The properties and functions of bacterial aminopeptidases.

Aminopeptidases are enzymes that release N-terminal amino residues from oligopeptides, polypeptides and proteins. The classification of aminopeptidases has often been based on mechanism of catalysis, structure of active site, substrate specificity kinetic and molecular properties. In terms of catalytic mechanism bacterial aminopeptidases can be divided into three main catalytic groups: metallo-, cysteine- and serine aminopeptidases. According to their substrate specificity the enzymes can be ordered into two sub-groups: having broad or narrow specificity. Almost half of the characterized aminopeptidases show a subunit structure. Enzymes having a quaternary structure are most often built of a combination of 2, 4, 6 subunits. Bacterial aminopeptidases may be localised in the cytoplasm, on membranes, associated with the envelope or secreted into the extracellular media. Regulation of the synthesis of aminopeptidases is assumed to take place mainly at the level of transcription. Most genes encoding the enzymes are monocistronic and contain a promotor characteristic for the genes transcribed by RNA polymerase associated with the factor sigma70. Aminopeptidases play an important role in the initial and final steps of protein turnover and they are involved in several specific regulatory functions.