Mapping of two tyrosine residues involved in the quinone‐ (QB) binding site of the D‐1 reaction center polypeptide of photosystem II

The property of the D‐1 subunit of photosystem II in binding herbicides in its quinone‐binding niche has provided important approaches to study its structure and function. In the D‐1 protein, amino acid residues Tyr‐254 and Tyr‐237 are labeled by an [14C]azido‐urea derivative, as identified by protein sequencing of proteolytic fragments. Whereas Tyr‐254 is in a parallel α‐helix already indicated to contribute to the herbicide‐binding site, Tyr‐237 is in a hydrophilic sequence that is partly accessible from the matrix space of the chloroplasts. This area has been implicated to contain a cleavage site for a protease involved in the rapid turnover of the D‐1 polypeptide. The photoaffinity labeling results show that some of the amino acids in this cleavage site are actually part of the quinone‐binding niche. It allows a refined and extended prediction of the three‐dimensional folding of the reaction center of photosystem II.