Characterization of the High-spin Heme x in the Cytochrome b{sub 6}f Complex of Oxygenic Photosynthesis
X-ray structures at 3.0-3.1 {angstrom} resolution of the ccterizytochrome b{sub 6}f 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 b{sub 6} polypeptide. Heme x faces the inter-monomer quinone exchange cavity. The only axial ligand associated with this heme is a H{sub 2}O or OH{sup -} that is H-bonded to the propionate of the stromal side heme b{sub n}, 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 b{sub 6}f 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.
- Research Organization:
- Pacific Northwest National Lab., Richland, WA (US), Environmental Molecular Sciences Laboratory (US)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 15011663
- Report Number(s):
- PNWD-SA-6673; 2358
- Journal Information:
- Biochemistry, Journal Name: Biochemistry Journal Issue: 51 Vol. 43
- Country of Publication:
- United States
- Language:
- English
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