NAD tends to be an electron acceptor in catabolic reactions involving the degradation of carbohydrates, fatty acids, ketone bodies, amino acids, and alcohol. . NADP, which is cytosolic, tends to be involved in biosynthetic reactions. Reduced NADP is generated by the pentose phosphate pathway (cytosolic) and used by cytosolic pathways, such as fatty acid biosynthesis and cholesterol synthesis, and by ribonucleotide reductase. The niacin coenzymes are used for two-electron transfer reactions. The oxidized from of NAD is NAD+. There is a positive charge on the cofactor because the aromic amino group is a quaternary amine. A quaternary amine participates in four covalent bonds. NAD-dependent reactions involve the transfer of two electrons and two protons. NAD accepts two of the electrons and one of the protons; the remaining proton remains in solution. Hence, the reduced from of NAD is not written as NADH2, but as NADH + H+ (Figure 9.64).

The niacin coenzymes might be compared with the riboflavin coenzymes. Niacin coenzymes are used by enzymes for the transfer of two electron at a time, where both electrons are transferred without the accumulation of a one-electron reduced intermediate. The riboflavin coenzymes, in accepting two electrons, can accept one electron at a time, with a detectable free radical intermediate. Another difference is that niacin coenzymes do not readily react with molecular oxygen, whereas riboflavin coenzymes can form covalent bond with oxygen. Hence, flavoenzymes are used for introducing oxygen, from O2 into various metabolites. Another difference is that the reducing power of NADH + H+ is greater than that of FADH2. Electrons from reduced NAD are often transferred to flavoproteins, resulting in a reduced flavoproteins; that is, the FAD cofactor is converted to FADH2. The reverse event, namely the reduction of NAD by FADH2 does not tend to occur. This point is illustrated by the fact that NADH + H+ can generate more ATPs in the respiratory chain than can FADH2.

Source: Nutritional Biochemistry pages 594-595