Mutation analysis of carbamoyl phosphate synthetase: Does the structurally conserved glutamine amidotransferase triad act as a functional dyad?

Evolutionarily conserved triad glutamine amidotransferase (GAT) domains catalyze the cleavage of glutamine to yield ammonia and sequester the ammonia in a tunnel until delivery to a variety of acceptor substrates in synthetase domains of variable structure. Whereas a conserved hydrolytic triad (Cys/His/Glu) is observed in the solved GAT structures, the specificity pocket for glutamine is not apparent, presumably because its formation is dependent on the conformational change that couples acceptor availability to a greatly increased rate of glutamine cleavage. In Escherichia coli carbamoyl phosphate synthetase (eCPS), one of the best characterized triad GAT members, the Cys269 and His353 triad residues are essential for glutamine hydrolysis, whereas Glu355 is not critical for eCPS activity. To further define the glutamine-binding pocket and possibly identify an alternative member of the catalytic triad that is situated for this role in the coupled conformation, we have analyzed mutations at Gln310, Asn311, Asp334, and Gln351, four conserved, but not yet analyzed residues that might potentially function as the third triad member. Alanine substitution of Gln351, Asn311, and Gln310 yielded respective Km increases of 145, 27, and 15, suggesting that Gln351 plays a key role in glutamine binding in the coupled conformation, and that Asn311 and Gln310 make less significant contributions. None of the mutant k(cat) values varied significantly from those for wild-type eCPS. Combined with previously reported data on other conserved eCPS residues, these results strongly suggest that Cys269 and His353 function as a catalytic dyad in the GAT site of eCPS.