Identification of N-terminal autodigestion target site in subtilisin ALP I

The molecular surface of proteolytic enzymes has an important role in stability of the enzymatic activity in extraordinary environments

It is scientifically and industrially important to clarify the stabilizing mechanism of proteases in extraordinary environments. We used subtilisins ALP I and Sendai as models to study the mechanism. Subtilisin ALP I is extremely sensitive to highly alkaline conditions, even though the enzyme is produced by alkalophilic Bacillus, whereas subtilisin Sendai from alkalophilic Bacillus is stable under conditions of high alkalinity. We constructed mutant subtilisin ALP I enzymes by mutating the amino acid residues specific for subtilisin ALP I to the residues at the corresponding positions of amino acid sequence alignment of alkaline subtilisin Sendai. We observed that the two mutations in the C-terminal region were most effective for improving stability against surfactants and heat as well as high alkalinity. We predicted that the mutated residues are located on the surface of the enzyme structures and, on thebasis of three-dimensional modelling, that they are involved in stabilizing the conformation of the C-terminal region. As proteolytic enzymes frequently become inactive due to autocatalysis, stability of these enzymes in an extraordinary environment would depend on the conformational stability of the molecular surface concealing scissile peptide bonds. It appeared that the stabilization of the molecular surface structure was effective to improve the stability of the proteolytic enzymes.