Biophysical and Functional Characterization of a Thermally Stable Bifunctional Serine Protease Inhibitor from Cleome viscosa Seeds

Plant protease inhibitors (PPI) play a significant role against microbes, insects, and, to a considerable extent, human pathogens. PPIs inactivate hydrolase enzymes or depolarize the plasma membrane of the pathogens, thereby inhibiting their growth, replication, and invasion. Here, an active serine protease inhibitor was isolated and purified from the seeds of <i>Cleome viscosa</i>. The purified inhibitor was homogenous and exhibited a molecular weight of around 12 kDa as a monomer. The secondary structure analysis indicated that the inhibitor was predominantly composed of α-helical content. The kinetics experiments demonstrated a noncompetitive mode of inhibition towards serine protease when casein was used as the enzyme substrate. The inhibitor formed a stable complex with serine protease, having a likely 1:1 stoichiometry, as inferred from ITC, and the dissociation constant was examined to be K_d = 1.9 × 10^-6 M with a Gibbs free energy of ΔG = -8.079 (kcal/mol). The inhibitor exhibits stable protease inhibition up to 90 °C. Further, in vitro preliminary studies revealed its inhibitory effects against HSV-2 function, evidence that it may have a role in the treatment of viral infections.