Question

(a) Give a schematic representation of Kreb cycle. List the enzymes involved in each step.
(b) Mention the steps in Krebs cycle in which step NAD and FAD are reduced by accepting hydrogen from different substrates.

Answer 1

(a) List of enzymes involved in Krebs cycle includes:

1 Citrate synthase

2 Aconitase

3 Isocitrate dehydrogenase

4 α-ketoglutarate dehydrogenase

5 Succinyl coenzyme A synthetase

6 Succinate dehydrogenase

7 Fumarase

8 Malate dehydrogenase.

(b) In the third step of Kreb cycle, isocitrate is oxidized to form $\alpha$-ketoglutarate. In this step, a molecule of carbon dioxide is released. During this step, NAD${}^{+}$ is reduced to form NADH + H${}^{+}$. Isocitrate dehydrogenase enzyme catalyzes this step.

In the fourth step, alpha-ketoglutarate is oxidized to form succinyl CoA. In this step, NAD${}^{+}$ is reduced to form NADH + H${}^{+}$. A molecule of carbon dioxide is released. The $\alpha$-ketoglutarate dehydrogenase enzyme catalyzes this step.

In the sixth step, succinate is oxidized to form fumarate. In this step, FAD${}^{+}$ is reduced to form FADH${}_2$. Succinic dehydrogenase enzyme catalyzes this step.

In the last step of the Kreb cycle, oxaloacetate is regenerated by oxidation of malate. In this step, NAD${}^{+}$ is reduced to form NADH + H${}^{+}$. Malate dehydrogenase enzyme catalyzes this step.