Characterization of folding intermediates during urea-induced denaturation of human carbonic anhydrase II

Knowledge of folding/unfolding pathway is fundamental basis to study protein structure and stability. Human carbonic anhydrase II (HCAII) is a ∼29 kDa, β-sheet dominated monomeric protein of 259 amino acid residues. In the present study, the urea-induced denaturation of HCAII was carried out which was a tri-phasic process, i.e., N (native) ↔ X_I ↔ X_II ↔ D (denatured) with stable intermediates X_I and X_II populated around 2 and 4 M urea, respectively. The far-UV CD was used to characterize the intermediate states (X_I and X_II) for secondary structural content, near-UV CD for tertiary structure, dynamic light scattering for hydrodynamic radius and ANS fluorescence spectroscopy for the presence of exposed hydrophobic patches. Based on these experiments, we concluded that urea-induced X_I state has characteristics of molten globule state while X_II state bears characteristics features of pre-molten globule state. Characterization of the intermediates on the folding pathway will contribute to a deeper understanding of the structure-function relationship of HCAII. Furthermore, this system may provide an excellent model to study urea stress and the strategies adopted by the organisms to combat such a stress.