Kinetic studies on the redox reactions of Clostridium pasteurianum rubredoxin

Abstract

Electron-transfer reactions (25 °C) of the single iron protein rubredoxin from Clostridium pasteurianum with three inorganic complexes as redox partners have been studied at pH 7.0, I= 0.100 M (NaCl). With [Co(sep)]²⁺(sep = sepulchrate = 1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]icosane)(–300 mV) as reductant for the oxidised iron(III) form of rubredoxin (Rd^III)(–57 mV) a second-order rate law with k= 7.5 × 10⁵ M^–1 s^–1 has been determined. No dependence on pH 3.8–8.8 is observed. With [Co(terpy)₂]³⁺(terpy = 2,2′:6′,2″-terpyridine)(+260 mV) as oxidant for the Rd^III form saturation kinetic behaviour is obtained, with K= 470 ± 70 M^–1 for association prior to electron transfer, k_et= 515 ± 65 s^–1. Again no dependence on pH, in this case the range 5.2–7.0, is observed. With [Ru(NH₃)₆]³⁺(+70 mV) as oxidant for the Rd^II form saturation kinetics is also observed with K= 4430 ± 240 M^–1 and k_et= 240 ± 26 s^–1. No information is available concerning the site on rubredoxin at which these reactions occur, although it is noted that the Cys-9 and Cys-42 residues at the Fe(Cys)₄ active site are partially exposed to solvent.