FOLDING MEDIATED BY AN INTRAMOLECULAR CHAPERONE - AUTOPROCESSING PATHWAY OF THE PRECURSOR RESOLVED VIA A SUBSTRATE ASSISTED CATALYSIS MECHANISM

Subtilisin is synthesized with an N-terminal propeptide which has been demonstrated to function as an intramolecular chaperone that is only essential for the folding of the active enzyme. After folding, the propeptide is removed via an intramolecular autoprocessing mechanism. This mechanism is blocked when His64, a member of the catalytic triad is substituted with Ala. However, an additional mutation in the propeptide substituting Glu-2 with His was able to suppress the His64Ala mutation, allowing autoprocessing of the propeptide. This suppression is considered to be due to a ''substrate assisted catalysis'' mechanism and demonstrates that the cleavage to the subtilisin propeptide is an autocatalytic process.