On rate limitations of electron transfer in the photosynthetic cytochrome b6f complex.
Considering information in the crystal structures of the cytochrome b(6)f complex relevant to the rate-limiting step in oxygenic photosynthesis, it is enigmatic that electron transport in the complex is not limited by the large distance, approximately 26 Å, between the iron-sulfur cluster (ISP) and its electron acceptor, cytochrome f. This enigma has been explained for the respiratory bc(1) complex by a crystal structure with a greatly shortened cluster-heme c(1) distance, leading to a concept of ISP dynamics in which the ISP soluble domain undergoes a translation-rotation conformation change and oscillates between positions relatively close to the cyt c(1) heme and a membrane-proximal position close to the ubiquinol electron-proton donor. Comparison of cytochrome b(6)f structures shows a variation in cytochrome f heme position that suggests the possibility of flexibility and motion of the extended cytochrome f structure that could entail a transient decrease in cluster-heme f distance. The dependence of cyt f turnover on lumen viscosity is consistent with a role of ISP - cyt f dynamics in determination of the rate-limiting step under conditions of low light intensity. Under conditions of low light intensity and proton electrochemical gradient present, for example, under a leaf canopy, it is proposed that a rate limitation of electron transport in the b(6)f complex may also arise from steric constraints in the entry/exit portal for passage of the plastoquinol and -quinone to/from its oxidation site proximal to the iron-sulfur cluster.
Department of Biological Sciences, Hockmeyer Hall of Structural Biology, Purdue University, West Lafayette, IN 47907, USA.
S Saif
SS
Hasan
William A
WA
Cramer
D000595:Amino Acid Sequence; D016825:Chlamydomonas reinhardtii / Q000737:chemistry / Q000201:enzymology*; D018360:Crystallography, X-Ray; D000458:Cyanobacteria / Q000737:chemistry / Q000201:enzymology*; D045346:Cytochrome b6f Complex / Q000737:chemistry* / Q000378:metabolism; D004579:Electron Transport; D004583:Electrons; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D010788:Photosynthesis; D016415:Sequence Alignment
D045346:Cytochrome b6f Complex