Crystal structure of oxidized cytochrome c6A from Arabidopsis thaliana

Compared with algal and cyanobacterial cytochrome c6, cytochrome c6A from higher plants contains an additional loop of 12 amino acid residues. We have determined the first crystal structure of cytochrome c6A from Arabidopsis thaliana at 1.5 Å resolution in order to help elucidate its function. The overall structure of cytochrome c6A follows the topology of class I c-type cytochromes in which the heme prosthetic group covalently binds to Cys16 and Cys19, and the iron has octahedral coordination with His20 and Met60 as the axial ligands. Two cysteine residues (Cys67 and Cys73) within the characteristic 12 amino acids loop form a disulfide bond, contributing to the structural stability of cytochrome c6A. Our model provides a chemical basis for the known low redox potential of cytochrome c6A which makes it an unsuitable electron carrier between cytochrome b6f and PSI.