Endopeptidase
 
Peptidases are enzymes that catalyze the hydrolysis of the peptide bonds that link amino acids into linear polypeptide chains. The Enzyme Commission of the International Union of Biochemistry and Molecular Biology classifies them as E.C. 3.4, peptide hydrolases. The chemical reaction that occurs can be depicted in its most simplified form as R – CO – NH – R′ + H₂O ↔ R – CO – OH + H₂N-R′ . In a linear polymer of x amino acids (x can range from 2 to >1000 ), there are x – 1 peptide bonds. Enzymes that catalyze the hydrolysis of the first or second (or sometimes even the third) peptide bond in such a linear polymer are called exopeptidases and, more specifically, aminopeptidases (E.C. 3.4.11-15) . This is because the amino group of the first amino acid in a linear polymer of amino acids is not part of a peptide bond, and hence this end of the chain of x amino acids is called the amino terminus (or N-terminus). Enzymes that catalyze the hydrolysis of the last or the penultimate peptide bond in this chain are also exopeptidases, but in this case they are carboxypeptidases (E.C. 3.4.16-19). The carboxyl group of the last amino acid in the chain is similarly not part of a peptide bond, and this end of the chain is therefore the carboxy terminus (or C-terminus. (
Peptidases that catalyze the hydrolysis of internal peptide bonds in the polypeptide chain are called endopeptidases (E.C. 3.4.21–24, 99). The subclass to which they are assigned depends on the particular type of mechanism they employ to achieve catalysis.