Crystal structure of low-potential cytochrome c549 from Synechocystis sp PCC 6803 at 1.21 Å resolution

The crystal structure of low-potential cytochrome C-549, an extrinsic component of the photosystem II (PS II) from Synechocystis sp. PCC 6803, was obtained directly from single-wavelength 1.21 Angstrom resolution diffraction data. This is the first monodomain bis-histidinyl monoheme cytochrome c to be structurally characterized. The extended N-terminal region of c(549) builds up a two-strand antiparallel beta -sheet in a hairpin motif, which extends through two molecules owing to crystal packing. Both peptide termini are involved in crystal contacts, which may explain their protrusion out of the globular fold. The C-terminus is preceded by a 9 Angstrom -long hydrophobic finger extending from a positively charged base and could be involved in PSII interactions, as well as a protruding negative patch built by a set of conserved acidic residues among C-549 sequences.