Citric Acid Cycle || Krebs Cycle || Tricarboxylic Acid (TCA) Cycle

Citric acid cycle is also known as the Krebs cycle or Tricarboxylic acid cycle. It was discovered by H.A.Krebs, a German born British Biochemist, who received the Nobel prize in 1953. This cycle occurs in the Mitochondrial matrix (cytosol in prokaryotes). The net result of citric acid cycle is that for each acetyl group entering the cycle as acetyl-CoA, two molecules of CO2 are produced.

Firstly Glycolysis is take place from which Glucose is converted into Pyruvate. This Pyruvate is then converted into Acetyl CoA by removing 2 moles of carbon dioxide and NADH. Then this acetyl CoA act in TCA cycle.

This is 8 steps reaction followed by various different enzymes.


Step 1 : The cycles begins with the condensation of an Oxaloacetate (four carbon unit) and the acetyl group of acetyl-CoA (two carbon unit). Oxaloacetate reacts with acetyl-CoA and H2O to yield citrate and coenzyme A. This reaction, which is an aldol condensation followed by a Hydrolysis, is catalyzed by Citrate Synthase. Citrate has no chiral center but has the potential to react asymmetrically if an enzyme with which it interacts has a active site that is asymmetric. Such molecules is called Prochiral molecule.

Step 2 : An Isomerization reaction, in which water is first removed and then added back, moves the hydroxyl group from one carbon atom to its neighbour. The enzyme catalyzing this step, Aconitase (non heam iron proteins), is the the target site for the toxic compound Fluoroacetate (used as a pesticide). Fluoroacetate blocks the TCA cycle by its metabolic conversion of fluorocitrate, which is a potent inhibitor of Aconitase.

Step 3 : Isocitrate is oxidized and decarboxylated to alpha-Ketoglutarate ( also called oxoglutarate). In the first of four oxidation steps in the cycle, the carbon carrying the hydroxyl group is converted to a carbonyl group. The intermediate product is unstable, losing co2 while still bound to the enzyme. The oxidative decarboxylation of Isocitrate is catalyzed by isocitrate dehydrogenase.

Step 4 : A second oxidative decarboxylation reaction results in the formation of succinyl CoA [SCA] from alpha-ketoglutarate. Alpha-ketoglutarate dehydrogenase catalyzes this oxidative step and produces NADH, CO2, and a high energy thioester bond to coenzyme A.

                     

                          Fig. Citric Acid Cycle

Step 5 : The cleavage of the thioester bond of SCA is coupled with the phosphorylation of an ADP or a GDP (substrate level phosphorylation). This is catalyzed by SCA synthetase (succinate thiokinase). ATP and GTP are energetically equivalent. This is the only step in the TCA that directly yields a compound with high phosphoryl transfer potential through a substrate level phosphorylation.

Animal cells have two isozymes of SCA synthetase, one specific for ADP and other for GTP. This GTP formed by SCA synthetase can donate its terminal phosphoryl group to ADP to form ATP, in a reversible reaction catalyzed by nucleoside diphosphate kinase. In the cells of plants, bacteria and some animal tissues , an ATP molecule forms directly by substrate level phosphorylation

Step 6 : In the third oxidation step in the cycle, FAD removes two hydrogen atoms from succinate. The enzyme catalyzing this step, succinate dehydrogenase, is strongly inhibited by Malonate, A structural analog of succinate and a classic example of a competitive inhibitor.

Step 7 : The addition of water to Fumarate residues a hydroxyl group next to a Carbonyl carbon by Fumarase.

Step 8 : In the last of four oxidation steps in the cycle, the carbon carrying the hydroxyl group is converted to a carbonyl group, regenerating the Oxaloacetate needed for step one NAD+ linked Malate dehydrogenase catalyzed the oxidation of malate to oxaloacetate.

Regulation Of Citric Acid Cycle

The TCA is regulated at its three strongly exergonic steps catalyzed by enzyme Citrate synthase, Isocitrate dehydrogenase and Alpha-ketoglutarate dehydrogenase. The regulatory enzyme of the TCA cycle seems to control flux primarily by three simple mechanism:

1. Substrate availability 2. Product inhibition

3. Allosteric feedback inhibition of the enzyme.

The TCA can be limited by the availability of the citrate synthase substrates, acetyl CoA and oxaloacetate. Enzyme isocitrate dehydrogenase is allosterically activated by ADP and inhibited by reaction product, NADH. Enzyme alpha-ketoglutarate dehydrogenase catalyzes the rate limiting step in the TCA. It is allosterically inhibited by succinyl CoA and NADH the products of the reaction that is catalyzes.