Question

COA The acetate of acetyl COA undergoes a stepwise oxidation to carbon dioxide and water in a cyclic pathway Explain in detail the ca (

Answer 1

Ans- Kreb cycle also known as Citric acid cycle, is a metabolic pathway that connects carbohydrate, fat, and protein metabolism. The reactions of the cycle are carried out by eight enzymes that completely oxidize acetate (a two carbon molecule), in the form of acetyl-CoA, into two molecules each of carbon dioxide and water.

It occurs in the matrix of mitrochondria.

Following are the steps of Kreb cycle-

1. The kreb cycle begins wth the condenstaion of an oxaloactetae, and the acetyl group of acetyl coenzyme A. Oxaloacetate reacts with acetyl coenzyme A and water two yeild citrate and coenzyme A. This reaction, which is an aldol condensation followed by hydrolysis, is catalysed by citrate synthase.

2. An isomerisation action, in which water is first removed and then added back, move the hydroxyl group from one carbon atom to its neighbour. The enzyme catalyzing this step is aconitase.

3.Isoitrate is oxidized decarboxylated to alpha ketoglutrate. in the first of four oxidation step in the cycle, the carbon carrying the hydroxyl group is converted to carbonyl group. the immediate product is unstable losing CO2 while still bound to the enzyme. Oxidative decarboxylation of isocitrate is catalyzed by isocitrate dehydrogenase.
4.Second oxidative decarboxylation reaction results in the formation of succinyl coenzyme A from alpha ketoglutarate. Alpha ketoglutarate dehydrogenase catalyzes the oxidative step and produces NADH, CO2 and high energy thioester bond to coenzyme A.

5. The cleavage of the thioester bond of succinyl CoA is coupled with the phosphorylation of an ADP or a GDP.this step is catalyzed by succinyl CoA synthase. ATP or GTP energetically equivalent. This is the only step in the citric acid cycle that directly its a compound with high Phosphorus transfer potential through a substrate level phoshorylation.

6. In the third oxidation step in the cycle, FAD remove to hydrogen atoms from succinate. The enzyme catalyzing this steps is succinate dehydrogenase.

7. the addition of water to furmate places a hydroxyl group next to a carbonyl carbon.

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 1.NAD+ linked malate dethydrogenase catalyzes the oxidation of malate to oxaloacetate.

The overall reaction for the citric acid cycle is:

2 acetyl groups + 6 NAD+ + 2 FAD + 2 ADP + 6 H2O-------> 4 CO2 + 6 NADH + 6 H+ + 2 FADH2 + 2 ATP + H2O