Protease K is a stable and highly active serine protease. Evidence from crystal and molecular structure studies indicates that this enzyme belongs to the subtilisin family and has an active site catalytic triad (Asp39 His69 Ser224). It is stable in a wide range of environments: pH, buffer salts, detergent (SDS), and temperature. In the presence of 0.1-0.5% SDS, protease K can maintain its activity and digest various proteins and nucleases in DNA preparations without damaging the integrity of the separated DNA.