Solution structure of cytochrome c6 from the thermophilic cyanobacterium Synechococcus elongatus

Cytochrome c(6) is a small, soluble electron carrier between the two membrane-bound complexes cytochrome b(6)f and photosystem I (PSI) in oxygenic photosynthesis, We determined the solution structure of cytochrome c(6) from the thermophilic cyanobacterium Synechococcus elongatus by NMR spectroscopy and molecular dynamics calculations based on 1586 inter-residual distance and 28 dihedral angle restraints. The overall fold exhibits four alpha-helices and a small antiparallel beta-sheet in the vicinity of Met58, one of the axial heme ligands. The flat hydrophobic area in this cytochrome c(6) is conserved in other c(6) cytochromes and even in plastocyanin of higher plants. This docking region includes the site of electron transfer to PSI and possibly to the cytochrome b(6)f complex. The binding of cytochrome c(6) to PSI in green algae involves interaction of a negative patch with a positive domain of PSI. This positive domain has not been inserted at the evolutionary level of cyanobacteria, but the negatively charged surface region is already present in S. elongatus cytochrome c(6) and may thus have been optimized during evolution to improve the interaction with the positively charged cytochrome f. As the structure of PSI is known in S. elongatus, the reported cytochrome c(6) structure can provide a basis for mutagenesis studies to delineate the mechanism of electron transfer between both.