NADH

NADPH, nicotinamide adenine dinucleotide phosphate (reduced), is the major source of electrons for biosynthesis. NADP+ and NADPH are identical to NAD+ and NADH, respectively, with the exception the former have an additional phosphate esterfied at C-2’ on the adenylate moiety.

Consider the structure above. When R = H, this is the structure of NAD+. When R = PO3-2 (phosphate), the structure is that of NADP+. NADP+ has a very different function than NAD+. Recall that NAD+ is converted to NADH during the oxidation of glucose, and is then reoxidized, yielding ATP. For this reason, the cellular ratio of NAD+/NADH is kept at 1000, to favor oxidative reactions such as those in glycolysis. By contrast, NADP+ is converted to NADPH by the pentose phosphate shunt, and his compound is then reoxidized during reductive biosynthesis, such as certain reactions in fatty acid metabolism. The cell keeps the ratio of NADP+/NADPH at 0.01 to favor reductive biosynthesis. In some tissues such as adipose and liver, where fatty acid and cholesterol synthesis are rapid, as much as 30% of glucose is metabolized by the pentose phosphate shunt.

The pentose phosphate shunt pathway is shown in the diagram above. It occurs in three stages: oxidative reactions that produce NADPH, isomerizations and epimerizations, and carbon-carbon bond cleaving and forming reactions. Note that the shunt begins at the glycolytic intermediate glucose-6-P, and ends at another glycolytic intermediate, glyceraldehyde-3-phosphate. Hence the name "shunt".

Most importantly, the shunt is responsible for the formation of two main products, ribose-5-P, which is used in nucleotide biosynthesis, and NADPH, which is used in reductive biosynthesis.