It is also important to remember that the PAG molecules belong to a group of
proteolytic enzymes known as aspartic proteinases (AP) (8, 32), having more than
50% amino acid sequence identity with pepsin, cathepsin D and cathepsin E.
Comparative molecular modeling studies have demonstrated that PAG have
retained the well-known bilobed structure of AP, containing a peptide binding
cleft between the two lobes capable of accommodating peptides up to 7 amino
acids (33). In spite of the fact that most PAG are assumed to be enzymatically
inactive due to key mutations within their binding cleft, some reports (33, 34)
have claimed that they can bind to pepstatin A (a 6 amino acid peptide), which is
an extremely powerful inhibitor of AP. It remains to be determined whether the
PAG expressed in the ungulate placenta are catalytically active and, even if they
are, whether their physiological main function is that of proteinases.