Effects of Temperature and ΔG° on Electron Transfer from Cytochrome c2 to the Photosynthetic Reaction Center of the Purple Bacterium Rhodobacter sphaeroides

Abstract

The kinetics of electron transfer from cytochrome c2 to the primary donor (P) of the reaction center from the photosynthetic purple bacterium Rhodobacter sphaeroides have been investigated by time-resolved absorption spectroscopy. Rereduction of P+ induced by a laser pulse has been measured at temperatures from 300 K to 220 K in a series of specifically mutated reaction centers characterized by altered midpoint redox potentials of P+/P varying from 410 mV to 765 mV (as compared to 505 mV for wild type). Rate constants for first-order electron donation within preformed reaction center-cytochrome c2 complexes and for the bimolecular oxidation of free cytochrome c2 have been obtained by multiexponential deconvolution of the kinetics. At all temperatures the rate of the fastest intracomplex electron transfer increases by more than two orders of magnitude as the driving force -deltaGo is varied over a range of 350 meV. The temperature and deltaGo dependences of the rate constant fit the Marcus equation well. Global analysis yields a reorganization energy lambda = 0.96 +/- 0.07 eV and a set of electronic matrix elements, specific for each mutant, ranging from 1.2 10(-4) eV to 2.5 10(-4) eV. Analysis in terms of the Jortner equation indicates that the best fit is obtained in the classical limit and restricts the range of coupled vibrational modes to frequencies lower than approximately 200 cm(-1). An additional slower kinetic component of P+ reduction, attributed to electron transfer from cyt c2 docked in a nonoptimal configuration of the complex, displays a Marcus type dependence of the rate constant upon deltaGo, characterized by a similar value of lambda (0.8 +/- 0.1 eV) and by an average electronic matrix element smaller by more than one order of magnitude. In all of the mutants, as the temperature is decreased below 260 K, both intracomplex reactions are abruptly inhibited, their rate being negligible at 220 K. The free energy dependence of the second-order rate constant for oxidation of cyt c2 in solution suggests that the collisional reaction is partially diffusion controlled, reaching the diffusion limit at exothermicities between 150 and 250 meV over the temperature range investigated.

Intramolecular electron transfer in [4Fe-4S] proteins: estimates of the reorganization energy and electronic coupling in Chromatium vinosum ferredoxin

“…A value below 0.5 eV for the reorganization energy of C. vinosum ferredoxin lies at the lower edge of the available estimates (by other methods) in solution [36,37,38,39] for other soluble ET proteins, such as cytochromes c or blue copper proteins. However, very small reorganization energies have been obtained for some of these proteins (even smaller than that estimated here) by electrochemical measurements [40], and they have been attributed to a minimal reorientation of solvent molecules upon ET involving these proteins adsorbed on electrodes.…”

Guidelines for tunneling in enzymes

“…A complication arises when the reorganization energy is estimated not from the direct driving force variation but from the experimentally easier temperature variation of the reaction rate [31]. It is often assumed that a classical, non-quantum Marcus theory description of the activation energy is appropriate.…”

Continuum Electrostatic Model for the Binding of Cytochromec2to the Photosynthetic Reaction Center fromRhodobacter sphaeroides

“…A water-soluble cyt c 2 transfers an electron to the RC, connecting the RC with the cytochrome bc 1 complex and completing the electron transfer cycle, resulting in proton pumping across the membrane and generation of a membrane potential driving ATP synthesis. The mechanism of the binding process and electron transfer reactions between cyt c 2 and the detergent-solubilized RCs have been extensively studied (4,(38)(39)(40)(41)(42)(43)(44). The reaction scheme can be described as follows (37):…”