Mechanism of Enhanced Superoxide Production in the Cytochrome b6f Complex of Oxygenic Photosynthesis

The specific rate of superoxide (O-2(center dot-)) production in the purified active crystallizable cytochrome b(6)f complex, normalized to the rate of electron transport, has been found to be more than an order of magnitude greater than that measured in isolated yeast respiratory bc(1) complex. The biochemical and structural basis for the enhanced production of O-2(center dot-) in the cytochrome b(6)f complex compared to that in the bc(1) complex is discussed. The higher rate of superoxide production in the b(6)f complex could be a consequence of an increased residence time of plastosemiquinone/plastoquinol in its binding niche near the Rieske protein iron-sulfur cluster, resulting from (i) occlusion of the quinone portal by the phytyl chain of the unique bound chlorophyll, (ii) an altered environment of the proton-accepting glutamate believed to be a proton acceptor from semiquinone, or (iii) a more negative redox potential of the heme bp on the electrochemically positive side of the complex. The enhanced rate of superoxide production in the b(6)f complex is physiologically significant as the chloroplast-generated reactive oxygen species (ROS) functions in the regulation of excess excitation energy, is a source of oxidative damage inflicted during photosynthetic reactions, and is a major source of ROS in plant cells. Altered levels of ROS production are believed to convey redox signaling from the organelle to the cytosol and nucleus.