The rate constants (25°, M-1 see-'), activation enthalpies (keal/mol), and activation entropies (e.u.) for the second-order reduction of oxidized clostridial rubredoxin by Ru(NH3)s0+, V(H20),?+, and Cr(H20)e' + at I = 0.10 have been determined or estimated to be 9.5 X 104, '-1.4, -31 (pH 6.3-7.0); 1.6 X 104, 0.1, -40 (pH 3.5-4.5); and 1.2 X 108, -0, --44 (pH 3.5-4.0), respectively. Ionic strength dependencies for the vanadium reaction are suggestive of a direct interaction of the reductants with the Fe(SR)4-1 site of oxidized rubredoxin.The results are consistent with outer-sphere mechanisms for all three reductants and an especially high inherent outer-sphere reactivity of rubredoxin. This high reactivity level is attributed to the low activation enthalpy demands of the iron-sulfur site of rubredoxin. Thus, the possibility is raised that the rate of reaction of rubredoxin with its physiological counterparts may be determined largely by the activation entropy demands imposed by the physiological reactant. Evidence is presented in support of an absolute entropy decrease of about 7.5 e.u. on going from oxidized to reduced rubredoxin, which is presumably attributable to the charge increase from Fe(SR)41-to Fe(SR)42-at the redox site.Over the past decade iron-sulfur proteins have emerged as an important class of redox metalloproteins, functioning in such diverse processes as respiration, photosynthesis, nitrogen fixation, biosynthesis, and degradative metabolism (1-5). The simpler proteins of this class appear to function physiologically as electron carriers; e.g., ferredoxins, adrenodoxin, and putidaredoxin (1-5). The structural characteristics of their metal sites have been reasonably well defined by extensive studies of their static properties (6), including refined x-ray determinations for the structures of crystalline clostridial rubredoxin (1 Fe) (7), high potential iron protein (4 Fe) (8), and a ferredoxin (8 Fe) (8). In constrast, little is known of their kinetic properties.We have begun a systematic investigation of the reactivity patterns of these proteins with a kinetic study of the reaction of the oxidized form of clostridial rubredoxin, Rd",', with simple inorganic reductants which have been well characterized in their mechanistic behavior (1).Rubredoxin was chosen because it is the least complex of the iron-sulfur proteins (molecular weight -6,000) and offers the advantage of an established structure for Rd"I (7). The Abbreviations: RdIII, oxidized rubredoxin; RdII, reduced rubredoxin. * To whom correspondence should be addressed at San Diego State University. single iron center is coordinated close to the surface of the crystalline protein by four cysteinyl sulfurs at the apices of a distorted tetrahedron (7). Tetrahedral coordination is retained in solution (9). From a preliminary difference map it appears that the dimensions of the immediate metal environment in the crystal change by no more than about 0.1 A in the reduced form, RdV, from those in the oxidized form (L. H.
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