Introduction of a free cysteinyl residue at position 68 in the subtilisin Savinase, based on homology with proteinase K.

Two subfamilies of the subtilisins, distinguished by the presence or absence of a free cysteinyl residue near the essential histidyl residue of the catalytic triad, are known. In order to evaluate the significance of the presence of this -SH group a cysteinyl residue has been introduced by site-directed mutagenesis into the cysteine-free subtilisin-like enzyme from Bacillus lentus, i.e. Savinase. The free cysteine affects the enzyme activity only slightly but renders it sensitive to mercurials presumably due to an indirect effect. The results indicate that the -SH group is not involved in catalysis.