Enzyme Thermostability and its Enhancement by Genetic Engineering

Abstract

Enzymes are observed to lose their catalytic activity as temperature is increased. Such thermal inactivation can be either reversible or irreversible, depending on whether return to ambient temperature restores enzymatic activity (1). Reversible conformational transitions that result in the denaturation (partial unfolding) of the enzyme can account for the loss of enzyme activity that is regained upon cooling; this phenomenon has been the subject of extensive investigation and is now well characterized (2–7). We have recently elucidated the major processes accounting for the irreversible thermoinactivation of enzymes. In studies of hen egg-white lysozyme (8), bovine pancreatic ribonuclease (9), and yeast triosephosphate isomerase (10), we have shown that the processes of irreversible enzyme inactivation at 90 – 100 C are deamidation of asparagine residues, hydrolysis of peptide bonds at aspartic acid residues, destruction of S-S bonds, and formation of incorrect (scrambled) structures. The relative contribution of each process depends on the nature of the protein and the pH.