Natively Oxidized Amino Acid Residues in the Spinach Cytochrome b6f Complex

The cytochrome b6f complex of oxygenic photosynthesis produces substantial levels of reactive oxygen species (ROS). It has been observed that the ROS production rate by b6f is 10-20 fold higher than that observed for the analogous respiratory cytochrome bc 1 complex. The types of ROS produced (O2•−,1O2, and, possibly, H2O2) and the site(s) of ROS production within the b6f complex has been the subject of some debate. Proposed sources of ROS have include the heme bp , PQp•− (possible sources for O2•−), the Rieske iron-sulfur cluster (possible source of O2•− and/or H2O2), Chl a (possible source of 1O2) and heme Cn (possible source of O2•− and/or H2O2). Our working hypothesis is that amino acid residues proximal to the ROS production sites will be more susceptible to oxidative modification than distant residues. In the current study, we have identified natively oxidized amino acid residues in the subunits of the spinach cytochrome b6f complex. The oxidized residues were identified by tandem mass spectrometry using the MassMatrix Program. Our results indicate that numerous residues, principally localized near p -side cofactors and Chl a , were oxidatively modified. We hypothesize that these sites are sources for ROS generation in the spinach cytochrome b6f complex.