Cytochrome c6A of Chloroplasts

Cytochrome c6A must be an important protein, as it is conserved across plant and green algal lineages, yet its function remains as mysterious as when it was discovered over 10 years ago. The protein is similar in sequence to conventional cytochrome c6 proteins, which are well known as alternatives to plastocyanin in transferring electrons from the cytochrome b6f complex to Photosystem I in the light reactions of photosynthesis. Yet cytochrome c6A has a dramatically lower redox midpoint potential than conventional cytochrome c6, meaning that it is an unsuitable electron acceptor from cytochrome f. The structure of cytochrome c6A has been determined, and mutagenesis studies have pinpointed a single substitution compared to cytochrome c6 that is responsible for at least 100 mV of the lowering of the midpoint potential—a remarkable effect for a single residue. Another striking feature of cytochrome c6A, absent from cytochrome c6, is a 12-residue insertion in a loop region of the protein. This insertion has two conserved cysteine residues. The function of these is unknown, but may be structural rather than catalytic. Cytochrome c6A is able to transfer electrons to plastocyanin. Transcriptomic and proteomic studies have provided remarkably little insight into the protein’s function. Ironically, the recognition of cytochrome c6A in plants—from which cytochrome c6 was long thought to be absent—has now led to the recognition of a previously unsuspected class of cytochrome c6-like proteins in cyanobacteria, designated cytochrome c6B and cytochrome c6C.