THE SELECTIVITY OF CHYMOSIN ACTION ON ALPHA(S1)-CASEINS AND BETA-CASEINS IN SOLUTION IS MODULATED IN CHEESE

The primary proteolysis of caseins by chymosin in Gouda cheese was ass essed by identifying the peptides present in the water-soluble fractio n of a starter-free cheese. The results show that the selectivity of c hymosin, as observed in solution, is affected by the cheese environmen t. Of all known preferred cleavage sites in alpha(sl)- and beta-casein s in solution, the enzyme failed to cleave susceptible bonds in the C- terminal part of alpha(sl)-casein in cheese. While alpha(sl)-casein is a poorly structured molecule in buffered solution, a specific structu ral arrangement is believed to occur in cheese and to exclusively affe ct the accessibility of the segment which includes the bonds 164-165, 156-157 and 149-150 that are highly susceptible in solution. Initially unevenly distributed salt in Gouda cheese is responsible for a limite d extent of aggregation of beta-casein; it allows a relatively rapid d egradation of the monomeric form during the first weeks of ripening. A broader specificity of chymosin on the accessible part of alpha(sl)-c asein as well as on beta-casein is observed in cheese than in solution ; this most probably results from an effect of the cheese environment on the substrate and/or the enzyme which affects the interactions betw een forces regulating binding. (C) 1997 Elsevier Science Limited.