Computer Simulations Explain the Anomalous Temperature Optimum in a Cold-Adapted Enzyme

Cold-adapted enzymes from psychrophilic species show the general characteristics of being more heat labile, and having a different balance between enthalpic and entropic contributions to free energy barrier of the catalyzed reaction compared to mesophilic orthologs. Among cold-adapted enzymes, there are also examples that show an enigmatic inactivation at higher temperatures before unfolding of the protein occurs. Here, we analyze these phenomena by extensive computer simulations of the catalytic reactions of psychrophilic and mesophilic α-amylases. The calculations yield temperature dependent reaction rates in good agreement with experiment, and also elicit the anomalous rate optimum for the cold-adapted enzyme, which occurs about 15 °C below the melting point. This result allows us to examine the structural basis of thermal inactivation, which turns out to be caused by breaking of a specific enzyme-substrate interaction. This type of behaviour is also likely to be relevant for other enzymes displaying such anomalous temperature optima. Enzymes from organisms inhabiting cold environments (psychrophiles) have adapted to catalyzing chemical reactions at near freezing temperatures. Here – using molecular dynamics simulations – the authors analyze cold adaptation of psychrophilic α-amylase and provide the structural basis for its low anomalous temperature optimum: the increased mobility of a surface loop involved in substrate interaction.