Relationship of Catalysis and Active Site Loop Dynamics in the (beta alpha)(8)-Barrel Enzyme Indole-3-glycerol Phosphate Synthase

Abstract

It is important to understand how the catalytic activity of enzymes is related to their conformational flexibility. We have studied this activity-flexibility correlation using the example of indole-3-glycerol phosphate synthase from Sulfolobus solfataricus (ssIGPS), which catalyzes the fifth step in the biosynthesis of tryptophan. ssIGPS is a thermostable representative of enzymes with the frequently encountered and catalytically versatile (beta alpha)(8)-barrel fold. Four variants of ssIGPS with increased catalytic turnover numbers were analyzed by transient kinetics at 25 degrees C, and wild-type ssIGPS was likewise analyzed both at 25 degrees C and at 60 degrees C. Global fitting with a minimal three-step model provided the individual rate constants for substrate binding, chemical transformation, and product release. The results showed that in both cases, namely, the application of activating mutations and temperature increase, the net increase in the catalytic turnover number is afforded by acceleration of the product release rate relative to the chemical transformation steps. Measurements of the solvent viscosity effect at 25 degrees C versus 60 degrees C confirmed this change in the rate-determining step with temperature, which is in accordance with a kink in the Arrhenius diagram of ssIGPS at similar to 40 degrees C. When rotational diffusion rates of electron paramagnetic spin-labels attached to active site loop beta 1 alpha 1 are plotted in the form of an Arrhenius diagram, kinks are observed at the same temperature. These findings, together with molecular dynamics simulations, demonstrate that a different degree of loop mobility correlates with different rate-limiting steps in the catalytic mechanism of ssIGPS.