Effects of surface charge on denaturation of bovine carbonic anhydrase

This work compares the denaturation of two proteins-bovine carbonic anhydrase II (BCA) and its derivative with all lysine groups acetylated (BCA-Ac(18))-by urea, guanidinium chloride (GuHCl), heat, and sodium dodecyl sulfate (SDS). It demonstrates that increasing the net negative charge of the protein by acetylation of lysines reduces its stability to urea, GuHCl, and heat, but increases its kinetic stability (its thermodynamic stability cannot be measured) towards denaturation by SDS. Increasing the ionic strength of the buffer improves the stability of BCA-Ac(18) to urea and heat, but still leaves it less stable than unacetylated BCA to those denaturants. In urea, the large change in electrostatic interactions not only modifies the free energy of denaturation, but also introduces a stable intermediate into the unfolding pathway. This work shows that modifications of charges on the surfaces of proteins can have a large effect--positive or negative, depending on the denaturant--on the stability of the proteins despite the exposure of these charges to high dielectric solvent and buffer ions.