LACTOCOCCAL PEPTIDASES

Lactococci are nutritionally fastidious and are auxotrophic for severa l amino adds. In order to find the nitrogen source necessary to grow t o high cell density, these bacteria need to hydrolyze milk proteins. A cell envelope-associated proteinase (which is now well characterized) releases peptides composed of > .5 residues. These peptides are furth er hydrolyzed into free amino acids and small peptides which can be tr ansported through the cytoplasmic membrane by active transport systems . In addition to their role in nitrogen supply, peptidases are respons ible for the degradation of bitter peptides in cheese and for the rele ase of free amino adds and small peptides which are precursors of arom atic compounds. The lactococcal peptidasic system is composed of at le ast 3 aminopeptidases, one dipeptidase, one tripeptidase, one X-prolyl -dipeptidylaminopeptidase, one prolidase and 3 endopepitidases. Up to now, the genes coding for 3 different peptidases have been cloned and sequenced. pepN and pepC, each of them encoding an aminopeptidase with broad specificity, and pepX encoding the X-prolyl-dipeptidyl-aminopep tidase. The properties, the cellular location and the role of these en zymes in bacterial nutrition are discussed in this review.