Abstract
Cytochrome b6f catalyzes quinone redox reactions within photosynthetic membranes to generate a transmembrane proton electrochemical gradient for ATP synthesis. A key step involves the transfer of an electron from the [2Fe-2S] cluster of the iron-sulfur protein (ISP) extrinsic domain to the cytochrome f heme across a distance of 26 Å, which is too large for competent electron transfer but could be bridged by translation-rotation of the ISP. Here we report the first crystallographic evidence of significant motion of the ISP extrinsic domain. It is inferred that extensive crystallographic disorder of the ISP extrinsic domain indicates conformational flexibility. The ISP disorder observed in this structure, in contrast to the largely ordered ISP structure observed in the b6f complex supplemented with neutral lipids, is attributed to electrostatic interactions arising from anionic lipids.