Question

Describe NAD+, NADH, NADP+, NADPH. And how NADH is used for catabolism mainly, while NADPH is used for biosynthesis.

Answer 1

NAD⁺ / NADH is mostly used in catabolic pathways while NADP ⁺ / NADPH is used in anabolic pathways. These cofactors are essential in metabolic reactions as they are involved in important ster of reactions accomplishing the electron transfer from one substrate to other.

Suppose if NAD⁺ / NADH is involved in both catabolic and anabolic processes, in catabolic process NAD⁺ would be reduced to NADH. In anabolic process, the NADH is oxidized to NAD⁺. The NADH concentration may excees as the catabolic process is progressed and this may activate the anabolic process. Both catabolic and anabolic processes csannot take place simultaneously.

Thus, in cells, both NAD⁺ / NADH and NADP⁺ / NADPH are readily available for catabolic and anabolic processes. Most of catabolic processes dominate the cellular metabolism therefore, the NAD⁺ / NADH are maintained at high rate. At the same time NADP⁺ / NADPH are maintained at low rate and cell ensures to maintain sufficient NADPH required for anabolic reactions. Moreover, the phosphate group of NADPH cannot readily involved electron tranfer because of its far position in the molecule. Thus, phosphate group enables the enzymes to choose between NADH and NADPH according to type of metabolic reaction. Therefore, NAD⁺ / NADH is preferred in catabolic reactions while NADP⁺ / NADPH is preferred in anabolic reactions.