IMP cyclohydrolase-like protein
crystal structure of puro from methanothermobacter thermoautotrophicus
Identifiers
Symbol	IMP_cyclohyd
Pfam	PF07826
InterPro	IPR020600
Available protein structures: Pfam   structures / ECOD   PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

In enzymology, an IMP cyclohydrolase (EC 3.5.4.10) is an enzyme that catalyzes the chemical reaction

IMP + H₂O ${\displaystyle \rightleftharpoons }$ 5-formamido-1-(5-phospho-D-ribosyl)imidazole-4-carboxamide

Thus, the two substrates of this enzyme are IMP and H₂O, whereas its product is 5-formamido-1-(5-phospho-D-ribosyl)imidazole-4-carboxamide.

This enzyme belongs to the family of hydrolases, those acting on carbon-nitrogen bonds other than peptide bonds, specifically in cyclic amidines. The systematic name of this enzyme class is IMP 1,2-hydrolase (decyclizing). Other names in common use include inosinicase, and inosinate cyclohydrolase. This enzyme catalyses the cyclisation of 5-formylamidoimidazole-4-carboxamide ribonucleotide to IMP, a reaction which is important in de novo purine biosynthesis in archaeal species.^[1]

In most cases this single-domain protein is arranged to form an overall fold that consists of a four-layered alpha-beta-beta-alpha core structure. The two antiparallel beta-sheets pack against each other and are covered by alpha-helices on one face of the molecule. The protein is structurally similar to members of the N-terminal nucleophile (NTN) hydrolase superfamily. A deep pocket was in fact found on the surface of IMP cyclohydrolase in a position equivalent to that of active sites of NTN-hydrolases, but an N-terminal nucleophile could not be found. Therefore, it is thought that this enzyme is structurally but not functionally similar to members of the NTN-hydrolase family.^[2]

As of late 2007, 14 structures have been solved for this class of enzymes, with PDB accession codes 1G8M, 1M9N, 1OZ0, 1P4R, 1PKX, 1PL0, 1THZ, 2B1G, 2B1I, 2IU0, 2IU3, 2NTK, 2NTL, and 2NTM.

• FLAKS JG, ERWIN MJ, BUCHANAN JM (1957). "Biosynthesis of the purines. XVIII 5-Amino-1-ribosyl-4-imidazolecarboxamide 5'-phosphate transformylase and inosinicase". J. Biol. Chem. 229 (2): 603–12. doi:10.1016/S0021-9258(19)63668-9. PMID 13502325.