Imidazole glycerol phosphate (IGP) synthase, a triad glutamine amidotransferase, catalyzes the fifth step in the histidine biosynthetic pathway, where ammonia from glutamine is incorporated into N1- [(5′-phosphoribulosyl)-formimino]-5-aminoimidazole-4-carboxamide ribonucleotide (PRFAR) to yield IGP and 5′-(5-aminoimidazole-4- carboxamide) ribonucleotide (AICAR). The triad family of glutamine amidotransferases is formed by the coupling of two disparate subdomains, an acceptor domain and a glutamine hydrolysis domain. Each of the enzymes in this family share a common glutaminase domain for which the glutaminase activity is tightly regulated by an acceptor substrate domain. In IGP synthase the glutaminase and PRFAR binding sites are separated by 30 Å. Using kinetic analyses of site-specific mutants and molecular dynamic simulations, we have determined that an interdomain salt bridge in IGP synthase between D359 and K196 (approximately 16 Å from the PRFAR binding site) plays a key role in mediating communication between the two active sites. This interdomain contact modulates the glutaminase loop containing the histidine and glutamic acid of the catalytic triad to control glutamine hydrolysis. We propose this to be a general principle of catalytic coupling that may be applied to the entire triad glutamine amidotransferase family.
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