Structure of the cytochrome b6f complex of oxygenic photosynthesis: tuning the cavity

The cytochrome b6f complex provides the electronic connection between the photosystem I and photosystem II reaction centers of oxygenic photosynthesis and generates a transmembrane electrochemical proton gradient for adenosine triphosphate synthesis. A 3.0 angstrom crystal structure of the dimeric b6f complex from the thermophilic cyanobacterium Mastigocladus laminosus re-veals a large quinone exchange cavity, stabilized by lipid, in which plastoqui-none, a quinone-analog inhibitor, and a novel heme are bound. The core of the b6f complex is similar to the analogous respiratory cytochrome bc1 complex, but the domain arrangement outside the core and the complement of pros-thetic groups are strikingly different. The motion of the Rieske iron-sulfur protein extrinsic domain, essential for electron transfer, must also be different in the b6f complex. In oxygenic photosynthesis, three integral membrane protein complexes accomplish electron transport and generate the trans