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 intracellular 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,[2][3][4] composed of monomeric subunits with molecular masses of approximately 55 kDa[5] and generally consist of 400-500 residues.[6]
Visual representation of the active site with IMP (green) and NAD (purple) bound.[7] Key residues (white) of the protein and the catalytic cysteine (cyan) are shown. Dashed lines represent polar contacts.
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.[6][8] Monovalent cations have been shown to activate IMPDH enzymes and may serve to stabilize the conformation of the active-site loop.[9]
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.[10][11]Anin vivo deletion of the CBS subdomain in E. coli suggests that the domain can act as a negative transregulator of adenine nucleotide synthesis.[12] IMPDH has also been shown to bind nucleic acids,[13] and this function can be impaired by mutations that are located in the subdomain.[14] The CBS subdomain has also been implicated in mediating IMPDH association with polyribosomes,[15] 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.[16] 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.[16]
The mechanism of IMPDH involves a sequence of two different chemical reactions: (1) a fast redox reaction involving a hydride transfer to NAD+ which generates NADH and an enzyme-bound XMP intermediate (E-XMP*) and (2) a hydrolysis step that releases XMP from the enzyme. IMP binds to the active site and a conserved cysteine residue attacks the 2-position of the purine ring. A hydride ion is then transferred from the C2 position to NAD+ and the E-XMP* intermediate is formed. NADH dissociates from the enzyme and a mobile active-site flap element moves a conserved catalytic dyad of arginine and threonine into the newly unoccupied NAD binding site. The arginine residue is thought to act as the general base that activates a water molecule for the hydrolysis reaction.[6] Alternatively, molecular mechanics simulations suggest that in conditions where the arginine residue is protonated, the threonine residue is also capable of activating water by accepting a proton from water while transferring its own proton to a nearby residue.[18]
Humans express two distinct isozymes of IMPDH encoded by two distinct genes, IMPDH1 and IMPDH2.
Both isozymes contain 514 residues, have an 84% similarity in peptide sequence, and have similar kinetic properties.[19] Both isozymes are constitutively expressed in most tissues, but IMPDH1 is predominately expressed in the spleen, retina, and peripheral blood leukocytes.[6]IMPDH1 is generally expressed constitutively at low levels, and IMPDH2 is generally upregulated in proliferating cells and neoplastic tissues.[20][21][22] Homozygous IMPDH1knockout mice demonstrate a mild retinopathy in which a slow, progressive form of retinal degeneration gradually weakens visual transduction,[23] while homozygous IMPDH2 knockout mice display embryonic lethality.[24]
Guanine nucleotide synthesis is essential for maintaining normal cell function and growth, and is also important for the maintenance of cell proliferation and immune responses. IMPDH expression is found to be upregulated in some tumor tissues and cell lines.[21] B and T lymphocytes display a dependence on IMPDH for normal activation and function,[25][26] and demonstrate upregulated IMPDH expression.[22] Therefore, IMPDH has been addressed as a drug target for immunosuppressive and cancerchemotherapy.
^Gan L, Seyedsayamdost MR, Shuto S, Matsuda A, Petsko GA, Hedstrom L (February 2003). "The immunosuppressive agent mizoribine monophosphate forms a transition state analogue complex with inosine monophosphate dehydrogenase". Biochemistry. 42 (4): 857–63. doi:10.1021/bi0271401. PMID12549902.
^Zhang R, Evans G, Rotella FJ, Westbrook EM, Beno D, Huberman E, Joachimiak A, Collart FR (April 1999). "Characteristics and crystal structure of bacterial inosine-5'-monophosphate dehydrogenase". Biochemistry. 38 (15): 4691–700. CiteSeerX10.1.1.488.2542. doi:10.1021/bi982858v. PMID10200156.
^Whitby FG, Luecke H, Kuhn P, Somoza JR, Huete-Perez JA, Phillips JD, Hill CP, Fletterick RJ, Wang CC (September 1997). "Crystal structure of Tritrichomonas foetus inosine-5'-monophosphate dehydrogenase and the enzyme-product complex". Biochemistry. 36 (35): 10666–74. doi:10.1021/bi9708850. PMID9271497.
^Prosise GL, Luecke H (February 2003). "Crystal structures of Tritrichomonasfoetus inosine monophosphate dehydrogenase in complex with substrate, cofactor and analogs: a structural basis for the random-in ordered-out kinetic mechanism". J. Mol. Biol. 326 (2): 517–27. doi:10.1016/S0022-2836(02)01383-9. PMID12559919.
^Sintchak MD, Nimmesgern E (May 2000). "The structure of inosine 5'-monophosphate dehydrogenase and the design of novel inhibitors". Immunopharmacology. 47 (2–3): 163–84. doi:10.1016/S0162-3109(00)00193-4. PMID10878288.
^Nimmesgern E, Black J, Futer O, Fulghum JR, Chambers SP, Brummel CL, Raybuck SA, Sintchak MD (November 1999). "Biochemical analysis of the modular enzyme inosine 5'-monophosphate dehydrogenase". Protein Expression and Purification. 17 (2): 282–9. doi:10.1006/prep.1999.1136. PMID10545277.
^Senda M, Natsumeda Y (1994). "Tissue-differential expression of two distinct genes for human IMP dehydrogenase (E.C.1.1.1.205)". Life Sciences. 54 (24): 1917–26. doi:10.1016/0024-3205(94)90150-3. PMID7910933.
^ abZimmermann AG, Gu JJ, Laliberté J, Mitchell BS (1998). Inosine-5'-monophosphate dehydrogenase: regulation of expression and role in cellular proliferation and T lymphocyte activation. Progress in Nucleic Acid Research and Molecular Biology. Vol. 61. pp. 181–209. doi:10.1016/S0079-6603(08)60827-2. ISBN978-0-12-540061-9. PMID9752721.
^Jonsson CA, Carlsten H (January 2003). "Mycophenolic acid inhibits inosine 5'-monophosphate dehydrogenase and suppresses immunoglobulin and cytokine production of B cells". International Immunopharmacology. 3 (1): 31–7. doi:10.1016/s1567-5769(02)00210-2. PMID12538032.
^Clinical trial number NCT04356677 for『Study to Evaluate the Safety and Efficacy of VIRAZOLE® in Hospitalized Adult Participants With Respiratory Distress Due to COVID-19』at ClinicalTrials.gov
Wang J, Yang JW, Zeevi A, Webber SA, Girnita DM, Selby R, Fu J, Shah T, Pravica V, Hutchinson IV, Burckart GJ (May 2008). "IMPDH1 gene polymorphisms and association with acute rejection in renal transplant patients". Clin. Pharmacol. Ther. 83 (5): 711–7. doi:10.1038/sj.clpt.6100347. PMID17851563. S2CID12718828.
xanthosine 5'-phosphate + NADH + H+
