Evidence for a unique Rieske iron-sulphur centre in Heliobacterium chlorum

An iron sulphur centre (g_z = 2.035, g_y =1.89, g_x =1.81), which can be observed in both whole cells and isolated plasma membranes of Heliobacterium chlorum, was identified as a Rieske centre on the basis of its sensitivity to the inhibitors 2,5-dibromo-3-methyl-6-isopropyl-benzoquinone (DBMIB), 5-(n-undecyl)-6-hydroxy-4,7-dioxobenzothiazole (UHDBT) and stigmatellin as well as on the basis of its orientation (gx, is oriented perpendicular and g_y is oriented parallel to the membrane plane). Its midpoint potential is unusually low (E_m,7= +120 ± 10 mV) and does not depend on pH in the range between pH 6.5 and pH 8.0. The effects of the inhibitors on the EPR spectrum are altered compared to Rieske centres in other systems. The significance of the low E_m is discussed with regard to the overall midpoint potentials of the electron transfer chain in Heliobacterium chlorum.