Resistance of bromelain to SDS binding

Interaction of the plant cysteine protease bromelain with SDS has been studied using CD spectroscopy, intrinsic fluorescence emission, extrinsic fluorescence probe pyrene, isothermal calorimetric (ITC) investigations and inhibition of hydrolyzing activity. Results exhibit number of synchronous transitions when plotted against the total SDS concentration. SDS at submicellar level caused conformation change of bromelain leading to a stable entity. ITC and pyrene experiments suggest that the structural modifications below 5 mM, the cmcapp of SDS solutions containing bromelain, are the result of alterations of solvent hydrophobicity or non-specific weak binding and/or adsorption of SDS monomers. Melting temperature (Tm) and the free energy change for thermal unfolding (ΔGunf) of the SDS induced conformers was decreased by 5 °C and 0.5 kcal/mol respectively, compared to native bromelain. Below 5 mM, SDS caused large decrease in Vmax without affecting Km for the substrate Z-Arg-Arg-NHMec. Analysis of kinetic data imply that SDS acts as a partial non-competitive inhibitor since even at 100 mM, the residual activity of bromelain was retained by 3%. Inhibition studies show an IC50 of 0.55 mM and a high Ki of 0.145 mM. These demonstrate that bromelain is resistant to SDS binding and denaturation, a property known for β-sheet rich kinetically stable proteins.