The evolution of 02 from H202 catalyzed by the oxygen-evolving complex (OEC) in darkness was examined with photosystem II reaction center complex preparations from spinach. Flash illumination of dark-adapted reaction centers was used to make S0-enriched or Srenriched complexes. The membranes catalyzed 02 evolution from H202 when preset to either the S0or S1state. However, only the S0-state reaction was inhibited by carbonyl cyanide m-chlorophenylhydrazone and dependent on chloride. These results indicate that (1) the S0-dependent and S1-dependent catalytic cycles can be separated by flash illumination, (2) the S0-dependent reaction involves the formation of the S2 state, and (3) the S1-dependent reaction does not involve the formation of the S2 or S3 states. A kinetic study of the S1-dependent reaction revealed a rapid equilibrium ordered mechanism in which (1) the binding of Ca(II) must precede the binding of H202 to the OEC and (2) the reaction of Ca(II) with the free enzyme is at thermodynamic equilibrium such that Ca(II) does not necessarily dissociate after each catalytic cycle.
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