These pairs of molecules are identical except for the presence of a phosphate group at the 2′ position on the ribose moiety. This is not a high-energy phosphate but rather a molecular tag that enables enzymes to discriminate between the two forms of redox compound. In higher animals there do not appear to be any NADH transferase enzymes that catalyze direct transfer of hydrogen atoms from NADH to NADP+ or from NADPH to NAD+.
NADH and NADPH are equivalent in terms of their standard redox potentials, but because redox enzymes are usuallyselective for one or the other of them, two distinct pools of reductants exist. NADH is used as a source of reducing equivalents for the electron transport chain (ETC) while NADPH provides reducing equivalents for many biosyntheticreactions. Hence, even within a single spatial compartment such as the cytoplasm, the NADH to NAD+ ratio can be very low, favoring oxidation of fuels, while simultaneously the NADPH to NADP+ ratio can be very high, facilitating biosynthesis.
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