Abstract
Homodimeric photosynthetic reaction centers (RCs) in green sulfur bacteria and heliobacteria are functional homologs of Photosystem (PS) I in oxygenic phototrophs. They show unique features in their electron transfer reactions; however, detailed structural information has not been available so far. We mutated PscA-Leu688 and PscA-Val689 to cysteine residues in the green sulfur bacterium Chlorobaculum tepidum; these residues were predicted to interact with the special pair P840, based on sequence comparison with PS I. Spectroelectrochemical measurements showed that the L688C and V689C mutations altered a near-infrared difference spectrum upon P840 oxidation, as well as the redox potential of P840. Light-induced Fourier transform infrared difference measurements showed that the L688C mutation induced a differential signal of the S-H stretching vibration in the P840+/P840 spectrum, as reported in P800+/P800 difference spectrum in a heliobacterial RC. Spectral changes in the 131-keto C=O region, caused by both mutations, revealed corresponding changes in the electronic structure of P840 and in the hydrogen-bonding interaction at the 131-keto C=O group. These results suggest that there is a common spatial configuration around the special pair sites among type 1 RCs. The data also provided evidence that P840 has a symmetric electronic structure, as expected from a homodimeric RC.
Highlights
High-resolution X-ray structures were first reported for the type 2 reaction center (RC) in purple bacteria (PbRC)[8], and subsequently for PS I9,10 and PS II11–13
The P840+/P840 Fourier transform infrared (FTIR) difference spectrum for GbRC showed 2 prominent bands for P840+; these bands were assigned to the 131-keto C= O stretching vibrations of constituent BChl a molecules[21,26], and their presence implied that there is an asymmetric charge distribution, at least on the infrared (IR) timescale determined by the uncertainty principle
To identify amino acid residues that are predicted to interact with P840 in the GbRC, the amino acid sequence of the core polypeptide of GbRC, PscA, was aligned with those of other type 1 RCs, that is, PshA of HbRC, and PsaA/B of PS I
Summary
High-resolution X-ray structures were first reported for the type 2 RCs in purple bacteria (PbRC)[8], and subsequently for PS I9,10 and PS II11–13 These RCs are known as heterodimeric RCs and consist of 2 highly homologous but not identical polypeptides. The P800+/P800 FTIR difference spectrum for HbRC showed a single positive 131-keto C= O signal; this observation suggests that the positive charge is significantly delocalized over the BChl g dimer on the IR timescale[21,25]. A novel technique for site-directed mutagenesis of the homodimeric GbRC, which was designated the ‘pscA gene duplication method’[16], has been developed in the thermophilic green sulfur bacterium Chlorobaculum tepidum This method duplicates the pscA gene coding for the RC core polypeptide by introducing another mutated pscA gene into the recA gene locus. The structural homology around the special pair sites among type 1 RCs, together with the electronic structure of P840 in the homodimeric GbRCs, is discussed
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