Food Chemistry and Fish in Nutrition

6.3.2. Citric acid cycle

The citric acid cycle (Kreb’s cycle, Tricarboxylic acid cycle) is a series of reactions in mitochondria that bring about the catabolism of acetyl residues, liberating hydrogen equivalents, which, upon oxidation, lead to the release of most of the free energy which is captured as ATP of most of the available energy of tissue fuels. The acetyl residues are in the form of acety1- CoA (CH₃-CO-S-CoA, active acetate), an ester of coenzyme A. Coenzyme A contains the vitamin pantothenic acid.

             The major function of the citric acid is to act as the final common pathway for the oxidation of carbohydrate, lipids, and protein. This is because glucose, fatty acids, and many amino acids are all metabolized to acetyl-CoA or intermediates of the cycle. It also plays a major role in gluconeogenesis, transamination, deamination, and lipogenesis. Several of these processes are carried out in many tissues but the liver is the only tissue in which all occur to a significant extent. Reactions of the citric acid cycle liberate reducing equivalents and CO2. The reactions of citric acid cycle the following 9 steps:

1. Condensation of acety1- CoA with oxaloacetate to form citrate

            The initial condensation of acety1- CoA with oxaloacetate to form citrate is catalyzed by condensing enzyme, citrate synthase, which effects synthesis of a carbon to carbon bond between the methyl carbon of acety1-CoA and the carbony1 carbon of oxaloacetate.

2. Conversion of citrate to isocirtrate via cis-aconitate

            Citrate is converted to isocitrate by the enzyme aconitase (aconitate hydratase), which contains iron in the Fe2+ state in the form of an iron- sulfur protein (Fe:S) This conversion takes place in two steps: dehydration to cis-aconitate, some of which remains bound to the enzyme, and rehydration to iocitrate. 

3. Dehydrogenation of isocitrate to oxalosuccinate

Isocitrate undergoes dehydrogenation in the presence of isocitrate dehydrogenase to form oxalosuccinate. The linked oxidation of isocitrate proceeds almost completely through the NAD+ dependent enzyme isocitrate dehydrogenase. 

4. Decarboxylation  of oxalosuccinate to α -ketoglutarate

       There follows decarboxylation of oxalosuccinate to α -ketoglutarate, also catalyzed by isocitrate dehydrogenase. A CO₂ molecule is liberated.  Mn2+(or Mg2+) is an important component of the decarboxylation .

5. Decarboxylation of α-ketoglutarate to succiny1-CoA

            Next, α -ketoglutarate undergoes oxidative decarboxylation. The reaction is catalyzed by an α -ketoglutarate dehydrogenase complex, which requires cofactors thiamin pyrophosphote, lipoate, NAD+, FAD and CoA results in the formation of succiny1-CoA, a high- energy thioester and NADH. Arsentic inhabits the reaction, causing the substrate, α -ketoglutarate to accumulate.

 6.  Conversion of succinyl-CoA to succinate

            Succinyl-CoA is converted to succinate by the enzyme succinate thiokinase   (succiny1CoA synthetase). High-energy phosphate (ADP+Pi →ATP) is synthesized at the substrate level because the release of free energy from the oxidative decarboxylation of α -ketoglutarate is sufficient to generate a high- energy phosphate..

7. Dehydrogenation of succinate to fumarate        

Succinate is metabolized further by undergoing a dehydrogenation catalyzed by succinate dehydrogenase to form fumerate. It is the only dehydrogenation in the citric acid cycle that involves the direct transfer of hydrogen from the substrate to a flavorprotein without the participation of NAD+.

8. Addition of water to furmarate to give malate.

Furmarase (furmarate hydratase) catalyzes the addition of water to furmarate to give malate. In addition to being specific for the L-isomer of malate, furmarase catalyzes the addition of the elements of water to the double bond of furmarate in the tans configuration. 

9. Dehydrogenation of malate to form oxaloacetate

Malate is converted to oxaloacetate by dehydrogenation catalysed by the  enzyme malate dehydrogenase, a reaction requiring NAD+.

Thus the citric acid cycle is completed. An acetyl group, containing two carbon atoms, is fed into the cycle by combining it with oxaloacetate. At the end of the cycle a molecule of oxaloacetate is generated.

            The enzymes of the citric acid cycle, except for the α-ketoglutarate and succinate dehydrogenase, are also found outside the mitochondria. As a result of oxidations catalyzed by dehydrogenase enzymes of the citric acid cycle, three molecules of NADH and one molecule of FADH2 are produced for each molecule of acety1-CoA catabolized in one revolution of the cycle. These reducing equivalents are transferred to the respiratory chain in the inner mitochondrial membrane for reoxidation.