ELECTROSTATIC EFFECTS IN ELECTRON-TRANSFER REACTIONS OF [2FE-2S] FERREDOXINS WITH INORGANIC REAGENTS

The kinetics of electron transfer from the reduced [2Fe-2S] ferredoxin s from the cyanobacterium Anabaena 7120 and the protozoan Trichomonas vaginalis to select cobalt coordination compounds have been studied in order to gain insight into the mechanism of electron transfer and int rinsic reactivity of [2Fe-2S] active sites. With tripositive cobalt co mplexes, reactions of both proteins displayed saturation kinetics; val ues of association constants of 12,900 and 1,400 M(-1) and limiting ra te constants of 7.6 and 3.5 s(-1) were found for oxidation of T. vagin alis and Anabaena ferredoxins, respectively, by Co(NH3)(6)(3+) at room temperature and I = 0.1 M. An activation enthalpy of 12.1 kcal/mol an d activation entropy of -14.3 cal/mol K for oxidation of T. vaginalis ferredoxin by Co(NH3)(6)(3+)) contrasted with corresponding values of 13.4 kcal/mol and - 10.5 cal/mol K for the Spirulina platensis protein , which is homologous to Anabaena ferredoxin. The dependence of the re action rates on ionic strength were measured to probe the importance o f electrostatics on the reactivity of the proteins. Analysis of the io nic strength dependence of the oxidation of the proteins by Co(NH3)(6) (3+) by the ''parallel plate'' model of Watkins et al. (1994, Protein Sci 3:2104-2114) afforded values for active site charges of -0.7 and - 1.1 and limiting rate constants at infinite ionic strength of 25,800 a nd 76 M(-1)s(-1) for T. vaginalis and Anabaena ferredoxins, respective ly. These results suggest that the [2Fe-2S] center of the protozoal fe rredoxin is more accessible and adjacent to a less highly charged, mor e compact patch of negative charges than the photosynthetic protein.