13 kDa), but are devoid of a b-type cytochrome [7]. The QPs Abstract The axial ligands of low potential cytochrome b560 in in SQRs from bovine heart mitochondria [8,9] and E. coli the five subunit bovine heart succinate-ubiquinone reductase com- [10,11] both contain one cytochrome b, but differ in the number plex and in the isolated quinone binding proteins have been invesand size of the constituent subunits (14-, 11-and 9-kDa subtigated using EPR and near-infrared magnetic circular dichroism spectroscopies. The results are consistent with bis-histidine ligaunits for bovine heart QPs [3,4]; 14-and 13-kDa subunits for tion with near-perpendicular imidazole rings for cytochrome b56oE. coli QPs [12,13]). Moreover, the cytochrome bs in these in the four-subunit complex. The pronounced changes in EPR enzymes differ greatly in their midpoint potentials; E m = +36 properties that accompany isolation of the cytochrome-b560 conmV and -180 mV for E. coli and bovine heart SQR, However, this was before the presence of distinct low and high Bovine heart succinate-quinone reductase (SQR) is a mempotential hemes was established in B. subtilis cytochrome b558 brane-bound component of the mitochondrial respiratory [14]. Therefore, although these results taken together attest to chain. It catalyzes the oxidation of succinate to fumarate and bis-histidyl ligation for the high potential cytochrome bs (charthe reduction of ubiquinone, thereby contributing electrons acterized by HALS EPR signals with ramped line shapes for ultimately to oxidative phosphorylation and energy transducthe low-field component), the axial ligation of the low potential tion. The purified complex can be resolved into two reconstitucytochrome bs remains ill-defined. The low potential cytotively active fractions [1,2]. One fraction, composed of the two chrome b560 of bovine heart SQR exhibits a HALS-type EPR larger subunits (70 and 27 kDa), is the soluble succinate dehysignal with a Gaussian-shaped low-field component at g = 3.46, drogenase (SDH) which contains a flavin moiety as well as but this resonance is replaced by two new resonances with low three Fe-S clusters. The other fraction, termed the quinone field g-values at 3.07 and 2.92 in purified QPs [8]. In this work, binding proteins (QPs), anchors SDH to the mitochondrial the two complementary techniques of EPR and MCD have inner membrane and comprises three smaller subunits (14, 11, been used to investigate the axial ligation of the low potential and 9 kDa) [3,4]. This membrane-bound fraction contains the cytochrome b560 in bovine heart SQR and the isolated QPs. cytochrome b560 that is the focus of this paper [5,6].As their name suggests, the QPs facilitate interaction of 2. Materials and methods SDHs and the highly homologous fumarate reductases (FRDs) with quinones. However, a specific role for the heme group in Bovine heart SQR and QPs were purified and assayed as previously mediating electron transfer to or from quinone has yet to be described [8]. Samples for spectroscopic measurements were prepar...