title: Studies of Ca2+ binding in spinach photosystem II using 45Ca2+.
abstract: The Ca(2+)-binding properties of photosystem II were investigated with radioactive 45Ca2+. PS II membranes, isolated from spinach grown on a medium containing 45Ca2+, contained 1.5 Ca2+ per PS II unit. Approximately half of the incorporated radioactivity was lost after incubation for 30 h in nonradioactive buffer. About 1 Ca2+/PS II bound slowly to Ca(2+)-depleted membranes in the presence of the extrinsic 16- and 23-kDa polypeptides in parallel with restoration of oxygen-evolving activity. The binding was heterogeneous with dissociation constants of 60 microM (0.7 Ca2+/PS II) and 1.7 mM (0.3 Ca2+/PS II), respectively, which could reflect different affinities of the dark-stable S-states for Ca2+. The reactivation of oxygen-evolving activity closely followed the binding of Ca2+, showing that a single exchangeable Ca2+ per PS II is sufficient for the water-splitting reaction to function. In PS II, depleted of the 16- and 23-kDa polypeptides, about 0.7 exchangeable Ca2+/PS II binds with a dissociation constant of 26 microM, while 0.3 Ca2+ binds with a much weaker affinity (Kd > 0.5 mM). The rate of binding of Ca2+ in the absence of the two extrinsic polypeptides was significantly higher than with the polypeptides bound. The rate of dissociation of bound Ca2+ in the dark, which had a half-time of about 80 h in intact PS II, increased in the absence of the 16- and 23-kDa polypeptides and showed a further increase after the additional removal of the 33-kDa protein and manganese. The rate of dissociation was also significantly faster in weak light than in the dark regardless of the presence or absence of the 16- and 23-kDa polypeptides.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Biochemistry and Biophysics, Göteborg University, Sweden.
authors.forename: P
authors.initials: P
authors.lastname: Adelroth
authors.forename: K
authors.initials: K
authors.lastname: Lindberg
authors.forename: L E
authors.initials: LE
authors.lastname: Andréasson
mesh_terms: D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002132:Calcium Radioisotopes; D007700:Kinetics; D008345:Manganese / Q000378:metabolism; D008970:Molecular Weight; D010100:Oxygen / Q000378:metabolism; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism*; D045332:Photosystem II Protein Complex; D011487:Protein Conformation; D018724:Spinacia oleracea / Q000378:metabolism
chemical_list: D002132:Calcium Radioisotopes; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D008345:Manganese; D010100:Oxygen; D002118:Calcium