Caesalpinia bonduc serine proteinase inhibitor CbTI-2: Exploring the conformational features and antimalarial activity

Seeds of tropical legumes posses a repertoire of proteinase inhibitors (PI) and the current study highlights some structural/functional features of a strong serine PI from the seeds of Caesalpinia bonduc (CbTI-2). Following purification, N-terminal sequence of CbTI-2 revealed over 40% similarity with a few serine PIs of Caesalpinioideae subfamily. Upon exposure to metal ions and ionic/non ionic surfactants, CbTI-2 showed immense variation in the levels of antitryptic activity. Exposure of CbTI-2 to 1,4-Dithiothreitol, Guanidinium HCl, H2O2 and Dimethyl sulfoxide led to a steady loss of inhibitory activity. Chemical modification of amino acids suggested an arginine as the active site residue. Circular Dichroism spectrum of native CbTI-2 revealed an unordered state. Secondary structure composition of CbTI-2 following exposure to extreme conditions (heat, acidic/alkaline environment, Guanidine hydrochloride and DTT) showed considerable perturbations that caused severe loss of antiproteolytic activity. DLS studies yielded a hydrodynamic radius of ∼2.2 nm for CbTI-2 and also reconfirmed 1:1 stoichiometry for the trypsin-CbTI-2 complex. Initial studies indicated CbTI-2 to be a potent antiplasmodial agent by being highly toxic towards growth, schizont rupture process and erythrocytic invasion of Plasmodium falciparum.