Inhibition of horseradish peroxidase activity through conformational change in surfactant solution

Surfactants have a considerable potential for inhibiting horseradish peroxidase (HRP) activity; yet, more research is needed to understand the inhibition mechanism to allow for choosing the suitable surfactant candidate as inhibitor. In this study, three traditional surfactants, sodium dodecyl sulfate (SDS), N-dodecyltrimethyl ammonium bromide (DTAB) and N-dodecyldimethyl (3-sulfopropyl) ammonium hydroxide (SB3-12), that share the same (C₁₂) hydrophobic tail but possess different charged head groups, were taken as model surfactant inhibitors, and the enzymatic activity of HRP was assessed in these surfactant solutions. The activity of HRP was inhibited by both anionic SDS and cationic DTAB at dilute concentration even below their critical micelle concentration (cmc), and the electrostatic interaction of the ionic surfactants with some amino residues of HRP was mainly responsible for the inhibition of activity. HRP in a dilute solution of SB3-12, with the latter's concentration being below its cmc, retained a high level of enzymatic activity; but, at concentrations above the cmc of SB3-12, HRP was inhibited owing to the synergetic interaction of the SB3-12 micelle with HRP. Results from circular dichroism (CD) and intrinsic fluorescence spectroscopic analyses of HRP showed the unfolding of the tertiary structure around HRP's heme active site played a critical role in the inhibition, whereas the changes of HRP in the secondary structure and the tertiary structure near its tryptophan residue (Trp117) induced by these surfactants were minor for the inhibition. Based on the experimental results, a relationship between conformation and activity for HRP was suggested.