Crystal Structure of Methanobacterium thermoautotrophicum Phosphoribosyl-AMP Cyclohydrolase HisI

The metabolic pathway for histidine biosynthesis is interesting from an evolutionary perspective because of the diversity of gene organizations and protein structures involved. Hydrolysis of phosphoribosyl-AMP, the third step in the histidine biosynthetic pathway, is carried out by PR-AMP cyclohydrolase, the product of the hisI gene. The three-dimensional structure of PR-AMP cyclohydrolase from Methanobacterium thermoautotrophicum was solved and refined to 1.7 Å resolution. The enzyme is a homodimer. The position of the Zn2+-binding site that is essential for catalysis was inferred from the positions of bound Cd2+ ions, which were part of the crystallization medium. These metal binding sites include three cysteine ligands, two from one monomer and the third from the second monomer. The enzyme remains active when Cd2+ is substituted for Zn2+. The likely binding site for Mg2+, also necessary for activity in a homologous cyclohydrolase, was also inferred from Cd2+ positions and is comprised of aspartic acid side chains. The putative substrate-binding cleft is formed at the interface between the two monomers of the dimer. This fact, combined with the localization of the Zn2+-binding site, indicates that the enzyme is an obligate dimer.