REMARKABLE ACTIVITY ENHANCEMENT OF THERMOLYSIN MUTANTS

Most attempts to modify the properties of enzymes by amino acid substi tution around the active sites have resulted in suppression of the bio logical activity, suggesting that the structure of natural enzymes sho uld be almost optimized evolutionally to show the highest activity. In contrast, we found an interesting site of a well-known metalloendopep tidase, thermolysin (EC. 3.4.24.4), where almost all the amino acid re placement causes a remarkable increase in the hydrolytic activity. Neg ative correlation between the activity and the thermal stability was o bserved. The flexibility around the substrate binding site is suggeste d to be a key to the correlation. Nature may have selected the amino a cid at this site, which suppresses the flexibility of the molecule, to get the highest thermal stability at the expense of the activity.