Question

1. Refer to the structure of NAD in order to be sure you know where the "pieces" are coming from for these reactions ?

2. Why are flavoproteins, as a class, involved in a greater variety of reactions than are NAD-linked enzymes?

Answer 1

I.

NAD+ is synthesized from two pathways;

1. Denovo pathway from amino acids

2. Salvage pathway from nicotinamide

Trp or Asp catabolism produces quinolinic acid which is converted to nicotinic acid mononucleotide (NaMN) by the transfer of phosphoribose moiety.

An adenylate moiety is added to NaMN to form NaAD which is further amidated to form NAD.

NAD+ is converted to NADP+ by the transfer of a phosphate by NAD+ kinase.

II.

FAD (or FMN) and NAD+ (or NADP+) are cofactors present in various enzymes that catalyze redox reactions.

The oxidized form of NAD+ (or NADP+) accepts two electrons and a proton.

Flavoproteins can participate in the transfer of one or two electrons.

The fully reduced form accepts two electrons.

The fully oxidized form accepts one electron.

This plasticity allows flavoproteins to catalyze a diverse set of reactions compared to NAD-proteins.