How the features of three-dimensional structure of aspartate proteinases determine their properties

There is given a brief account of the specific interactions of some amino acid residues in aspartic proteases of both higher organisms and retroviruses that determine their important properties: an anomalously low isoelectric point of pepsin and its stability at pH close to unity; the ability of one of the carboxyl groups in the active site of proteases of higher organisms to retain charged state at any pH value and protonated state of another carboxyl, which is necessary for their enzymatic activity. It is also explained how such states can be induced in retroviral proteases.