Structure and function of protein complexes in the photosynthetic membrane

This chapter discusses the structure and function of protein complexes in the photosynthetic membrane. Photosynthetic membranes of cyanobacteria, algae and plants contain four major protein complexes that harness light energy and convert it into chemical energy. Two of these protein complexes, the cytochrome b₆-f complex and the proton-ATPase (adenosine triphosphate) complex, contain no pigments and function in biochemical reactions. The other two, photosystem PS I reaction center and PS II reaction center, function in photochemical reactions and contain pigments involved in light-harvesting and primary energy transduction. There are a few ways to define membrane protein complexes. Because a minimal structure is usually difficult to resolve, copurification of various polypeptides was taken as a primordial criterion for the integrity of those polypeptides as part of the protein complex. Cyt b-c complex forms the evolutionary link between the respiratory and photosynthetic electron-transport pathways. In both systems, it oxidizes quinols and reduces metalloproteins while generating proton motive force. In cyanobacteria, the Cyt b₆-f complex is shared by both respiration and photosynthesis. The biogenesis of the Cyt b₆-f complex is not controlled by light, in contrast to the biosynthesis of the two reaction centers. Etiolated leaves contain substantial amounts of Cyt b₆-f complex and the relative amounts of the various subunits did not significantly change during the light-dependent greening of etiolated seedlings. During growth, additional complexes were assembled by a concerted mechanism in which all the four subunits were synthesized and assembled at the same rate.