The enzyme is involved in histidine biosynthesis, as well as purine nucleotide biosynthesis. The enzymes from archaea and bacteria are heterodimeric. A glutaminase component (cf. EC?3.5.1.2, glutaminase) produces an ammonia molecule that is transferred by a 25 ? tunnel to a cyclase component, which adds it to the imidazole ring, leading to lysis of the molecule and cyclization of one of the products. The glutminase subunit is only active within the dimeric complex. In fungi and plants the two subunits are combined into a single polypeptide.
