Proteinase K is a broad-spectrum serine protease originally isolated from the fungus Engyodontiumalbum (Tritirachium album). The enzyme was named "proteinase K" for its ability to digest keratin. Structural and molecular biology studies suggest that the enzyme belongs to the subtilisin family characterized by a catalytic triad (Asp39-His9-Ser2) in the active site. Proteinase K has no pronounced cleavage specificity and the preferential cleavage site is the peptide bond adjacent to hydrophobic amino acids.