Lysozyme refolding with cyclodextrins: structure–activity relationship

Cyclodextrins (CDs), in the presence or absence of detergents, have been reported to suppress aggregate formation during the refolding of a number of proteins. A structure–activity relationship study between CD chemistry and refolding of lysozyme was performed and compared to carbonic anhydrase, in order to better understand the mechanism of CD-assisted protein refolding and to identify CDs that could function as good protein folding agents. Among the natural CDs, which have only hydroxyl groups, alpha-CD, with a smaller cavity size was more effective than the oligosaccharide with a larger cavity, gamma-CD. Replacement of the hydroxyls with other functional groups did not improve, but could seriously interfere, with the lysozyme refolding ability of alpha-CD. In case of gamma-CD, substitution of its hydroxyls with other groups either enhanced or diminished its refolding capability towards lysozyme. In general, neutral CDs were better refolding agents than the charged sugars. The presence of anionic substituents like carboxyl and phosphate groups actually promoted aggregate formation and completely abolished the sugar's refolding ability. This effect was more pronounced with lysozyme than with carbonic anhydrase. CDs with cationic functional groups did not show any significant effects on lysozyme refolding. The presence of both anionic and cationic substituents on the same CD molecule was found to partially restore its renaturation ability. Electrophoresis data indicate that CDs, which promoted lysozyme refolding, arrested aggregation at the stage of smaller soluble aggregates. Interestingly, the structure–activity relationship observed with lysozyme was quite similar to that reported for a non-disulfide protein, carbonic anhydrase. These results suggest that the effects of CDs on protein refolding are attributed to their ability to suppress aggregation of proteins. CDs may show properties similar to chaotropic agents, which may help explain their anti-aggregation and protein refolding ability. Besides alpha-CD, a number of other neutral CDs were found to be effective protein folding aids.