Question

describe Krebs cycle

Answer 1

The Krebs cycle (or citric acid cycle) is a part of cellular respiration. Named after Hans Krebs, it is a series of chemical reactions used by all aerobic organisms to generate energy. Its importance to many biochemical pathways suggests that it was one of the earliest parts of cellular metabolism to evolve.

Answer 2

Kreb's Cycle

The oxidation of pyruvic acid into CO2 and water is called Krebs cycle. This cycle is also citric acid cycle because the cycle begins with the formation of citric acid. Citric acid is a carboxylic acid containing 3 COOH groups. Hence this cycle is also called as tri carboxylic acid cycle or TCA cycle. This cycle was first described by Kreb's in 1936. This cycle occurs only in the presence of oxygen. Hence it is an aerobic process. It takes place in the mitochondria.

Various Steps Involved in the Kerb's Cycle

Kreb's cycle involves the following steps:

Sub Topics
a) Formation of acetyl CoAb) Formation of citric acidc) Dehydrationd) Hydratione) Dehydrogenation If) Decarboxylationg) Oxidative decarboxylationh) Oxidationi) Hydrationj) Dehydrogenation a) Formation of acetyl CoABack to Top

The raw material for citric acid cycle is acetyl CoA. It is formed from fatty acids by b-oxidation and from pyruvic CoA to form Acetyl Co A. This reaction is catalysed by a set of enzymes called pyruvic acid dehydrogenase. In this reaction, 2 hydrogen atoms and one CO2 molecule are removed. Hence this reaction is called oxidative decarboxylation. The 2 hydrogen atoms are accepted by NAD and NAD is converted into NADH.

b) Formation of citric acidBack to Top

The acetyl- CoA combines with oxaloacetic acid to form citric acid. It contains 6 carbon atoms. This reaction is catalysed by an enzyme called citric acid synthetase.

The Citric acid Cycle

c) DehydrationBack to Top

Citric acid undergoes dehydration and forms cis-aconitic acid. This reaction is catalysed by the enzyme aconitase.

d) HydrationBack to Top

The aconitic acid is hydrated and it forms isocitric acid. This reaction is catalysed by the enzyme aconitase.

e) Dehydrogenation IBack to Top

Isocitric acid undergoes dehydrogenation in the presence of isocitric acid dehydrogenase to form oxalo succinic acid. In this reaction 2 hydrogen atoms are released. They are accepted by NAD+ to form NADH.

f) DecarboxylationBack to Top

The oxalo succinic acid undergoes decarboxylation to form a-ketoglutaric acid. This reaction is catalysed by decarboxylase. In this reaction one CO2 is eliminated. Hence the a-ketoglutaric acid has only 5 carbon atoms.

g) Oxidative decarboxylationBack to Top

During oxidative decarboxylation a-ketoglutaric acid is converted into succinyl CoA. This reaction is catalysed by a-ketoglutaric acid dehydrogenase. Two hydrogen atoms are released and they are transferred to NAD. The NAD is converted into NADH.

In the next step, the succinyl CoA is decarboxylated to succinic acid. This step is catalysed by succinic acid thiokinase. CoA is liberated.h) OxidationBack to Top

Succinic acid is oxidised to fumaric acid by the removal of 2 hydrogen atoms. The reaction is catalysed by succinic acid dehydrogenase. The hydrogen atoms are accepted by FAD and it forms FADH2.

i) HydrationBack to Top

Fumaric acid undergoes hydration to form malic acid. This reaction is catalysed by fumarase.

j) DehydrogenationBack to Top

It is the final step in Krebs cycle. Oxalo acetic acid is regenerated from malic acid by a process of dehydrogenation. This reaction is catalysed by malic acid dehydrogenase in the presence of NAD. The 2 hydrogen atoms removed are accepted by NAD and it forms NADH.

The oxaloacetic acid formed in the above reaction condenses with the acetyl CoA to form citric acid again and thus the cycle is repeated.