On rate limitations of electron transfer in the photosynthetic cytochromeb6f complex

Considering information in the crystal structures of the cytochrome b6f 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 bc1 complex by a crystal structure with a greatly shortened cluster–heme c1 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 c1 heme and a membrane–proximal position close to the ubiquinol electron–proton donor. Comparison of cytochrome b6f 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 b6f 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.