PARENT SESSION

Symposium S8A Cytochrome b-c complexes
Friday September 3rd, 2004 8:30 AM-10:30 AM Room 511D
Chair: Fevzi Daldal
Co-Chair: David Kramer

Spectroscopic properties of heme x in the cytochrome b₆f complex. Huamin Zhang¹, Micheal Bowman², David Kramer³, William Cramer¹, Jiusheng Yan^*,1, ¹ Purdue University, West Lafayette, IN, USA² Structural Biology and Microimaging, Richland, WA, USA³ Institute of Biological Chemistry, Pullman, WA, USA

ABSTRACT- X-ray structures at 3.0 -3.1 resolution of the cytochrome b₆f complex from the thermophilic cyanobacterium, Mastigocladus luminosus (Kurisu et al., 2003), and the green alga, C. reinhardtii (Stroebel et al., 2003), showed the presence of a heme facing the inter-monomer quinone exchange cavity. The heme is covalently linked by a covalent thioether bond to a single Cys residue (Cys35) on the n-side of the cytochrome b₆ polypeptide. The one axial ligand is a water or OH- that is H-bonded to the propionate of the stromal side heme bn. Electron coupling and rapid electron transfer must exist between the two hemes. The unique ligation of this heme has led to a tentative designation as ^"heme x^". Pyridine hemochromagen reduced minus oxidized difference spectra for heme x isolated from SDS-PAGE show a broad spectrum of low amplitude with a peak at 553 nm, similar to that of other hemes with a single thioether linkage. EPR spectra define heme x to be ferric high spin (g values at 6.7 and 7.4) in a rhombic environment. The strong field ligands, CO or a hydrophobic cyanide analogue, bind to dithionite-reduced b₆f complex and perturb the visible spectrum, and the latter inhibits electron transfer activity. (Supported by NIHGM-38323).

KEY WORDS: heme x, cytochrome b6f complex, spectroscopy