Oxidation-reduction properties of Chromatium vinosum high potential iron-sulfur protein

The oxidation-reduction properties of the high potential iron-sulfur protein (HIPIP) from Chromatium vinosum have been investigated. Both equilibrium and kinetic measurements demonstrate electron transport by HIPIP is pH independent in the pH range 7-11. The kinetics of reduction (potassium ferrocyanide, SO2, S2O42-, sodium ascorbate, and Rhodospirillum rubrum cytochrome c2) and oxidation (potassium ferricyanide and Rhodospirillium rubrum cytochrome c2) of HIPIP are reported. Based on the data obtained with different reactants and the influence of ionic strength, pH, and temperature on the kinetics of oxidation and reduction, a number of conclusions can be drawn. (1) HIPIP undergoes rapid outer-sphere electron transfer with no evidence of kinetic complexity and no indication of complex formation with various reactants. (2) The site of oxidation of reduced HIPIP has an apparent negative charge while the site of reduction of oxidized HIPIP is uncharged. (3) HIPIP appears to interact with a physiological reactant (R. rubrum cytochrome c2) at the same site as nonphysiological oxidants or reductants suggesting single minimum energy pathways for the oxidation and reduction processes. (4) Based on a comparison of the rates of oxidation and reduction with different reactants, it appears that steric restrictions and differences in oxidation-reduction potential are less important than electrostatic attraction and/or repulsion in determining the absolute rate constants. (5) The thermodynamic activation parameters indicate that both oxidation and reduction by the iron hexacyanides are driven entropically with the enthalpic terms making no contribution to HIPIP oxidation and a small contribution to HIPIP reduction. Based on the data reported here and available structural and physical-chemical information, possible mechanisms of the oxidation and reduction of HIPIP are discussed and their relative merits analyzed. The more likely mechanisms include electron transfer via a tyrosine residue, electron transfer through a nonaqueous media to the iron-sulfur chromophore, and direct interaction between the iron-sulfur chromophore and the different oxidants and reductants.