Study of heme Fe(III) ligated by OH − in cytochrome b-559 and its low temperature photochemistry in intact chloroplasts

Abstract

EPR properties of Cyt b-559 have been investigated in intact chloroplasts that are functionally competent in O 2 evolution and in CO 2 fixation. After chemical oxidation of Cyt b-559 by 10 mM 2,3-dicyano, 4, 5-dichloro- p-benzoquinone (DDQ) the major part of Cyt b-559 is found to be present in the high spin Fe(III) form. Only a small fraction of low spin heme Fe(III) (less than 5%) was formed by chemical or light-induced oxidation. This fraction increased during aging of intact chloroplasts. A comparison with the EPR signal of Fe(III) in myoglobin (Mb) reveals that the structure of the high spin signal in intact chloroplasts is indicative for the presence of an axial OH − ligand at the heme Fe(III). This type of ligation comprised a considerable part (∼ 40%) of the total Cyt b-559 content. Removal of the Mn-claster caused a change of the EPR parameters of OH − ligation. When in intact chloroplasts the heme Fe is chemically oxidized to FE (III) ligated by OH −, this OH − ligation disappeared after a subsequent illumination at 80K by red light. Upon illumination at 140K this disappearance was accompanied by the formation of a high spin Fe(III) that is not ligated by OH −. These results are discussed in terms of removal of OH − from Fe(III) caused by structural changes or photooxidafion at a complex of Cyt b-559 that could possibly also comprise the Mn-cluster. This photooxidation is assumed to be accompanied by the formation of a bound OH • radical. The possibility is discussed that this process is related to photosynthetic water oxidation.