Crystal structure of imidazole glycerol phosphate synthase: A tunnel through a (beta/alpha)(8) barrel joins two active sites

Background: Imidazole glycerol phosphate synthase catalyzes a two-step reac tion of histidine biosynthesis at the bifurcation point with the purine de novo pathway. The enzyme is a new example of intermediate channeling by glu tamine amidotransferases in which ammonia generated by hydrolysis of glutam ine is channeled to a second active site where it acts as a nucleophile. In this case, ammonia reacts in a cyclase domain to produce imidazole glycero l phosphate and an intermediate of purine biosynthesis. The enzyme is also a potential target for drug and herbicide development since the histidine p athway does not occur in mammals. Results: The 2.1 Angstrom crystal structure of imidazole glycerol phosphate synthase from yeast reveals extensive interaction of the glutaminase and c yclase catalytic do mains. At the domain interface, the glutaminase active site points into the bottom of the (beta/alpha)(8) barrel of the cyclase do main. An ammonia tunnel through the (beta/alpha)(8) barrel connects the glu taminase docking site at the bottom to the cyclase active site at the top. A conserved "gate" of four charged residues controls access to the tunnel. Conclusions: This is the first structure in which all the components of the ubiquitous (beta/alpha)(8) barrel fold, top, bottom, and interior, take pa rt in enzymatic function. Intimate contacts between the barrel domain and t he glutaminase active site appear to be poised for crosstalk between cataly tic centers in response to substrate binding at the cyclase active site. Th e structure provides a number of potential sites for inhibitor development in the active sites and in a conserved interdomain cavity.