This is a family of bifunctional enzymes catalysing the last two steps in de novo purine biosynthesis. The bifunctional enzyme is found in both prokaryotes and eukaryotes. The second last step is catalysed by 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase EC:2.1.2.3 (AICARFT), this enzyme catalyses the formylation of AICAR with 10-formyl-tetrahydrofolate to yield FAICAR and tetrahydrofolate [1]. This is catalysed by a pair of C-terminal deaminase fold domains in the protein [3], where the active site is formed by the dimeric interface of two monomeric units [3]. The last step is catalysed by the N-terminal IMP (Inosine monophosphate) cyclohydrolase domain EC:3.5.4.10 (IMPCHase), cyclizing FAICAR (5-formylaminoimidazole-4-carboxamide ribonucleotide) to IMP [1].

1: Molecular cloning and expression of a rat cDNA encoding 5- aminoimidazole-4-carboxamide ribonucleotide formyltransferase/IMP cyclohydrolase [published erratum appears in Gene 1998 Feb 27;208(2):337] Akira T, Komatsu M, Nango R, Tomooka A, Konaka K, Yamauchi M, Kitamura Y, Nomura S, Tsukamoto I; Gene 1997;197:289-293. PMID:9332377

2: The human purH gene product, 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase/IMP cyclohydrolase. Cloning, sequencing, expression, purification, kinetic analysis, and domain mapping. Rayl EA, Moroson BA, Beardsley GP; J Biol Chem 1996;271:2225-2233. PMID:8567683

3: Evolution of the deaminase fold and multiple origins of eukaryotic editing and mutagenic nucleic acid deaminases from bacterial toxin systems. Iyer LM, Zhang D, Rogozin IB, Aravind L; Nucleic Acids Res. 2011; [Epub ahead of print] PMID:21890906