Abstract
We study, by flash kinetic spectrophotometry on the microsecond time scale, the effects of ionic strength and viscosity on the kinetics of oxidative quenching of the triplet state of zinc cytochrome c, ^Zncyt, by the wild-type form and the following nine mutants of cupriplastocyanin: Leul2Glu, Leul2Asn, Phe35Tyr, Gln88Glu, Tyr83Phe, TyrSSHis, Asp42Asn, Glu43Asn, and the double mutant Glu59Lys/GIu60Gln. The unimolecular rate constants for the quenching reactions within the persistent diprotein complex, which predominates at low ionic strength, and within the transient diprotein complex, which is involved at higher ionic strength, are equal irrespective of the mutation. Evidently, the two complexes are the same, fo both reactions the rate-limiting step is rearrangement of the diprotein complex from a configuration optimal for docking to the one optimal for the subsequent electron-transfer step, which is fast. We investigate the effects of plastocyanin mutations on this rearrangement, which gates the overall electron-transfer reaction.
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