Eliminating Positively Charged Lysine ε-NH3+ Groups on the Surface of Carbonic Anhydrase Has No Significant Influence on Its Folding from Sodium Dodecyl Sulfate

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Abstract
This study compares the folding of two polypeptidesbovine carbonic anhydrase (BCA) and peracetylated BCA (BCA-Ac18)having the same sequence of amino acids but differing by 18 formal units of charge, from a solution containing denaturing concentrations of sodium dodecyl sulfate (SDS). Acetylation of BCA with acetic anhydride converts all 18 lysine-ε-NH3+ groups to lysine-ε-NHCOCH3 groups and generates BCA-Ac18. Both BCA and BCA-Ac18 are catalytically active, and circular dichroism spectroscopy (CD) suggests that they have similar secondary and tertiary structures. SDS at concentrations above ∼10 mM denatured both proteins. When the SDS was removed by dialysis, both proteins were regenerated in native form. This study suggests that large differences in the net charge of the polypeptide have no significant influence on the structure, the ability to refold, or the rate of refolding of this protein from solutions containing SDS. This study reinforces the idea that charged residues on the surface of BCA do not guide protein folding and raises the broader question of why proteins have charged residues on their surface, outside of the region of the active site.