Biochemical Foundations of Human Vital Activity - Volkov N.I., Nesen E.N. 2000

Biochemical Foundations of Human Vital Activity
Energy Metabolism in the Body
The Citric Acid Cycle as the Central Pathway of Aerobic Nutrient Oxidation

The Citric Acid Cycle (or Tricarboxylic Acid Cycle), discovered by the British biochemist Krebs in 1937, is the central metabolic pathway (the "furnace") for CARBOHYDRATES, fats, and Amino Acids, as well as for ENERGY EXTRACTION FROM oxidized substances. It takes place in the mitochondrial matrix and consists of 8 main reactions that gradually oxidize acetyl-CoA (the active form of acetic acid) into the end product of METABOLISM, CO₂, while storing energy in the form of three NADH molecules, two FADH₂ molecules, and one GTP molecule. Two carbon atoms from the acetyl-CoA molecule are converted into two CO₂ molecules during one full turn of the cycle. The sequence of transformations in The Tricarboxylic Acid Cycle is shown in Fig. 18 (intermediates are highlighted in bold, and Enzymes catalyzing the transformations of substances located in the mitochondrial matrix are shown in regular text).

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Fig. 78 Citric acid cycle

Entering the oxidation cycle, acetyl-CoA interacts with oxaloacetate (oxaloacetic acid) to form tricarboxylic citric acid. This is why the cycle is named the "tricarboxylic acid cycle, or citric acid cycle." Citric acid undergoes a multistep oxidation process in this cycle, The final stage of which regenerates oxaloacetate, capable of combining with a new acetyl-CoA molecule.

The first reaction of the cycle is the interaction of acetyl-CoA (a two-carbon (C₂) substance) with oxaloacetate (C₄) in the presence of the enzyme citrate synthase and Water. As noted above, this reaction yields citrate (citric acid). This reaction regulates The rate of the cycle, as the enzyme's activity depends on the concentration of ATP and certain cycle products.

Citrate undergoes dehydration (loss of H₂O) catalyzed by the enzyme aconitase. This reaction yields cis-aconitic acid (C₆), which then adds a molecule of water to become isocitrate (isocitric acid). The Active Site of aconitase contains an iron ion (Fe²⁺), which can interact with various Mineral Substances, such as fluorine, thereby regulating the enzyme's activity.

Under the Influence of the NAD-dependent isocitrate dehydrogenase enzyme, isocitrate is converted into oxalosuccinate (oxalosuccinic acid), while NAD is reduced to NADH + H⁺. This is the first oxidation-reduction reaction in the cycle.

Oxalosuccinate undergoes decarboxylation, releasing a CO₂ molecule and forming a-ketoglutarate (a-ketoglutaric acid).

Next, a-ketoglutarate undergoes oxidative decarboxylation involving the a-ketoglutarate dehydrogenase multienzyme complex, yielding the high-energy compound succinyl-CoA and a second CO₂ molecule. During this oxidation, two hydrogen atoms are bound by the coenzyme NAD. The enzyme catalyzing this reaction contains five Coenzymes: NAD, FAD, HS-CoA, lipoamide, and TPP. The sources of these coenzymes are Vitamins (PP, B₂, B₃, F, B₁), the Functions of which are discussed in Chapter 7.

Succinyl-CoA is converted into succinate (succinic acid) with the participation of the enzyme succinate thiokinase. In this process, The energy released from breaking the high-energy chemical bond in the succinyl-CoA molecule drives the synthesis of high-energy GTP from GDP and inorganic phosphate. A GTP molecule is energetically equivalent to an ATP molecule.

Succinate is oxidized to fumarate (fumaric acid) by the action of succinate dehydrogenase. The coenzyme for this enzyme is FAD, which binds two hydrogen atoms and is reduced to FADH₂.

Under the action of the enzyme fumarate hydratase (fumarase), fumaric acid loses a molecule of water and is converted into malic acid (malate).

Malate is oxidized to oxaloacetate by NAD-dependent malate dehydrogenase, releasing two hydrogen atoms that are accepted by NAD. Oxaloacetate serves as the initial substrate for the tricarboxylic acid cycle. Following this reaction, a new cycle begins with another acetyl-CoA molecule.

Thus, one full turn of the cycle results in The oxidation of a single acetyl-CoA molecule into two molecules of CO₂ and four pairs of hydrogen atoms (3NADH₂ and FADH₂) carrying high-energy electrons. These electrons are subsequently transferred to the Respiratory Chain of The inner mitochondrial membrane to reduce O₂.

The formation of CO₂ in this cycle utilizes oxygen produced during The breakdown of water molecules. CO₂ molecules diffuse out of the Cell/35.html">Mitochondria and leave The Cell. The overall reaction of The citric acid cycle is expressed as

CH₃CO-CoA + 3NAD + FAD + GDP + P_i + 2H₂O →

→ 2CO₂ + 3NADH + FADH₂ + GTP + 2H + CoA

Thus, the carriers NADH₂ and FADH₂ accumulate the energy of carbohydrate, fat, and protein oxidation, which can be released only when electrons are transferred to molecular oxygen.

The citric acid cycle functions exclusively under aerobic conditions. Therefore, its operation depends on the rate of oxygen delivery to the Organism and its utilization rate by Cells, as well as on the concentrations of the oxidized forms of NAD and FAD. The rate of many biochemical reactions in the cycle also depends on the levels of ATP and ADP within the mitochondria. When cellular ATP utilization decreases, or when ADP is deficient, the rate of the reactions occurring in the cycle drops.

An important role in the Regulation of the citric acid cycle is played by the activity and quantity of Enzymes and Coenzymes, which alter the concentrations of acetyl-CoA and various metabolic intermediates. For instance, an increased supply of acetyl-CoA and oxidation intermediates such as citrate, succinate, and fumarate increases the rate of the cycle's reactions and the overall rate of oxygen consumption. Many enzymes contain vitamins; therefore, their presence in the cell in adequate amounts also significantly affects the reaction rates of this cycle. Numerous cations (Fe²⁺, Mn²⁺, Mg²⁺, Cu²⁺) act as activators of mitochondrial enzymes and thus influence the rate of citric acid cycle reactions. Certain substances, such as fluorine-containing compounds, can decrease the rate of Biological Oxidation REACTIONS in this cycle by inhibiting enzyme activity.

Thus, the citric acid cycle mediates the oxidation of nutrients and the extraction of energy in the form of high-energy hydrogen (2H⁺ + 2e^-) via its carriers NAD and FAD. The reduced carriers (NADH and FADH₂) deliver hydrogen to the inner mitochondrial membranes, where they transfer it to the so-called respiratory chain. In this chain, electrons are transferred to molecular oxygen to form H₂O, generating an electrochemical H⁺ concentration gradient that drives ATP synthesis through Oxidative Phosphorylation.