Effects of hydrostatic pressure on the conformational equilibrium of tryptophan synthase from Salmonella typhimurium.

A wide range of parameters influence allosteric communications between the α- and β-subunits of the Trp synthase α2β2 multienzyme complex with L-Ser, including monovalent cations, pH, temperature, ligands, organic solvents, and hydrostatic pressure. The conformational change from closed to open can be monitored either by absorbance at 423 nm or fluorescence at 495 nm from the pyridoxal-5′-phosphate-L-Ser complex. Pressure perturbation was used to quantify the effects of monovalent cations, ligands, and mutations on the conformational equilibrium of Trp synthase. P-jump kinetics in the presence of Na+, NH4+, and Na+ together with benzimidazole were also examined. The plots of lnk versus P are nonlinear and require a compressibility (β‡o) term to obtain a good fit. β‡o is positive for the Na+ enzyme but negative for NH4+ and Na+ with benzimidazole. These results suggest that there is a large contribution of solvation to the kinetics of the conformational change of Trp synthase. The relaxation kinetics are also different if the P-jumps are made by increasing or decreasing pressure, suggesting that the enzyme conformations are ensembles of microstates.