Light-Induced Conformational Change at Low Temperature in the Protein Binding Site of Heme c556 in the Reaction Center of Rhodopseudomonas Viridis

Abstract

The photosynthetic reaction center (RC) of purple bacteria is a large membrane protein which performs the primary steps of photo-induced electron transfer. After light excitation, a dimer of bacteriochlorophyll molecules (P) transfers an electron to an acceptor, and the oxidized species P⁺ is then re-reduced by a secondary electron donor, a c-type cytochrome, in order to be reactivated. In some purple bacteria this secondary donor is a soluble c ₂ cytochrome, but in most cases it is a RC-bound tetraheme cytochrome c. The best studied example is Rhodopseudomonas (Rps.) viridis, in which the tridimensional structure of the RC has been solved with a high atomic resolution (1). Some of the electron transfer reactions from c-type cytochromes to P⁺ in the RC of purple bacteria occur even at cryogenic temperatures. Experimental observations of the effects of temperature on the rate and the yield of the cytochrome oxidation have contributed a lot to the development of current electron transfer theories (2). However, several of these observations, such as the extent of photooxidation of heme c556 at cryogenic temperature in Rps. viridis are still controversial (for a review see (3)). In order to understand some of these effects we have investigated the effect of flash and continuous illumination at low temperature on cytochrome hemes in RC isolated from Rps. viridis.