Relationship between rate and free energy difference for electron transfer from cytochrome c2 to the reaction center in Rhodobacter sphaeroides.

Abstract

The rate of electron transfer from cytochrome c2 to the bacteriochlorophyll dimer of the reaction center from the photosynthetic bacterium Rhodobacter sphaeroides has been investigated using time-resolved optical spectroscopy. Measurements were performed on a series of mutant reaction centers in which the midpoint potentials of the bacteriochlorophyll dimer vary over a range of 350 mV. Dramatic changes in the characteristic time of electron transfer were observed, with the measured values ranging from 7730 to 80 ns compared to 960 ns for wild type. The binding constants (0.15 to 0.25 microM-1) and the second-order rate constants for the slow component (5.5 x 10(8) to 9.4 x 10(8) M-1 s-1) for the mutants are similar to the corresponding values for wild type (0.35 microM-1 and 11 x 10(8) M-1 s-1), indicating that the binding of the cytochrome to the reaction center is not changed in the mutants. In the mutants with the fastest rates, an additional minor component was resolved that is probably due to formation of a reaction center-cytochrome complex in an unfavorable configuration with a binding constant an order of magnitude weaker than the major component. The altered midpoint potentials in the mutants result in values for the free energy difference for this electron transfer reaction ranging from -65 to -420 meV compared to -160 meV for wild type. The relationship between the rate and free energy difference was well fit by a Marcus equation using a reorganization energy of 500 meV.(ABSTRACT TRUNCATED AT 250 WORDS)