A comparative structural and functional analysis of cytochrome c M, cytochrome c 6 and plastocyanin from the cyanobacterium Synechocystis sp. PCC 6803

摘要

>Cytochrome c _M is a new c-class photosynthetic haem protein whose physiological role is still unknown. It has been proposed previously that cytochrome c _M can replace cytochrome c ₆ and plastocyanin in transferring electrons between the two membrane complexes cytochrome b ₆–f and photosystem I in organisms growing under stress conditions. The experimental evidence herein provided allows us to discard such a hypothesis. We report a procedure to overexpress cytochrome c _M from the cyanobacterium Synechocystis sp. PCC 6803 in Escherichia coli cells in mg quantities. This has allowed us to perform a comparative laser flash-induced kinetic analysis of photosystem I reduction by the three metalloproteins from Synechocystis. The bimolecular rate constant for the overall reaction is up to 100 times lower with cytochrome c _M than with cytochrome c ₆ or plastocyanin. In addition, the redox potential value and surface electrostatic potential distribution of cytochrome c _M are quite different from those of cytochrome c ₆ and plastocyanin. These findings strongly indicate that cytochrome c _M cannot be recognised by and interact with the same redox partners as the other two metalloproteins.