Experimental approach to study the effect of mutations on the protein folding pathway.

Is it possible to compare the physicochemical properties of a wild-type protein and its mutant form under the same conditions? Provided the mutation has destabilized the protein, it may be more correct to compare the mutant protein under native conditions to the wild-type protein destabilized with a small amount of the denaturant. In general, is it appropriate to compare the properties of proteins destabilized by different treatments: mutations, pH, temperature, and denaturants like urea? These issues have compelled us to search for methods and ways of presentation of experimental results that would allow a comparison of mutant forms of proteins under different conditions and lead to conclusions on the effect of mutations on the protein folding/unfolding pathway. We have studied equilibrium unfolding of wild-type bovine carbonic anhydrase II (BCA II) and its six mutant forms using different urea concentrations. BCA II has been already studied in detail and is a good model object for validating new techniques. In this case, time-resolved fluorescence spectroscopy was chosen as the basic research method. The main features of this experimental method allowed us to compare different stages of unfolding of studied proteins and prove experimentally that a single substitution of the amino acid in three mutant forms of BCA II affected the native state of the protein but did not change its unfolding pathway. On the contrary, the inserted disulfide bridge in three other mutant forms of BCA II affected the protein unfolding pathway. An important result of this research is that we have validated the new approach allowing investigation of the effect of mutations on the folding of globular proteins, because in this way it is possible to compare proteins in the same structural states rather than under identical conditions.