Photosystem I reduction by the soluble metalloproteins cytochrome c(6) and plastocyanin, which are alternatively synthesized by some photosynthetic organisms depending on the relative availability of copper and iron, has been investigated in cyanobacteria, green algae and plants. The reaction mechanism is classified in three different types on the basis of the affinity of the membrane complex towards its electron donor protein. The role of electrostatic interactions in forming an intermediate transient complex, as well as the structural and functional similarities of cytochrome c(6) and plastocyanin are analysed from an evolutionary point of view. The proposal made is that the heme protein was first "discovered" by nature, when iron was much more abundant on the Earth's surface, and replaced by plastocyanin when copper became available because of the oxidizing conditions of the new atmosphere.

Flavocytochrome b2 consists of two distinct domains. The N-terminal domain contains protohaem IX and the larger, C-terminal domain contains flavin mononucleotide (FMN). We describe here the isolation of the flavin-binding domain expressed in Escherichia coli independent of the cytochrome domain. The isolated domain is an efficient lactate dehydrogenase with ferricyanide as electron acceptor but reduces cytochrome c, the physiological oxidant for flavocytochrome b2, extremely poorly; electron transfer from the flavin-binding domain to the separately expressed cytochrome domain is undetectable. FMN reduction by lactate occurs as a single exponential process in the isolated flavin-binding domain, in contrast to the biphasic kinetics observed with native flavocytochrome b2.

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.