Kinetics of photo-induced electron transfer from high-potential iron-sulfur protein to the photosynthetic reaction center of the purple phototroph Rhodoferax fermentans

Abstract

The kinetics of photo-induced electron transfer from high-potential iron-sulfur protein (HiPIP) to the photosynthetic reaction center (RC) of the purple phototroph Rhodoferax fermentons were studied. The rapid photooxidation of heme c-556 belonging to RC is followed, in the presence of HiPIP, by a slower reduction having a second-order rate constant of 4.8 × 107 M-1·s-1. The limiting value of Kobs at high HiPIP concentration is 95 s-1. The amplitude of this slow process decreases with increasing HiPIP concentration. The amplitude of a faster phase, observed at 556 and 425 nm and involving heme c-556 reduction, increases proportionately. The rate constant of this fast phase, determined at 425 and 556 nm, is ≈3 × 105 s-1. This value is not dependent on HiPIP concentration, indicating that it is related to a first-order process. These observations are interpreted as evidence for the formation of a HiPIP-RC complex prior to the excitation flash, having a dissociation constant of ≈2.5 μM. The fast phase is absent at high ionic strength, indicating that the complex involves mainly electrostatic interactions. The ionic strength dependence of kobs for the slow phase yields a second-order rate constant at infinite ionic strength of 5.4 × 106 M-1·s-1 and an electrostatic interaction energy of -2.1 kcal/mol (1 cal = 4.184 J). We conclude that Rhodoferax fermentans HiPIP is a very effective electron donor to the photosynthetic RC.