Pigment-Protein Interactions in Reaction Centers of Rhodopseudomonas Viridis: Endor-Study of the Oxidized Primary Donor in Site-Directed Mutants

Abstract

In the light-induced electron transfer process in bacterial reaction centers (RCs) the primary donor P is a dimer of bacteriochlorophyll (BChl) molecules. The two dimer halves are denoted PL and Pm (Figure 1). The spatial structure of this dimer is known for Rhodopseudomonas viridis, containing BChl b, and Rhodobacter sphaeroides, containing BChl a, from X-ray crystallography of RCs [1]. Comparison of the structures revealed some differences of the pigment-protein interactions in the RCs of the two species. In particular, there are three H-bonds from histidine H(L168), threonine T(L248) and tyrosine Y(M195) to keto groups of PL and Pm in R. viridis (Figure 1), whereas there is only one H-bond from H(L168) to PL in R. sphaeroides [1]. Despite these differences the electronic structure of the light-induced radical cation P+ of the primary donor as determined from ENDOR and Special TRIPLE spectroscopy [3] has been found to be similar in the two species. In both cases —2/3 of the unpaired spin density is located on Pl. [4,5]. ENDOR/TRIPLE studies of mutant RCs of R. sphaeroides with specific changes of key amino acid residues (AARs) in the vicinity of P demonstrated a strong influence of H-bonds on the distribution of the spin density between PL and Pm [4]. In view of these facts the similarity of the electronic structure of 13-+ in wild type RCs of R. vtridis and R. sphaeroides is remarkable. In order to further elucidate the role of H-bonds to P in the two types of RCs, we performed ENDOR/TRIPLE experiments on 13 in mutants of R. viridis, in which Y(M195) near the 3-acetyl group of Pm is exchanged to either histidine (H) or phenylalanine (F) [6]. The observed effects are compared here with those of the corresponding mutations at position M197 in RCs of R. sphaeroides.