THE LACTOCOCCAL CELL-ENVELOPE PROTEINASES - DIFFERENCES, CALCIUM-BINDING EFFECTS AND ROLE IN CHEESE RIPENING

The lactococcal cell envelope proteinase (CEP) has been extensively ch aracterized during the last decade, both biochemically and genetically . The enzyme belongs to a family of subtilisin-like serine proteinases , the subtilases, and is characterized by a large extension C-terminal of the proteinase domain. Biochemical characterization has involved t he truncated so-called release-CEP obtained be the autoproteolytic rel ease from cells which occurs in the absence of calcium. A comparative study of the specificities of the release-CEP of different strains of Lactococcus lactis revealed a broad diversity. Differences could in pa rt be related to charge characteristics of the subtilisin-like substra te binding region and of other residues outside this region and outsid e the proteinase domain. The structural organization of the bound CEP at the periphery of the cell wall seems to be the main determinant for specific catalytic properties different from those of the release-CEP . Calcium plays a role in inducing and (or) stabilizing active conform ation of the bound CEP. Thermal stabilization is most obvious in type III CEP; it involves a change of conformation induced be binding of on e calcium ion to a relatively high affinity binding site and accompani ed by an increase of specific activity. A simultaneous anchoring of th e actual subtilisin-like proteinase domain to the cell wall surface is assumed to prevent autoproteolytic release. Binding of additional cal cium ions increases its stability and specific activity. The action of CEP during the ripening of cheese is essential to secondary proteolys is and flavour development.