FUNCTIONAL-CHARACTERIZATION OF A PURIFIED HOMOGENEOUS PHOTOSYSTEM-II CORE COMPLEX WITH HIGH OXYGEN EVOLUTION CAPACITY FROM SPINACH

The functional properties of a purified homogeneous spinach PS II-core complex with high oxygen evolution capacity (Haag et al. 1990a) were investigated in detail by measuring thermoluminescence and oscillation patterns of flash induced oxygen evolution and fluorescence quantum yield changes. The following results were obtained: a) Depending on the illumination conditions the PS II-core complexes exhibit several thermoluminescence bands corresponding to the A band, Q band and Z(v) band in PS II membrane fragments. The lifetime of the Q band (T(max) = 10-degrees-C) was determined to be 8 s at T = 10-degrees-C. No B band corresponding to S2Q(B)- or S3Q(B)- recombination could be detected. b) The flash induced transient fluorescence quantum yield changes exhibit a multiphasic relaxation kinetics which reflect the reoxidation of Q(A)-. In control samples without exogenous acceptors this process is markedly slower than in PS II membrane fragments. The reaction becomes significantly retarded by addition of 10 muM DCMU. After dark incubation in the presence of K3[Fe(CN)6], the electron transfer from Q(A)- to the oxidized endogenous iron center Fe3+ takes place with a half time of less than 10 mus. In the presence of phenyl-p-benzoquinone (Ph-p-BQ) a period two oscillation is observed, corresponding to photoreductant induced oxidation of Fe2+ by Ph-p-BQ. This finding supports the existence of a partially functioning binding site for Ph-p-BQ. c) Excitation of dark-adapted samples with a train of short saturating flashes gives rise to a typical pattern dominated by a high O2 yield due to the third flash and a highly damped period four oscillation. The decay of redox states S2 and S3 are dominated by short life times of 4.3 s and 1.5 s, respectively, at 20-degrees-C. The results of the present study reveal that in purified homogeneous PS II-core complexes with high oxygen evolution isolated from higher plants by beta-dodecylmaltoside solubilization the thermodynamic properties and the kinetic parameters of the redox groups leading to electron transfer from water to Q(A) are well preserved. The most obvious phenomenon is a severe modification of the Q(B) binding site. The implications of this findings are discussed.