Functional implications of disulfide bond, Cys206-Cys210, in recombinant prochymosin (chymosin)

Prochymosin (chymosin) contains three disulfide bonds: Cys45-Cys50, Cys206-Cys210, and Cys250-Cys283. We have demonstrated that Cys250-Cys283 is indispensable for correct refolding of prochymosin, whereas Cys45-Cys50 is dispensable but has some contribution to the stability and substrate specificity of the enzyme. Here, we report the results about the functions of Cys206-Cys210 by site-directed mutagenesis studies. In a glutathione redox system C206A/C210A mutant exhibited oxidative refolding kinetics and efficiency ( approximately 40% reactivation) similar to those of the wild-type prochymosin, indicating that Cys206-Cys210 is also dispensable for refolding. However, C206S/C210S and single-site mutants (C210A, C210S, and C206A) showed only about 3 and 0-0.4% reactivation, respectively. This is quite different from the Cys45-Cys50 deficient mutants (C45A, C50A, C45A/C50A, C45D, C50S, C45D/C50S, C45A/C50S), which have comparable refolding efficiencies, implying that the substituents at position 206 and 210 play more important role in determining correct refolding than those at position 45 and 50. Urea-induced denaturation and fluorescence quenching studies indicated that the prochymosin mutants C206A/C210A and C206S/C210S were 2.1 and 4.8 kJ/mol less stable than prochymosin and some tryptophan residue in the mutated molecules was less exposed. However, the wild-type and mutant prochymosins shared similar far-UV CD and fluorescence emission spectra and similar specific potential activity, suggesting that the overall conformation was maintained after mutation. Activity assay and kinetic analysis revealed that mutation did not change the specific milk-clotting activity significantly but resulted in an increase in K(m) and k(cat) toward a hexapeptide substrate. On the basis of the above-mentioned perturbance of tryptophanyl microenvironment and the three-dimensional structure of chymosin, we proposed that deletion of Cys206-Cys210 may induce a propagated conformational change, resulting in a perturbance of the local conformation around active-site cleft and in turn, an alteration of the substrate specificity.