Influence of pH on the structural changes of beta-lactoglobulin studied by tryptic hydrolysis

The attempt to use trypsin in order to monitor pH (7.5-9.0) induced beta-lactoglobulin conformation changes has revealed differences in the cleavage of specific sites. The tryptic cleavage of two dibasic X-Lys-Lys-Y sites (Lys 69, 70 and 100, 101) shows slighter predominance of symmetrical cut at pH 7.5 and 8.0. Mostly asymmetrical cleavage yielding two C-terminal lysines can be observed at pH 8.5 and 9.0. Atypical cleavage of the Tyr-20-Ser-21 site, which at pH 9.0 is relatively negligible, increases substantially in pH 7.5-8.5. This implies that Tyr-20 probably is the tyrosine reported to be exposed on the surface of the protein during transformation of beta-lactoglobulin molecule occurring in the studied pH range (Tanford et al. (1959) J. Am. Chem. Soc. 81, 4032-4036).