Question

Aerobic dehydrogenase in biological oxidation contains:

• A. NAD
• B. NADH
• C. NADP
• D. FMN & FAD

Answer 1

The Correct Option is D

The correct option is (D): FMN & FAD.

Answer 2

Dehydrogenases involved in biological oxidation are classified as aerobic or anaerobic based on whether they can react directly with molecular oxygen. Let's assess each option on this basis:

1. NAD: Nicotinamide adenine dinucleotide is the coenzyme of anaerobic (pyridine-linked) dehydrogenases. It cannot transfer electrons directly to oxygen and instead passes them to a flavin-linked carrier in the electron transport chain, so it does not qualify as the coenzyme of an aerobic dehydrogenase.
2. NADH: This is simply the reduced form of NAD generated after a dehydrogenation reaction. Since it belongs to the same pyridine nucleotide system as NAD, it is also confined to anaerobic dehydrogenases and cannot react with oxygen directly.
3. NADP: Nicotinamide adenine dinucleotide phosphate functions largely in reductive biosynthetic pathways rather than in oxidative catabolism, and like NAD it is a pyridine nucleotide that needs an intermediate carrier to hand off electrons to oxygen. It is therefore not the coenzyme of an aerobic dehydrogenase either.
4. FMN & FAD: Flavin mononucleotide and flavin adenine dinucleotide are flavoproteins that can accept hydrogen directly from a substrate and, unlike the pyridine nucleotides, can also transfer that hydrogen directly to molecular oxygen, generating hydrogen peroxide. This direct reactivity with oxygen is exactly what defines an aerobic dehydrogenase.

Since only flavin-linked coenzymes can interact with oxygen without needing an intermediate carrier, they are what characterise aerobic dehydrogenases.

Therefore, the correct answer is FMN & FAD.