Carbonic Anhydrase B Interactions with Water and Urea

High-resolution NMR spectroscopy has been used to study native carbonic anhydrase B unfolding with urea at pH 5.75 and T = 298 K. The rigidity parameter reflecting the effectiveness of spin diffusion (SD) displays a sigma-like dependence on urea concentration, which is characteristic of denaturing processes. The ratio between the integral intensities of urea and protein signals measured in SD spectra and normal 1D spectra are the same. This suggests the absence of a predominant interaction between urea and protein molecules. The concentration of large protein-solvent complexes rapidly increases at urea concentrations of 4.2-6.2 M, which is apparently related to the transition of the protein into the molten globule state. If the urea concentration is increased to 6.6 M, these complexes dissociate, and the polypeptide chain of carbonic anhydrase B becomes completely unfolded.[PUBLICATION ABSTRACT]