Quantitative effects of allosteric ligands and mutations on conformational equilibria in Salmonella typhimurium tryptophan synthase.

Allosteric communications are important in coordination of the reactions in the tryptophan (Trp) synthase alpha2beta2 multienzyme complex. We have measured the conformational equilibria of l-Ser and l-Trp complexes, using absorption and fluorescence spectrophotometry with hydrostatic pressure equilibrium perturbation. The effects of monovalent cations, disodium alpha-glycerophosphate (Na2GP), indoleacetylglycine (IAG), and benzimidazole (BZI), as well as of betaE109D and betaD305A mutations, on K(eq) for the conformational equilibria were determined. The l-Ser external aldimine-aminoacrylate equilibrium (K(eq)=[external aldimine]/[aminoacrylate]) has the largest value with Na+ (0.12), followed by K+ (0.04), Li+ (7.6 x 10(-4)), Rb+ (4.3 x 10(-4)), NH4+ (2.3 x 10(-4)), no cation (2.0 x 10(-4)) and Cs+ (1.6x10(-5)). alpha-Site ligands, Na(2)GP and IAG, have modest 3- to 40-fold effects on K(eq) in the direction of aminoacrylate, but BZI in the presence of Na+ gives a low value of K(eq) comparable to that obtained with Cs(+). There is no additivity of free energy for Na2GP and BZI, suggesting a common pathway for allosteric communications for both ligands. The values of DeltaV(o) range from -126 mL/mol for the Na+ complex to -204 mL/mol for the Na+ complex with BZI. The betaD305A mutation changes the K(eq) by a factor of at least 10(5) (26.7kJ/mol) and nearly abolishes allosteric communications. There are also dramatic decreases in the magnitude of both DeltaV(o) and DeltaS for the l-Ser external aldimine-aminoacrylate equilibrium for betaD305A Trp synthase, consistent with a large decrease in solvation accompanying the conformational change in betaD305A Trp synthase relative to wild-type Trp synthase. The betaE109D mutation has more modest but significant effects on K(eq), which differ with the ligand, ranging from 40-fold for GP to 2200-fold for BZI, even though betaGlu-109 is not directly involved in allosteric communications. The effect of GP on the external aldimine-quinonoid intermediate equilibrium of the Trp synthase-l-Trp complex is similar to that of GP on the Trp synthase-l-Ser external aldimine-aminoacrylate equilibrium. These results have allowed a quantitative comparison of the allosteric effects of ligand and mutations in Trp synthase. These allosteric effects are finely tuned to control the synthesis of l-Trp without resulting in substrate or product inhibition.