cd04723

Phosphoribosylformimino-5-aminoimidazole carboxamide ribonucleotide (ProFAR) isomerase (HisA) and the cyclase subunit of imidazoleglycerol phosphate synthase (HisF). The ProFAR isomerase catalyzes the fourth step in histidine biosynthesis, an isomerisation of the aminoaldose moiety of ProFAR to the aminoketose of PRFAR (N-(5'-phospho-D-1'-ribulosylformimino)-5-amino-1-(5''-phospho-ribosyl)-4-imidazolecarboxamide). In bacteria and archaea, ProFAR isomerase is encoded by the HisA gene. The Imidazole glycerol phosphate synthase (IGPS) catalyzes the fifth step of histidine biosynthesis, the formation of the imidazole ring. IGPS converts N1-(5'-phosphoribulosyl)-formimino-5-aminoimidazole-4-carboxamide ribonucleotide (PRFAR) to imidazole glycerol phosphate (ImGP) and 5'-(5-aminoimidazole-4-carboxamide) ribonucleotide (AICAR). This conversion involves two tightly coupled reactions in distinct active sites of IGPS. The two catalytic domains can be fused, like in fungi and plants, or peformed by a heterodimer (HisH-glutaminase and HisF-cyclase), like in bacteria.