Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 2 H2O
-->3 NADH + FADH2 + CoA + GTP + 2 CO2 + 2H+
The citric acid cycle reactions occur in the matrix of mitochondria. All enzymes required for these reactions are present in the matrix. Each step is controlled by a specific enzyme and is reversible.
Each molecule of 2 carbon acetyl Co-A that enters the citric cycle first combines with 4 carbon oxaloacetic acid, and a 6 carbon citric acid is formed. One molecule of H2O is used in the reaction. In the next step, 6 carbon citric acid is first converted into 6 carbon aconitic acid (removal of H2O) and then into 6 carbon isocitric acid (addition of H2O).
The next reaction involves the oxidation of isocitric acid (by removal of hydrogen) to form 6 carbon oxalosuccinic acid. NADH2 is formed in the process. Decarboxylation of 6 carbon Oxalosuccinic acid results in the formation of 5 carbon α -Ketoglutaric acid with the liberation of carbon dioxide. α-Ketoglutaric acid (5 carbon) then undergoes oxidation (by removal of hydrogen) and decarboxylation to form 4 carbon succinyl Co-A. The reaction is highly complicated and takes place in the presence of Co-A and NAD. NADH2 is formed and carbon dioxide is released. 4 carbon Succinyl Co-A is hydrolyzed to succinic acid (4 carbon) in the next step. One molecule of H2O is used and Co-A is regenerated. The reaction is exergonic. Energy released is used for the formation of GTP (guanosine triphosphate) from GDP and inorganic phosphate. Subsequently, ATP is formed when GTP reacts with ADP
GTP + ADP → ATP + GDP
(Thus, there is the direct formation of one ATP when a 5 carbon acid is converted to a 4 carbon acid.)
In the next step, 4-C Succinic acid is oxidized (by removal of hydrogen) to 4 carbon fumaric acid in the presence of co-enzyme FAD (flavin adenine dinucleotide). A reduced FADH2 is formed. Fumaric acid (4 carbon) is converted to another 4 carbon acid, malic acid, by the addition of H2O. In the final step of the citric acid cycle, 4 carbon malic acid is oxidized (by removal of hydrogen) to 4 carbon oxaloacetic acid. NADH2 is formed in the process. Thus, oxaloacetic acid is regenerated in the last step. It can combine with another 2 carbon Acetyl Co-A to form citric acid and participate in the citric acid cycle again.
During various steps of the citric acid cycle, oxidation of substrate takes place by the removal of an electron hydrogen ion from substrate. It is accepted at each step by a suitable co-enzyme such as NAD or FAD to form a reduced co-enzyme molecule, NADH2 or FADH2, respectively. Electrons in this form are transferred then to oxygen through the electron transport chain, seen below:
The cycle, as a cycle, is purely catabolic, there are 2 carbons in (as acetyl group of acetyl CoA) and 2 carbons out (as CO2), and no net production or loss of any of the 9 chemical intermediates.
However, parts of the cycle can participate in linear metabolic pathways, and be used in anabolism.
For example, several amino acids can be catabolised to α-ketoglutarate, and act as a source of glucose, a carbon skeleton to be used in metabolic pathways. The synthesis of this and other glucogenic amino acids is biologically feasible because they are citric acid cycle intermediates (and pyruvate) which can be converted into phosphoenol pyruvate and then into glucose. Only leucine and lysine are solely ketogenic as when degraded give rise to ketone bodies or fatty acids, whereas other amino acids are both ketogenic and glucogenic.
If the cell is actively synthesizing protein, the citric acid cycle participates in biosynthesis (anabolism) by providing precursors to the required amino acids. Oxaloacetate is removed from the cycle for several purposes in the cell; it can be transaminated to aspartate, go down the gluconeogenic pathway and be converted into glucose, and converted into pyruvate. All of these are possible as well as the function of the compound in the citric acid cycle of being condensed with Acetyl Co-A to form citrate. Depletion of oxaloacetic acid under these conditions is prevented by its net synthesis in the pyruvate carboxylase reaction. Pyruvate carboxylase provided oxaloacetic acid in cells actively synthesizing carbohydrate.
Most of the oxaloacetic acid destined for gluconeogenesis (in the cytoplasm) leaves the mitochondrion as malate. In the cytoplasm, malate is reconverted to oxaloacetic acid + NADH and then to PEP. This indirect "shuttle" route is required because neither NAD+ nor NADH can be transported across the mitochondrial membranes.
The pathway for fatty acid synthesis in the cytoplasm requires a similar shuttle system involving transport of malate and citrate out of the mitochondrion. If the available (and storage) supply of carbohydrate is sufficient, the malate is converted to pyruvate + CO2 + NADPH in the malic enzyme-catalyzed reaction. This variant of the malate shuttle is significant because the NADPH is used as reducing agent in the fatty acid synthesis pathway.
There are several other, similar feed-in reactions, enabling cycle intermediates to be tapped off for anabolic purposes. The ‘feed-in’ reactions are called anaplerotic (or replenishing) reactions. Because of this capacity for anabolic activity, the citric acid cycle is sometimes referred to as being amphibolic, having both catabolic and anabolic activity.
Reference:
References were made to ‘Biochemistry’ - Stryer chapters 17, 23, 30.
Figure 1: accessed on 3/1/04
Figure 2, 4, and 5:
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Figure 3:
accessed on 20/12/03
