NADPH + NAD+ + 2H+(inside) => NADP+ + NADH + 2H+(outside)
Despite the exact equivalence of their redox potentials, most cells maintain the mitochondrial NADPH / NADP couple about 500 times more reduced than NADH / NAD. (The situation also obtains in the cytosol, but it is achieved by different means, since there is no direct communication between the mitochondrial and the cytosolic pools of pyridine nucleotides.)
In general, the substrates for enzymes which react with NADP are often better reducing agents than the substrates for enzymes which react with NAD. Most dehydrogenases are specific for either NAD or NADP, and their substrates interact with only one of the pyridine nucleotide pools. There are exceptions to this rule: glutamate dehydrogenase works equally well with either coenzyme, and some substrates such as isocitrate and malate are metabolised by two alternative decarboxylation pathways: either using NADP via a small, non-regulatory dehydrogenase which seems close to equilibrium in vivo, or with NAD and a large allosteric enzyme with extensive regulatory properties. We do not presently understand the reason for this, but some biological advantage must be derived from this highly conserved arrangement..
The transhydrogenase is a single 97kDa polypeptide which is imported from the cytosol and inserted into the mitochondrial inner membrane. The enzyme has no prosthetic groups.
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