Natively oxidized amino acid residues in the spinach cytochrome b 6 f complex

The cytochrome b 6 f complex of oxygenic photosynthesis produces substantial levels of reactive oxygen species (ROS). It has been observed that the ROS production rate by b 6 f is 10–20 fold higher than that observed for the analogous respiratory cytochrome bc 1 complex. The types of ROS produced (O 2 •−, 1 O 2 , and, possibly, H 2 O 2 ) and the site(s) of ROS production within the b 6 f complex have been the subject of some debate. Proposed sources of ROS have included the heme b p , PQ p •− (possible sources for O 2 •− ), the Rieske iron–sulfur cluster (possible source of O 2 •− and/or 1 O 2 ), Chl a (possible source of 1 O 2 ), and heme c n (possible source of O 2 •− and/or H 2 O 2 ). 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 b 6 f 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 b 6 f complex.