NADP/NADPH

The coenzyme for the oxidation-reduction reactions of fatty acid synthesis is NADP /NADPH, whereas degradation involves the NAD /NADH couple. 

No living cells can store large amounts of ATP. There is a finite amount of adenine distributed between AMP, ADP and ATP. Thus if the cell has a relatively high concentration of ATP, the concentrations of AMP and/or ADP must be lowered. The balance alters like a "see-saw", as one goes up the other must come down. In addition the total amount of NAD+/NADH and NADP+/NADPH in the cell is constant... 

Flavoprotein dehydrogenases usually accept electrons from reduced pyridine nucleotides and donate them to a suitable electron acceptor. The oxidation-reduction midpoint potential of the FAD of the oxidase has been determined by ESR spectroscopy and shown to be -280 mV. The NADP+/ NADPH redox potential is -320 mV and that of the cytochrome b is -245 mV hence, the flavin is thermodynamically capable of accepting electrons from NADPH and transferring them to cytochrome b. As two electrons are transferred from NADPH, although O2 reduction requires only one electron, the scheme of electron transfer shown in Figure 5.8 has been proposed by Cross and Jones (1991). 

Examples of coenzymes vitamin-derived nucleotides for example adenosine phosphates ATP, ADP, AMP nicotinamide derivatives NAD+, NADH, NADP+, NADPH flavin derivatives FAD, FADH2 coenzyme A (abbreviated to CoA, CoASH or CoA-SH). 

This enzyme [EC 2.7.1.23] catalyzes the reaction of ATP with NAD+ to produce ADP and NADP+. This reaction has the potential for altering the intracellular [NADP+]/ [NADPH] redox potential by converting NAD+ to NADP+, but details on the regulation of this enzyme have not been worked out. 

The coenzyme NADP+ differs from NAD+ only by the presence of a phosphate group (-P04=)on one of the ribose units (Figure 13.4). This seemingly small change in structure allows NADP+ to interact with NADP+-specific enzymes that have unique roles in the cell. For example, the steady-state ratio of NADP+/NADPH in the cytosol of hepato-cytes is approximately 0.1, which favors the use of NADPH in reductive biosynthetic reactions. This contrasts with the high ratio of NAD7NADH (approximately 1000 in the cytosol of hepatocytes), which favors an oxidative role for NAD+. This section summarizes some important NADP+ or NADPH-specific functions. 

Why are there two pyridine nucleotides, NAD+ and NADP+, differing only in the presence or absence of an extra phosphate group One important answer is that they are members of two different oxidation-reduction systems, both based on nicotinamide but functionally independent. The experimentally measured ratio [NAD+] / [NADH] is much higher than the ratio [NADP+] / [NADPH]. Thus, these two coenzyme systems also can operate within a cell at different redox potentials. A related generalization that holds much of the time is that NAD+ is usually involved in pathways of catabolism, where it functions as an oxidant, while NADPH is more often used as a reducing agent in biosynthetic processes. See Chapter 17, Section I for further discussion. 

See also NAD+, NADH NADP+, NADPH daily requirements 756, 769 Nicotinamide adenine dinucleotide. See NAD+, NADH... 

Still another difference between biosynthesis of fatty acids and oxidation (in mammals) is that the former has an absolute requirement for NADPH (Fig. 17-12) while the latter requires NAD+ and flavo-proteins (Fig. 17-1). This fact, together with many other observations, has led to the generalization that biosynthetic reduction reactions usually require NADPH rather than NADH. Many measurements have shown that in the cytosol of eukaryotic cells the ratio [NADPH]/[NADP+] is high, whereas the ratio [NADH]/[NAD+] is low. Thus, the NAD+/NADH system is kept highly oxidized, in line with the role of NAD+ as a principal biochemical oxidant, while the NADP+/NADPH system is kept reduced. 

If the ratio [NAD+] / [NADH] in a cell were 500 and the ratio [NADP+] / [NADPH] were 0.002, what concentrations of fructose and sorbitol would be in equilibrium with 0.1 mM glucose ... 

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