Structure-function of the cytochrome bf lipoprotein complex: a scientific odyssey and personal perspective.

Structure-function studies of the cytochrome bf complex, the central hetero-oligomeric membrane protein complex in the electron transport chain of oxygenic photosynthesis, which formed the basis for a high-resolution (2.5 Å) crystallographic solution of the complex, are described. Structure-function differences between the structure of subunits of the bc complexes, bf, and bc from mitochondria and photosynthetic bacteria, which are often assumed to function identically, are discussed. Major differences which suggest that quinone-dependent electron transport pathways can vary in bf and bc complexes are as follows: (a) an additional c-type heme, c, and bound single copies of chlorophyll a and β-carotene in the bf complex; and (b) a cyclic electron transport pathway that encompasses the bf and PSI reaction center complexes. The importance of including lipid in crystallization of the cytochrome complex, or with any hetero-oligomeric membrane protein complex, is emphasized, and consequences to structure-function of bf being a lipoprotein complex discussed, including intra-protein dielectric heterogeneity and resultant pathways of trans-membrane electron transport. The role of the bf complex in trans-membrane signal transduction from reductant generated on the p-side of the electron transport chain to the regulation of light energy to the two photosystems by trans-side phosphorylation of the light-harvesting chlorophyll protein is presented. Regarding structure aspects relevant to plastoquinol-quinone entrance-egress: (i) modification of the p-side channel for plastoquinone access to the iron-sulfur protein would change the rate-limiting step in electron transport; (ii) the narrow niche for entry of plastoquinol into bf from the PSII reaction center complex would seem to require close proximity between the complexes.