Catalyzes the conversion of inosine 5'-phosphate (IMP) to xanthosine 5'-phosphate (XMP), the first committed and rate-limiting step in the de novo synthesis of guanine nucleotides, and therefore plays an important role in the regulation of cell growth. H2O + IMP + NAD(+) = H(+) + NADH + XMP Mycophenolic acid (MPA) is a non-competitive inhibitor that prevents formation of the closed enzyme conformation by binding to the same site as the amobile flap. In contrast, mizoribine monophosphate (MZP) is a competitive inhibitor that induces the closed conformation. MPA is a potent inhibitor of mammalian IMPDHs but a poor inhibitor of the bacterial enzymes. MZP is a more potent inhibitor of bacterial IMPDH. Purine metabolism; XMP biosynthesis via de novo pathway; XMP from IMP: step 1/1. Homotetramer. Can form fiber-like subcellular structures termed 'cytoophidia' in response to intracellular guanine-nucleotide depletion. Belongs to the IMPDH/GMPR family.