Site-specific mutations of calf chymosin B which influence milk-clotting activity

Zymogen forms of wild type and three mutant calf chymosin B enzymes were heterologously expressed in Escherichia coli under control of a T7 promoter as inclusion bodies. The chaperone-like protein, α-crystallin, was used as a possible aid to unfolding. Prochymosin formed a complex with the chaperone-like protein α-crystallin, before and after folding; after activation, free chymosin was recovered without bound α-crystallin. Following solubilisation, refolding and activation, steady-state kinetic comparisons were determined using the synthetic substrate Leu-Ser-Phe(NO2)-Nle-Ala-Leu-OMe. The mutation by deletion of 34 residues from the C-terminus (PC289) caused the loss of stability of the mature enzyme after activation. Insertion of histidine-glycine residues at the C-terminus produced a mutant (PC + 2) with a lower kcat (2.52s−1 compared to 18.9s−1 for the recombinant wild type) and kcatKm (3.8mM−1s−1 compared to 49.7mM−1s−1 for the recombinant wild type), suggesting functional involvement of this region. Exchange of threonine 77 on the flap of the enzyme for an aspartyl residue (T77D — pepsin numbering) caused little change in kcat or kcatKm values. Both PC + 2 and T77D mutants showed reduced milk clotting activity (151.5 U mg−1 and 303 U mg−1, respectively) compared to the recombinant wild type enzyme (909 U mg−1), and reductions in the C/P (milk-clotting activity over proteolytic activity) ratios (3.0 and 3.03, respectively) compared to the recombinant wild type (6.09).