An approach for the assessment of the order of disruption of the elements of protein structure upon protein unfolding: A study of carbonic anhydrase B

An experimental approach named μ-analysis has been developed in order to elucidate the sequence of the loss of ordered structure by elements of a protein during the denaturation of the molecule. This approach is applicable for the analysis of proteins that fold (unfold) in a multistep process that involve the formation (destruction) of a range of intermediate states. The concept of the approach consists in systematic analysis of mutagenized forms of the protein with point substitutions of hydrophobic amino-acid residues and additional cysteine bridges. Importantly, the substitutions of the amino-acid residues must be localized to the same structural elements of the protein. Point substitutions of hydrophobic amino-acid residues mainly provide information on the structural elements of the protein that are disrupted at the final stages of protein denaturation. The addition of cysteine bridges to the surface of the protein molecule allows investigation of structural elements of the protein that are the first to unfold upon protein denaturation. Calorimetric studies of non-equilibrium melting of bovine carbonic anhydrase B yielded information on the rate constants of the unfolding of ten mutant forms of the protein. The analysis of the effects of mutations on the rates of different stages of protein unfolding allowed for elucidation of the order of disruption of structural elements of carbonic anhydrase B upon thermal denaturation.