Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Subtilisin (serine endopeptidase) is a protease (a protein-digesting enzyme) initially obtained from Bacillus subtilis. Subtilisins belong to subtilases, a group of serine proteases which initiate the nucleophilic attack on the peptide (amide) bond through a serine residue at the active site. They are physically and chemically well characterized enzymes. Subtilisins typically have molecular weights of about 20,000 to 45,000 dalton. They can be obtained from soil bacteria, for example Bacillus amyloliquefaciens. Subtilisins are secreted in large amounts from many Bacillus species.