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

• Published in: Bulletin of the Chemical Society of Japan Vol. 83; no. 6; pp. 703 - 708
• Main Authors: Singh, Beena G, Bag, Partha P, Kumakura, Fumio, Iwaoka, Michio, Priyadarsini, K. Indira
• Format: Journal Article
• Language: English
• Published: Tokyo The Chemical Society of Japan 15.06.2010; Chemical Society of Japan
• ISSN: 0009-2673; 1348-0634
• DOI: 10.1246/bcsj.20090348

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 (kcat) and the relative reactivity parameters toward thiol (φG) and hydroperoxide (φH), were determined by applying Dalziel kinetics for a bi-substrate model in the presence of hydrogen peroxide (H2O2), t-butyl hydroperoxide, or α-cumyl hydroperoxide and glutathione or dithiothreitol (DTTred). The intermediates formed during the reaction of CysSeSeCys with H2O2 and DTTred were characterized by 77Se NMR spectroscopy. Ab initio calculation at HF/6-31G(d) indicated that the reactions with H2O2 are exothermic, while those with DTTred 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.