Nucleophile specificity in anthranilate synthase, aminodeoxychorismate synthase, isochorismate synthase, and salicylate synthase.
Anthranilate synthase (AS), aminodeoxychorismate synthase (ADCS), isochorismate synthase (IS), and salicylate synthase (SS) are structurally homologous chorismate-utilizing enzymes that carry out the first committed step in the formation of tryptophan, folate, and the siderophores enterobactin and mycobactin, respectively. Each enzyme catalyzes a nucleophilic substitution reaction, but IS and SS are uniquely able to employ water as a nucleophile. Lys147 has been proposed to be the catalytic base that activates water for nucleophilic attack in IS and SS reactions; in AS and ADCS, glutamine occupies the analogous position. To probe the role of Lys147 as a catalytic base, the K147Q IS, K147Q SS, Q147K AS, and Q147K ADCS mutants were prepared and enzyme reactions were analyzed by high-performance liquid chromatography. Q147K AS employs water as a nucleophile to a small extent, and the cognate activities of K147Q IS and K147Q SS were reduced approximately 25- and approximately 50-fold, respectively. Therefore, Lys147 is not solely responsible for activation of water as a nucleophile. Additional factors that contribute to water activation are proposed. A change in substrate preference for K147Q SS pyruvate lyase activity indicates Lys147 partially controls SS reaction specificity. Finally, we demonstrate that AS, ADCS, IS, and SS do not possess chorismate mutase promiscuous activity, contrary to several previous reports.