Peptide substrates for chymosin (rennin). Kinetic studies with peptides of different chain length including parts of the sequence 101-112 of bovine k-casein.

Kinetic parameters have been determined for the reaction between milk-clotting chymosin (EC 3.4.23.4) and a series of peptides (or their methyl esters) including the amino acid sequence around the enzyme-sensitive Phe(105)-Met (106) bond the bovine k-casein. In particular, the influence of the substrate's chain length on the kinetic parameters has been studied. Evidence is presented that in the model peptides studied the sequence -Ser-Phe-Met-Ala with a further residue added to either end (in casu Leu(103) or Ile(108)) is necessary to induce any cleavage by the enzyme. When both the Leu(103) and Ile(108) residues form part of the peptide chain, a marked improvement of the substrate properties is observed. It is suggested that prolyl residues on either side of the sensitive peptide bond form additional sites for secondary enzyme-substrate interactions.