SDS-induced conformational transitions of ervatamin B: Evidence of greater stability of α-rich domain compared to β-rich domain of the SDS derived state

The sodium dodecyl sulfate (SDS)-induced conformational and functional changes of ervatamin B were monitored by optical methods (circular dichroism (CD), fluorescence), 8-anilino-1-napthalenesulfonic acid (ANS) binding and proteolytic activity. An enhancement in the α-helicity of protein molecule is observed as a function of increasing concentration of SDS along with a blue shift in the fluorescence emission maximum and a decrease in fluorescence intensity. At a certain concentration of SDS (≈2 mM) ervatamin B loses all the proteolytic activity and rigid tertiary structure but possesses considerable amount of secondary structure along with strong ANS binding, indicating the presence of an intermediate state with different conformation from the native and unfolded states. This state may be the SDS-induced molten globule state seen in the case of ervatamin B. The SDS-induced state is more stable then the native state towards chemical and thermal denaturation and shows biphasic transition curves as compared with the native state in the absence of the detergent where the transitions are cooperative. Further, the cooperative unfolding transition curve of ervatamin B in the absence of SDS intersects at the point where the second transition of SDS-induced state starts suggesting that the molecule of ervatamin B consist of at least two structural domains which are stabilized differentially in presence of SDS and thus unfolds sequentially. As the α-helicity of ervatamin B enhances in the presence of SDS, it can be postulated that the α-rich domain of the protein was stabilized due to interaction with SDS, and therefore, it may unfold later as compared with β-rich domain during temperature and chemical induced denaturation. Thus, the study of equilibrium unfolding pathway of ervatamin B in the presence of SDS provides a better understanding about the stability of this plant cysteine protease at the domain level.