Multiple sequence alignment of homoserine-acetyltransferases, serine-acetyltransferases and homoserine-succinyltransferases show they all belong to MetX family, having evolved from a common ancestor by conserving the catalytic site and substrate binding residues. The discrimination in the substrate selection arises due to the presence of substrate-specific residues lining the substrate-binding pocket. Mutation of Ala59 and Gly62 to Gly and Pro respectively in homoserine-acetyltransferase from M. tuberculosis resulted in a serine-acetyltransferase like enzyme as it acetylated both L-homoserine and L-serine. Homoserine-acetyltransferase from M. tuberculosis when mutated at positon 322 where Leu was converted to Arg, resulted in succinylation over acetylation of L-homoserine. Our studies establish the importance of the substrate binding residues in determining the type of activity possessed by MetX family, despite all of them having the same catalytic triad Ser-Asp-His. Hence key residues at the substrate binding pocket dictate whether the given enzyme shows predominant transferase or hydrolase activity.
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