The selectivity of chymosin action on αs1- and β-caseins in solution is modulated in cheese

The primary proteolysis of caseins by chymosin in Gouda cheese was assessed by identifying the peptides present in the watersoluble fraction of a starter-free cheese. The results show that the selectivity of chymosin, as observed in solution, is affected by the cheese environment. Of all known preferred cleavage sites in αs1- and β-caseins in solution, the enzyme failed to cleave susceptible bonds in the C-terminal part of αs1-casein in cheese. While αs1-casein is a poorly structured molecule in buffered solution, a specific structural arrangement is believed to occur in cheese and to exclusively affect 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 limited extent of aggregation of β-casein; it allows a relatively rapid degradation of the monomeric form during the first weeks of ripening. A broader specificity of chymosin on the accessible part of αs1-casein as well as on β-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 between forces regulating binding.