title: Mutation of arginine 357 of the CP43 protein of photosystem II severely impairs the catalytic S-state cycle of the H2O oxidation complex.
abstract: Basic amino acid side chains situated in active sites may mediate critical proton transfers during an enzymatic catalytic cycle. In the case of photosynthetic water oxidation, a strong base is postulated to facilitate the deprotonation of the active site Mn4-Ca cluster, thereby allowing the otherwise thermodynamically constrained transfer of an electron away from the Mn4-Ca cluster to the oxidized redox active tyrosine radical, YZ*, generated by photosynthetic charge separation. Arginine 357 of the CP43 polypeptide may be located in the second coordination shell of the O2-evolving Mn4-Ca cluster of photosystem II (PSII) according to current structural models. An ostensibly conservative substitution mutation, CP43-357K, was investigated using polarographic and fluorescence techniques in evaluating its potential impact on S-state cycling. Cells containing the CP43-357K mutation lost their capacity for autotrophic growth and exhibited a drastic reduction in O2 evolving activity ( approximately 15% of that of the wild type) despite the fact that mutant cells contained more than 80% of the concentration of charge-separating PSII reaction centers and more than half of these contained photooxidizable Mn. Fluorescence kinetics indicated that acceptor side electron transfer, dominated by the transfer of electrons from QA- to QB, was unaffected, but the fraction of centers containing Mn clusters capable of forming the S2 state was reduced to approximately 40% of that of the wild type. Analysis of O2 yields using a bare platinum electrode indicated a severe defect in the S-state cycling properties of the mutant H2O oxidation complexes. Although O2 evolution was delayed to the third flash during a train of single-turnover saturating flashes, the pattern of O2 emission did not exhibit a discernible periodicity indicating a very high miss factor, which was estimated to be approximately 45% compared to the wild-type value of approximately 10%. On the other hand, the multiflash fluorescence measurements indicate that the yield of formation of the S2 state from S1 is diminished by approximately 20%, although this latter estimate is complicated by the presence of damaged PSII centers. Taken together, the experiments indicate that the high miss factor observed during S-state cycling is likely due to a defect in the higher S-state transitions. These results are discussed in relation to the idea that CP43-R357 may serve as a ligand to bicarbonate or as the catalytic base proposed to mediate proton-coupled electron transfer (PCET) in the higher S states of the catalytic cycle of H2O oxidation.
authors.affiliation: Department of Microbiology and Molecular Genetics, Oklahoma State University, Stillwater, Oklahoma 74078, USA.
authors.forename: Hong Jin
authors.initials: HJ
authors.lastname: Hwang
authors.forename: Preston
authors.initials: P
authors.lastname: Dilbeck
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Debus
authors.forename: Robert L
authors.initials: RL
authors.lastname: Burnap
mesh_terms: D001120:Arginine / Q000235:genetics*; D002384:Catalysis; D008958:Models, Molecular; D009154:Mutation; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D045332:Photosystem II Protein Complex / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D014867:Water / Q000378:metabolism*
chemical_list: D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; C083100:photosystem II, chlorophyll binding protein, CP-43; D014867:Water; D001120:Arginine