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 (PubMed:20491506, PubMed:21081761). Does not catalyze the reverse reaction, i.e. the conversion of XMP to IMP (PubMed:21081761). Appears to be essential for the optimal growth of M.tuberculosis (PubMed:12657046). 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 (By similarity). Inhibited by the products XMP and NADH. Significantly inhibited in vitro by a panel of diphenyl urea-based derivatives and a series of novel classes of inhibitors, which are also potent anti-mycobacterial agents against M.tuberculosis and M.smegmatis (PubMed:21081761) (PubMed:22479467). Is also inhibited by a mycophenolic adenine dinucleotide (MAD) derivative in which a 1,2,3-triazole linker was incorporated as isosteric replacement of the pyrophosphate linker, thereby mimicking NAD (PubMed:20491506). Other inhibitors with modular structures consisting of two aromatic moieties connected by different linkers (such as urea and amide) have been identified and shown to exhibit antitubercular activity (PubMed:26440283). kcat is 0.53 sec(-1). Optimum pH is 8-8.5. Optimum temperature is 37 degrees Celsius. Purine metabolism; XMP biosynthesis via de novo pathway; XMP from IMP: step 1/1. Homotetramer. Cells lacking this gene display impaired growth. Belongs to the IMPDH/GMPR family.