PNNL

Abstract

X-ray structures at 3.0 -3.1 Å resolution of the cytochrome b6f complex from the cyanobacterium, Mastigocladus laminosus (1) and the green alga, Chlamydomonas reinhardtii (2) showed the presence of a unique heme, heme x, that is covalently linked by a single thioether bond to a Cys residue (Cys35) on the electrochemically negative (n) side of the cytochrome b6 polypeptide. Heme x faces the inter-monomer quinone exchange cavity. The only axial ligand associated with this heme is a H2O or OH? that is H-bonded to the propionate of the stromal side heme bn, showing that is penta-coordinate. The spectral properties of this heme were hardly defined at the time of the structure determination. The pyridine hemochromagen redox difference spectrum for heme x covalently bound to the cytochrome b polypeptide isolated from SDS-PAGE displays a broad spectrum of low amplitude with a peak at 553 nm, similar to that of other hemes with a single thioether linkage. The binding of CO and a hydrophobic cyanide analogue, butyl isocyanide (BIC), to dithionite-reduced b6f complex perturbs and significantly shifts the redox difference visible spectrum. Together with EPR spectra displaying ‘g’ values of the oxidized complex at 6.7 and 7.4, the character of heme x is defined to be ferric high spin in a rhombic environment. In addition to a possible function in photosystem I-linked cyclic electron transport, the 5-coordinate state implies that there is at least one more function of heme x that is related to axial binding of a physiological ligand.