KINETICS OF PHOTOOXIDATION OF SOLUBLE CYTOCHROMES, HIPIP, AND AZURIN BY THE PHOTOSYNTHETIC REACTION CENTER OF THE PURPLE PHOTOTROPHIC BACTERIUM RHODOPSEUDOMONAS-VIRIDIS

The photosynthetic reaction center of Rhodopseudomonas viridis contains a bound tetraheme cytochrome c subunit which is the primary electron donor to the photooxidized special pair bacteriochlorophyll. We have tested a variety of soluble electron-transfer proteins for their ability to serve as secondary electron donors to the bacteriochlorophyll via the bound cytochrome by measuring the kinetics of reaction center heme reduction following photooxidation by a laser flash, as a function of soluble protein concentration and ionic strength. All of the soluble redox proteins utilized appear to interact with a negatively charged region on the reaction center and to transfer electrons to the 300-mV heme c-556 of the bound cytochrome. Rps. viridis cytochrome c2 was the best electron donor among those proteins tested, with a second-order rate constant extrapolated to infinite ionic strength of 1.2 x 10(6) M-1 s-1, which is two orders of magnitude larger than that of horse cytochrome c. Rps. viridis cytochrome c2 apparently binds to the reaction center at low ionic strength, as evidenced by a nonlinear dependence of k(obs) on protein concentration. The limiting first-order electron-transfer rate constant at 6 mM ionic strength is approximately 1300 s-1. Horse cytochrome c and the reaction center also form a complex, with a limiting first-order rate constant for electron transfer which is 5 times smaller than for cytochrome c2. Other cytochromes c2 are intermediate in reactivity. More distantly related cytochromes, HiPIP, and azurin are relatively poor electron donors under the conditions of assay. We conclude from these experiments that (i) soluble cytochrome binding to the reaction center does not correlate with the magnitude of the ionic strength effect on the electron-transfer kinetics; thus there may be more than one site of interaction, or forces other than electrostatics (e.g., hydrophobic interactions) may also be involved; (ii) electron-transfer rate constants do not correlate with thermodynamic driving force, and therefore parameters such as distance and orientation may also contribute; and (iii) the function of the bound cytochrome is to rapidly reduce the photooxidized reaction center bacteriochlorophyll special pair in order to prevent back-reaction from photoreduced quinone species.