energy as heat, which shifts its equilibrium toward citrate formation.

At the subsequent stage, citrate is converted into cis-aconitate via a dehydration reaction. This reaction is catalyzed by the enzyme aconitate hydratase (aconitase), which contains iron in the Fe²⁺ state:

The subsequent formation of isocitrate is also catalyzed by aconitate hydratase in a Hydration reaction:

It is suggested that cis-aconitate may not necessarily be an obligatory intermediate between citrate and isocitrate, but rather formed on a side branch of the main pathway.

The enzyme isocitrate dehydrogenase catalyzes the dehydrogenation reaction that converts isocitrate into oxalosuccinate. The same enzyme catalyzes The conversion of the latter into α-ketoglutarate via a decarboxylation reaction involving Mn²⁺ (or Mg²⁺) ions. Three forms of this enzyme are known. One of them is NAD⁺-dependent, found exclusively in mitochondria, and plays a leading role in isocitrate oxidation. The other two are NADP⁺-dependent, with one localized in the mitochondria and the other in the Cytosol. They presumably play an auxiliary role.

Under the action of the α-ketoglutarate dehydrogenase complex (which requires thiamine pyrophosphate, lipoate, NAD+, FAD, and CoA), α-ketoglutarate undergoes oxidative decarboxylation to form succinyl-CoA containing a high-energy bond. Under physiological conditions, this reaction is irreversible. The Mechanism of this reaction is similar to The oxidative decarboxylation of Pyruvate.

Subsequently, the enzyme succinate thiokinase, with the participation of Mg²⁺, catalyzes the conversion of succinyl-CoA into succinate, a reaction coupled with the formation of GTP via GDP phosphorylation. This is the only step in the tricarboxylic acid cycle where a high-energy bond is formed at the so-called substrate level.

The formation of GTP in a reaction catalyzed by phosphokinase can lead to the synthesis of dTP:

The enzyme succinate dehydrogenase, which contains FAD and an iron-sulfur protein (Fc-S), catalyzes the dehydrogenation of succinate to fumarate:

The conversion of fumarate into malate via a hydration reaction is catalyzed by the enzyme fumarate hydratase (fumarase):

The resulting malate is converted into oxaloacetate in an NAD⁺-dependent reaction catalyzed by malate dehydrogenase, thereby completing one turn of the TCA cycle:

It should be noted that the Reactions of the TCA cycle are reversible; however, during the conversion of citryl-CoA to citrate and alpha-ketoglutarate to succinyl-CoA, the equilibrium is strongly shifted toward the Formation of the latter products. Therefore, these reactions can be considered essentially unidirectional under physiological conditions.

During each turn of the TCA cycle, carbon atoms enter it as acetyl-CoA and are released as CO₂. Through redox reactions, four pairs of hydrogen atoms are formed (three of which are accepted as NAD⁺ H⁺, and one as FAD₂), which then enter the mitochondrial Respiratory Chain.

It should be noted that TCA cycle Enzymes, with the exception of α-ketoglutarate dehydrogenase and succinate dehydrogenase, are also localized outside the mitochondria. However, some of these enzymes, notably malate dehydrogenase, differ structurally and functionally from their mitochondrial counterparts.

In plants and microorganisms, alongside the standard TCA cycle, a modified variant known as The Glyoxylate cycle Functions. The conversion of isocitrate in this cycle is catalyzed by isocitrate lyase, yielding glyoxylate and succinate as products. Subsequently, glyoxylate reacts with acyl-CoA, and the resulting compound is converted into malate. In turn, succinate is transformed into oxaloacetate via fumarate and malate. Microorganisms also exhibit other modifications of the tricarboxylic acid cycle.

During one full revolution of the tricarboxylic acid cycle, for every metabolized molecule of acetyl-CoA, three molecules of NADH and one molecule of FADH₂ are produced. In The process of Oxidative Phosphorylation, which takes place on The inner mitochondrial membrane, each molecule of NADH drives the synthesis of three ATP molecules from ADP and P_i, while FADH₂ yields two ATP molecules. Thus, 3 molecules of NADH and 1 molecule of FADH₂ contribute to the formation of 11 ATP molecules. In addition, ATP is generated from GTP (itself formed from GDP and P_i during the conversion of succinyl-CoA to succinate) in a reaction catalyzed by phosphokinase.

The total yield of ATP molecules generated via the TCA cycle and oxidative phosphorylation indicates a potential synthesis of 12 ATP molecules.

The Complete oxidation of a single molecule of acetyl-CoA releases approximately 898.7 kJ/mol of energy. Meanwhile, the synthesis of 12 ATP molecules requires 367.2 kJ/mol of energy. Consequently, the energy efficiency (η) of the TCA cycle is approximately 40%:

Beyond its energetic function, the TCA cycle acts as a key supplier of metabolites for The Biosynthesis of numerous compounds. Specifically, α-ketoglutarate can serve as a precursor for Proline, glutamine, and glutamate; succinyl-CoA as a precursor for the heme group in Hemoglobin; and oxaloacetate as a precursor for both carbohydrates as well as asparagine and aspartate.