Kinetics of electron transfer between plastocyanin and the soluble Cu-A domain of cyanobacterial cytochrome c oxidase

摘要

It has been shown that efficient functioning of photosynthesis and respiration in the cyanobacterium Synechocystis PCC 6803 requires the presence of either cytochrome c(6) or plastocyanin. In order to check whether the blue copper protein plastocyanin can act as electron donor to cytochrome c oxidase, we investigated the intermolecular electron transfer kinetics between plastocyanin and the soluble Cu-A domain (i.e. the donor binding and electron entry site) of subunit II of the aa(3)-type cytochrome c oxidase from Synechocystis. Both copper proteins were expressed heterologously in Escherichia coli. The forward and the reverse electron transfer reactions were studied yielding apparent bimolecular rate constants of (5.1 +/- 0.2) x 10(4) M-1 s(-1) and (8.5 +/- 0.4) x 10(5) M-1 s(-1), respectively (20 mM phosphate buffer, pH 7). This corresponds to an apparent equilibrium constant of 0.06 in the physiological direction (reduction of Cu-A) which is similar to K-eq values calculated for the reaction between c-type cytochromes and the soluble fragments of other Cu-A domains. The potential physiological role of plastocyanin in cyanobacterial respiration is discussed. (C) 2004 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.