Site-directed mutagenesis of cytochrome c(6) from Synechocystis sp. PCC6803 - The heme protein possesses a negatively charged area that may be isofunctional with the acidic patch of plastocyanin

This paper reports the first site-directed mutagenesis analysis of any cyto chrome c(6), a heme protein that performs the same function as the copper-p rotein plastocyanin in the electron transport chain of photosynthetic organ isms. Photosystem I reduction by the mutants of cytochrome c(6) from the cy anobacterium Synechocystis sp, PCC 6803 has been studied by laser flash abs orption spectroscopy. Their kinetic efficiency and thermodynamic properties have been compared with those of plastocyanin mutants from the same organi sm. Such a comparative study reveals that aspartates at positions 70 and 72 in cytochrome c(6) are located in an acidic patch that may be isofunctiona l with the well known "south-east" patch of plastocyanin, Calculations of s urface electrostatic potential distribution in the mutants of cytochrome c( 6) and plastocyanin indicate that the changes in protein reactivity depend on the surface electrostatic potential pattern rather than on the net charg e modification induced by mutagenesis, Phe-64, which is close to the heme g roup and may be the counterpart of Tyr-83 in plastocyanin, does not appear to be involved in the electron transfer to photosystem I, In contrast, Arg- 67, which is at the edge of the cytochrome c(6) acidic area, seems to be cr ucial for the interaction with the reaction center.