Structure and stability of thermophilic enzymes. Studies on thermolysin.

The molecular mechanisms responsible for the unusual stability of enzymes isolated from thermophilic microorganisms are much more complex and subtle than was originally thought. In particular, a general mechanism cannot be proposed, since individual enzymes can be stabilized by specific molecular interactions and forces. The results of studies on thermophilic enzymes obtained in recent years in our laboratory will be summarized, with particular emphasis being placed on those obtained with thermolysin, a stable metalloendopeptidase isolated from Bacillus thermoproteolyticus. Fragmentation of thermolysin by limited proteolysis by added protease (subtilisin) or autolysis mediated by heat or the ion-chelating agent EDTA leads to quite selective peptide bond fissions, allowing isolation of 'nicked' thermolysin species. Correlation of the sites of proteolytic cleavage with the known three-dimensional structure of thermolysin allowed us to infer some of the key characteristics of the structure, folding, dynamics and stability of the thermolysin molecule. The potential utility of these and other studies on thermophilic enzymes in devising strategies for enhancing the stability of mesophilic enzymes using genetic engineering techniques is discussed.