25. The core functional unit present in NADH is:
1. Pyridine
2. Dihydro Pyridine
3. Purine
4. Pyramidine

What Is the Core Functional Unit in NADH?

NADH (Nicotinamide Adenine Dinucleotide, reduced form) is a vital coenzyme found in all living cells. It plays a crucial role in cellular respiration, particularly in redox reactions where it serves as an electron carrier. But what gives NADH its redox properties? It all comes down to its core functional unit.

The Structure of NADH

NADH consists of two main parts:

1. Nicotinamide moiety (derived from vitamin B3 or niacin)

2. Adenine nucleotide moiety

The nicotinamide ring is the redox-active site—this is where NAD⁺ gets reduced to NADH and vice versa. The key to this redox behavior lies in a specific ring structure.

The Core Functional Unit: Pyridine

The nicotinamide ring in NAD⁺/NADH is structurally a pyridine ring—a six-membered aromatic ring with one nitrogen atom.

• In its oxidized form (NAD⁺), the ring is a pyridinium ion (positively charged).

• During reduction (conversion to NADH), the hydride ion (H⁻) is added to the ring, forming 1,4-dihydropyridine, a reduced version of the pyridine ring.

Thus, pyridine is the core functional unit, and its ability to reversibly accept electrons makes NADH such a powerful biochemical tool in metabolism.

Breakdown of Options:

1. Pyridine – ✅ Correct. This is the core ring in the nicotinamide part of NAD⁺/NADH.

2. Dihydropyridine – ❌ Incorrect. While NADH forms a dihydropyridine ring upon reduction, the core base structure is still pyridine.

3. Purine – ❌ Incorrect. Purines are found in the adenine portion of NADH, not the functional redox center.

4. Pyrimidine – ❌ Incorrect. Pyrimidines are not part of the nicotinamide moiety.

✅ Correct Answer:

(1) Pyridine

Conclusion:

The core functional unit in NADH is pyridine, present in the nicotinamide ring responsible for electron transfer. This structure allows NADH to serve as a key redox cofactor in countless metabolic reactions, making it indispensable in the bioenergetics of life.