On the structural role of the aromatic residue environment of the chlorophyll a in the cytochrome b6f complex.
Because light is not required for catalytic turnover of the cytochrome b 6 f 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 ( approximately 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 b 6 f complex was purified from three mutant strains, a double mutant Phe133Leu/Phe135Leu in subunit IV and single mutants Tyr112Phe and Trp125Leu in the cytochrome b 6 subunit. The purified b 6 f 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 Q y 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 b 6 f 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.
Yan
Jiusheng
J
Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907, USA.
Dashdorj
Naranbaatar
N
Baniulis
Danas
D
Yamashita
Eiki
E
Savikhin
Sergei
S
Cramer
William A
WA
D002734:Chlorophyll / Q000737:chemistry*; D000077194:Chlorophyll A; D045346:Cytochrome b6f Complex / Q000737:chemistry* / Q000235:genetics; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D010649:Phenylalanine / Q000737:chemistry; D013057:Spectrum Analysis; D046940:Synechococcus / Q000737:chemistry; D014443:Tyrosine / Q000737:chemistry
D002734:Chlorophyll; D014443:Tyrosine; D010649:Phenylalanine; D045346:Cytochrome b6f Complex; D000077194:Chlorophyll A