SDS denaturation of complement factor C3 as a model for allosteric modifications occurring during C3b binding: demonstration of a profound conformational change by means of circular dichroism and quantitative immunoprecipitation

The antigenic expression of bound but not fluid-phase C3b closely resembles that of sodium dodecyl sulphate (SDS) denatured C3. For this reason, denatured C3 has been used in this study as a model to characterize the conformational changes associated with bound C3b. It was shown in circular dichroism in the far UV spectrum that profound changes in the secondary structure occurred in denatured C3. Furthermore, quantitation by immunoprecipitation of the previously observed antigenic changes during denaturation demonstrated that C3 lost 2/3 of the antigens associated with native C3 whereas 1/3 were stable. The lost antigens were completely replaced by antigens that are specific for denatured and bound C3. We postulate that the binding of C3b is accompanied by a profound conformational change distinctive of that observed in fluid-phase C3b.