Although it is possible to consider that the most important
characteristic of proteases is their form of action on substrates,
sometimes their specificities are very complex and not clearly
defined. However the mode of action can define proteases as
exopeptidases or endopeptidases, which are the two main subclasses
based on substrate interaction. The peptide substrate runs
through the entire length of the active site of an endopeptidase
framework and is cleaved in the middle of the molecule. On the
other hand, exopeptidades act near the end of polypeptide chains.
Furthermore, exopeptidases are termed aminopeptidases if they
act at the n-terminus, and carboxypeptidases are those acting on
peptide bonds from the c-terminus. Some enzymes present both
carboxy- and aminopeptidase forms, such as cathepsins, since their
structure canhave structural elements thatprovidenegative charge
(cathepsin H) to bind the positively charged amino terminus of
the substrate, or positive charge (cathepsin X) to bind the negatively
charged carboxyl terminus of the substrate [28,29]. Finally,
in describing the specificity of endopeptidases,the term oligopeptidase
isusedto refer to those that act optimally onsubstrates smaller
than proteins.