Mutation of βGln114 to Ala Alters the Stabilities of Allosteric States in Tryptophan Synthase Catalysis

The tryptophan synthase (TS) bienzyme complexes found in bacteria, yeasts, and molds are pyridoxal 5′-phosphate (PLP)-requiring enzymes that synthesize l-Trp. In the TS catalytic cycle, switching between the open and closed states of the α- and β-subunits via allosteric interactions is key to the efficient conversion of 3-indole-d-glycerol-3′-phosphate and l-Ser to l-Trp. In this process, the roles played by β-site residues proximal to the PLP cofactor have not yet been fully established. βGln114 is one such residue. To explore the roles played by βQ114, we conducted a detailed investigation of the βQ114A mutation on the structure and function of tryptophan synthase. Initial steady-state kinetic and static ultraviolet–visible spectroscopic analyses showed the Q to A mutation impairs catalytic activity and alters the stabilities of intermediates in the β-reaction. Therefore, we conducted X-ray structural and solid-state nuclear magnetic resonance spectroscopic studies to compare the wild-type and βQ114A mutant enzymes. These comparisons establish that the protein structural changes are limited to the Gln to Ala replacement, the loss of hydrogen bonds among the side chains of βGln114, βAsn145, and βArg148, and the inclusion of waters in the cavity created by substitution of the smaller Ala side chain. Because the conformations of the open and closed allosteric states are not changed by the mutation, we hypothesize that the altered properties arise from the lost hydrogen bonds that alter the relative stabilities of the open (βT state) and closed (βR state) conformations of the β-subunit and consequently alter the distribution of intermediates along the β-subunit catalytic path.