Cooperative conformational transitions and the temperature dependence of enzyme catalysis

Abstract

Many enzymes display non-Arrhenius behaviour with curved Arrhenius plots in the absence of denaturation. There has been significant debate about the origin of this behaviour and recently the role of the activation heat capacity (Δ𝐶# ‡) has been widely discussed. If enzymecatalysed reactions occur with appreciable negative values of Δ𝐶# ‡ (arising from narrowing of the conformational space along the reaction coordinate), then curved Arrhenius plots are a consequence. To investigate these phenomena in detail, we have collected high precision temperature-rate data over a wide temperature interval for a model glycosidase enzyme MalL, and a series of mutants that change the temperature-dependence of the enzyme-catalysed rate. We use these data to test a range of models including macromolecular rate theory (MMRT) and an equilibrium model. In addition, we have performed extensive molecular dynamics (MD) simulations to characterise the conformational landscape traversed by MalL in the enzymesubstrate complex and an enzyme-transition state complex. We have crystallised the enzyme in a transition state-like conformation in the absence of a ligand and determined an X-ray crystal structure at very high resolution (1.10 Å). We show (using simulation) that this enzymetransition