Mutant reaction centers of Rhodobacter sphaeroides I(L177)H with strongly bound bacteriochlorophyll a: Structural properties and pigment-protein interactions

Methods of photoinduced Fourier transform infrared (FTIR) difference spectroscopy and circular dichroism were employed for studying features of pigment-protein interactions caused by replacement of isoleucine L177 by histidine in the reaction center (RC) of the site-directed mutant I(L177)H of Rhodobacter sphaeroides. A functional state of pigments in the photochemically active cofactor branch was evaluated with the method of photo-accumulation of reduced bacteriopheophytin H A ⁻ . The results are compared with those obtained for wild-type RCs. It was shown that the dimeric nature of the radical cation of the primary electron donor P was preserved in the mutant RCs, with an asymmetric charge distribution between the bacteriochlorophylls PA and PB in the P⁺ state. However, the dimers P in the wild-type and mutant RCs are not structurally identical due probably to molecular rearrangements of the PA and PB macrocycles and/or alterations in their nearest amino acid environment induced by the mutation. Analysis of the electronic absorption and FTIR difference P⁺Q⁻/PQ spectra suggests the 17³-ester group of the bacteriochlorophyll PA to be involved in covalent interaction with the I(L177)H RC protein. Incorporation of histidine into the L177 position does not modify the interaction between the primary electron acceptor bacteriochlorophyll BA and the bacteriopheophytin HA. Structural changes are observed in the monomer bacteriochlorophyll BB binding site in the inactive chromophore branch of the mutant RCs.