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On the Structural Role of the Aromatic Residue Environment of the Chlorophyll a in the Cytochrome b6f Complex

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journal contribution
posted on 25.03.2008, 00:00 by Jiusheng Yan, Naranbaatar Dashdorj, Danas Baniulis, Eiki Yamashita, Sergei Savikhin, William A. Cramer
Because light is not required for catalytic turnover of the cytochrome b6f complex, the role of the single chlorophyll a in the structure and function of the complex is enigmatic. Photodamage from this pigment is minimized by its short singlet excited-state lifetime (∼200 ps), which has been attributed to quenching by nearby aromatic residues (Dashdorj et al., 2005). The crystal structure of the complex shows that the fifth ligand of the chlorophyll a contains two water molecules. On the basis of this structure, the properties of the bound chlorophyll and the complex were studied in the cyanobacterium, Synechococcus sp. PCC 7002, through site-directed mutagenesis of aromatic amino acids in the binding niche of the chlorophyll. The b6f complex was purified from three mutant strains, a double mutant Phe133Leu/Phe135Leu in subunit IV and single mutants Tyr112Phe and Trp125Leu in the cytochrome b6 subunit. The purified b6f complex from Tyr112Phe or Phe133Leu/Phe135Leu mutants was characterized by (i) a loss of bound Chl and b heme, (ii) a shift in the absorbance peak and increase in bandwidth, (iii) multiple lifetime components, including one of 1.35 ns, and (iv) relatively small time-resolved absorbance anisotropy values of the Chl Qy band. A change in these properties was minimal in the Trp125Leu mutant. In vivo, no decrease in electron-transport efficiency was detected in any of the mutants. It was concluded that (a) perturbation of its aromatic residue niche influences the stability of the Chl a and one or both b hemes in the monomer of the b6f complex, and (b) Phe residues (Phe133/Phe135) of subunit IV are important in maintaining the short lifetime of the Chl a singlet excited state, thereby decreasing the probability of singlet oxygen formation.