Role of Substrate Reactivity in the Glutathione Peroxidase (GPx) Activity of Selenocystine

Selenocystine (CysSeSeCys), a diselenide, exhibits glutathione peroxidase (GPx) activity, where it catalyses the reduction of hydroperoxides using a thiol co-factor. To understand the relative reactivity of the two substrates, enzyme kinetic parameters, i.e., the turnover number (k(cat)) and the relative reactivity parameters toward thiol (Phi(G)) and hydroperoxide (phi(H)), were determined by applying Dalziel kinetics for a bi-substrate model in the presence of hydrogen peroxide (H2O2), t-butyl hydroperoxide, or alpha-cumyl hydroperoxide and glutathione or dithiothreitol (DTTred). The intermediates formed during the reaction of CysSeSeCys with H2O2 and DTT,, were characterized by Se-77 NMR spectroscopy. Ab initio calculation at HF/6-31G(d) indicated that the reactions with H2O2 are exothermic, while those with DTT d are endothermic. Based on these studies, the GPx activity of CysSeSeCys is likely to be initiated by the reaction with hydroperoxide and in the catalytic cycle, the reaction with thiol is the rate-determining step.