Proteolysis of αs1-casein by chymosin: influence of pH and urea

Abstract

Summary: The specificity of chymosin on α_s1-casein was shown to be dependent on the reaction pH and on the state of aggregation of the substrate. In aqueous solution α_s1-casein was optimally hydrolysed to α_s1-I at pH 5·8; if the casein was solubilized in the isoelectric region by the use of 5 M-urea, optimum proteolysis occurred at pH 2·8. Hydrolysis of α_s1-I to yield α_s1-II, α_s1-III and α_s1-IV occurred at pH values > 5·8 in the presence or absence of urea. In the isoelectric region α_s1-II, α_s1-III and α_s1-IV were not formed in the absence of urea where the substrate was aggregated: instead a peptide α_s1-V was produced; at the same pH and using urea as a solubilizing agent α_s1-II, α_s1-III and α_s1-IV were formed together with a further peptide α_s1-VI.