Inosine-5′-monophosphate dehydrogenase (IMPDH) is a tetrameric enzyme. Biologically, it catalyzes the oxidation of inosine monophosphate (IMP) to xanthosine monophosphate (XMP) in the presence of nicotinamide adenine dinucleotide (NAD+). The overall reaction is as follows:
Inosine 5'-phosphate + NAD+) + H2O ⥂ xanthosine 5'-phosphate + NADH + H+)
This particular step is crucial as it is the first committed step in the de novo guanine biosynthesis. Similar to adenine, guanine is derived from IMP. Both purines, i.e. adenine and guanine, can be produced from IMP but when IMP is first oxidized to XMP with the utilization of NAD and through the catalytic activity of IMPDH it commits the biosynthetic pathway into producing guanine rather than adenine. Thus, IMPDH can be regarded as a regulator of guanine levels inside the cell. In humans, there are two isozymes encoded by the genes IMPDH1 and IMPDH2. Abbreviation: IMPDH, IMP dehydrogenase.
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