Question

Describe Kreb's cycle. (Flow chart or Explanation)

Answer 1

Krebs cycle explains the aerobic phase of respiration. There are a series of cyclic reactions involved in converting pyruvic acid to carbon dioxide and water in mitochondria. It is also known as citric acid or tricarboxylic acid cycle (TCA cycle).

(i) In the first reaction of citric acid cycle, one molecule of acetyl Co-A combines with an oxaloacetic acid to form citric acid. This reaction is catalyzed by citric acid groups.

(ii) Citric acid is dehydrated to form cis-aconitic acid in the presence of aconitase.

(iii) The same enzyme aconitase catalyzes the formation of isocitric acid from the cis-aconitic acid by the addition of a molecule of water. Citric acid, cis-aconitic acid and isocitric acid contain three carboxylic acid groups.

(iv) The isocitric acid is oxidatively decarboxylated to $\alpha$ ketoglutaric acid. This reaction is catalyzed by an isocitric dehydrogenase. During this reaction, one NADH${}_2$ is formed.

(v) The $\alpha$-ketoglutaric acid is oxidatively decarboxylated to form succinyl Co-A. This reaction is catalyzed by $\alpha$-ketoglutaric dehydrogenase. The energy released during this reaction is conserved in NADH${}_2$.

(vi) The succinyl Co-A is hydrolyzed to succinic acid in the presence of succinyl Co-A synthetase. In this reaction, ADP is phosphorylated to ATP. This is called substrate level phosphorylation.

(vii) The succinic acid is oxidized to form fumaric acid by the succinic dehydrogenase. Here, FAD${}^{+}$ is reduced to FADH${}_2$.

(viii) The fumaric acid is converted to malic acid by the addition of a molecule of water. This reaction is catalyzed by fumarase.

(ix) The malic acid is oxidized to oxaloacetic acid by the enzyme malic dehydrogenase. Here, NAD${}^{+}$ is reduced to NADH${}_2$.