Inosine-5′-monophosphate dehydrogenase

Inosine-5′-monophosphate dehydrogenase (IMPDH) is a purine biosynthetic enzyme that catalyzes the nicotinamide adenine dinucleotide (NAD+)-dependent oxidation of inosine monophosphate (IMP) to xanthosine monophosphate (XMP), the first committed and rate-limiting step towards the de novo biosynthesis of guanine nucleotides from IMP. IMPDH is a regulator of the intracelluar guanine nucleotide pool, and is therefore important for DNA and RNA synthesis, signal transduction, energy transfer, glycoprotein synthesis, as well as other process that are involved in cellular proliferation. Inosine-5′-monophosphate dehydrogenase (IMPDH) is a purine biosynthetic enzyme that catalyzes the nicotinamide adenine dinucleotide (NAD+)-dependent oxidation of inosine monophosphate (IMP) to xanthosine monophosphate (XMP), the first committed and rate-limiting step towards the de novo biosynthesis of guanine nucleotides from IMP. IMPDH is a regulator of the intracelluar guanine nucleotide pool, and is therefore important for DNA and RNA synthesis, signal transduction, energy transfer, glycoprotein synthesis, as well as other process that are involved in cellular proliferation. IMPDH is a tetrameric enzyme, composed of monomeric subunits with molecular masses of approximately 55 kDa and generally consist of 400-500 residues. Most IMPDH monomers contain two domains: a catalytic (β/α)8 barrel domain with an active site located in the loops at the C-terminal end of the barrel, and a subdomain consisting of two, repeated cystathionine beta synthetase (CBS) domains that are inserted within the dehydrogenase sequence. Monovalent cations have been shown to activate IMPDH enzymes and may serve to stabilize the conformation of the active-site loop. The CBS domain is not required for catalytic activity. Mutations within the CBS subdomain or a complete deletion of the domains do not impair the in vitro catalytic activity of IMPDH. An in vivo deletion of the CBS subdomain in E. coli suggests that the domain can act as a negative transregulator of adenine nucleotide synthesis. IMPDH has also been shown to bind nucleic acids, and this function can be impaired by mutations that are located in the subdomain. The CBS subdomain has also been implicated in mediating IMPDH association with polyribosomes, which suggests a potential moonlighting role for IMPDH as a translational regulatory protein. Drosophila IMPDH has been demonstrated to act as a sequence-specific transcriptional repressor that can reduce the expression of histone genes and E2F. IMPDH localizes to the nucleus at the end of the S phase and nuclear accumulation is mostly restricted to the G2 phase. In addition, metabolic stress has been shown to induce the nuclear localization of IMPDH.