Catalysis of Electron Transfer by Selenocysteine

Selenium is an essential element that is involved in biological redox processes. The electrode potentials of the selenocysteine half-reactions RSe(*) + e(-) --> RSe-, (RSeSeR)(*)(-) + e(-) --> 2 RSe(-), and RSeSeR + 2 e(-) --> 2 RSe(-) E degrees' (pH 7) are +0.43, +0.18, and -0.38 V, respectively, at pH 7. The spectra of RSe(*) and (RSeSeR)(*)(-) are characterized by absorption maxima at 460 nm (epsilon = 560 M(-)(1) cm(-)(1)) and 455 nm (epsilon = 7100 M(-)(1) cm(-)(1)), respectively. The bond dissociation energy of RSe-H has been calculated, and the value of 310 kJ/mol is in agreement with literature values. In comparison with the sulfur analogue cysteine, the more facile accessibility of the radical oxidation state is striking and may have biological implications, such as in mediation of one-electron- and two-electron-transfer processes, as illustrated by catalysis by selenocysteine of the electron transfer between dithiothreitol and benzyl viologen.