Kinetics of β-casein hydrolysis by wild-type and engineered trypsin

Apparent rate constants of tryptic hydrolysis of amide bonds containing Arg and Lys residues in beta-casein were determined by the analysis of kinetics of accumulation of 17 major peptide components revealed by high performance liquid chromatography. When studying pH influence on Arg/Lys bond cleavage preference, averaged rate constants over several ArgbX and LysbX bonds were used for analysis of kinetics of wild-type trypsin, K188H, K188F, K188Y, K188W, and of K188D/D189K mutants. The pK(a1) value of 6.5 was found for all studied trypsins. For wild-type trypsin and its K188D/D189K mutant, pK(a2) was found to be 10. The lowest among studied engineered trypsins pK(a2) = 9.3 was determined for K188Y mutant. Considerable preference for the cleavage of Arg over Lys containing peptide bonds was demonstrated for all trypsins with engineered S2 site except for K188H and K188F. The comparison of individual rate constants for various bonds showed that during the hydrolysis by wild-type trypsin, the probabilities of splitting depend on secondary specificity and local hydrophobicity of amino acid residues, which are nearest to the hydrolyzed peptide bond (P2 site). The improvement of prediction of hydrolysis rates performed by the used program was achieved after considering the presence of hydrophobic neighborhood of Lys48--Ile49 and Arg202--Gly203 bonds.