Inosinate dehydrogenase reaction

In inosine monophosphate dehydrogenase, the monovalent metal ion accelerates the hydride transfer step of the reaction with apparently few other effects on the enzyme structure. Probably the monovalent cation is involved in helping position the nicotinamide cofactor. The active site and location of the potassium ion are shown in Figure 2. Mycophenolic acid in this diagram is an inhibitor that is thought to lock inosine monophosphate into the active site, as shown. Note the large distance between the inhibitor (in the active site) and the K+. 

Adenosine deaminase converts adenosine monophosphate back to inosine monophosphate, liberating ammonia. This sequence of reactions thus provides a pathway for the deamination of a variety of amino acids, linked to transamination, similar to those shown in Figure 9.9 for transamination linked to glutamate dehydrogenase or glycine oxidase. 

The conversion of the adenine ring to guanine takes place by the deamination of AMP to inosine monophosphate (IMP), followed by oxidation of IMP to xanthosine monophosphate (XMP) and amination of XMP to GMP. Bacterial mutants which lack either the dehydrogenase or the aminase cannot convert adenine to nucleic acid guanine (19,20). In general the interconversion of purines does not take place by reversible reactions operating in either direction. 

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