Surface-modulated motion switch: Capture and release of iron–sulfur protein in the cytochrome bc ₁ complex

Abstract

In the cytochrome bc1 complex, the swivel motion of the ironsulfur protein (ISP) between two redox sites constitutes a key component of the mechanism that achieves the separation of the two electrons in a substrate molecule at the quinol oxidation (Qo) site. The question remaining is how the motion of ISP is controlled so that only one electron enters the thermodynamically favorable chain via ISP. An analysis of eight structures of mitochondrial bc1 with bound Qo site inhibitors revealed that the presence of inhibitors causes a bidirectional repositioning of the cd1 helix in the cytochrome b subunit. As the cd1 helix forms a major part of the ISP binding crater, any positional shift of this helix modulates the ability of cytochrome b to bind ISP. The analysis also suggests a mechanism for reversal of the ISP fixation when the shape complementarity is significantly reduced after a positional reorientation of the reaction product quinone. The importance of shape complementarity in this mechanism was confirmed by functional studies of bc1 mutants and by a structure determination of the bacterial form of bc1. A mechanism for the high fidelity of the bifurcated electron transfer is proposed.crystal structures ͉ electron transfer ͉ inhibitor binding ͉ mechanism W ithin cellular energy-conserving membranes (the mitochondrial inner membrane in eukaryotes and the plasma membrane in prokaryotes), the only mobile carriers of redox equivalents are ubiquinone (Q), ubiquinol (QH 2 ), and their derivatives. In the cytochrome (cyt) bc 1 complex (cyt bc 1 or bc 1 ) of the respiratory chain, QH 2 is oxidized to Q, and concomitantly, protons are pumped against the gradient across the membrane [from the matrix to the intermembrane space (IMS) in mitochondria and from the cytoplasm to the periplasm in prokaryotes], thus contributing to the electrochemical potential that drives ATP synthesis. The proposed mechanism by which bc 1 performs this dual task, the Q-cycle hypothesis (1), has been widely accepted; it suggests that for the oxidation of two molecules of QH 2 , one molecule of Q is reduced at separate catalytic sites (Fig. 1A). According to this mechanism, the Q reduction site is located near the negative side of the membrane (Q i ͞Q n , the mitochondrial matrix or cytoplasmic side) and is electronically linked via two prosthetic groups [high-potential b heme (b H ) and low-potential b heme (b L )] to the quinol oxidation site located close to the positive side of the membrane (Q o ͞Q p , the mitochondrial IMS side or periplasmic side in prokaryotes). At the Q o site, one electron from a substrate enters the energetically favorable path that leads via the iron-sulfur protein (ISP) and cyt c 1 subunit (cyt c 1 ) to the substrate cyt c, whereas the other electron proceeds via the b L and b H heme groups to Q (or semiquinone) bound at the Q i site. The half-reduction of a Q per oxidized quinol is achieved by the bifurcation of the electron pathway at the Q o site. In a complete catalytic cycle, two protons are taken up from the ma...

Electronic Connection Between the Quinone and CytochromecRedox Pools and Its Role in Regulation of Mitochondrial Electron Transport and Redox Signaling

“…Comparative analysis of various crystal structures containing different inhibitors of the Q o site identified the region of quinone binding consistent in amino acid composition with earlier mapping by site-directed mutagenesis (39,59). In addition, it showed displacements of polypeptide backbone in the region expected for the quinone binding (94,268).…”

“…These studies allowed the description of several conformational states where the movement is affected by mutations and, unlike in the native system, becomes rate-limiting for electron transfer. There is an ongoing debate (22) as to whether the movement of the FeS head domain is controlled by specific conformational changes in the region of substrate binding (94,268) or by allostery (50,52,55,187) or whether the changes between the b-and c-state are rather stochastic (49,60,199).…”

Domain movement of iron sulfur protein in cytochrome bc1 complex is facilitated by the electron transfer from cytochrome bL to bH

“…Based on the effect on head domain movement of ISP, Qp site inhibitors [21] can be divided into two groups: Pm, such as MOAS, which enhances movement and Pf, such as stigmatellin, which fixes the head domain at the b-position [19,22]. By examining the redox state dependent inhibitory efficacy of these inhibitors, one can gain insight concerning the molecular event that facilitates the movement of the head domain of ISP.…”

Inhibitor-complexed Structures of the Cytochrome bc1 from the Photosynthetic Bacterium Rhodobacter sphaeroides

“…3A). However, it is functionally important, as the point mutation S322A or deletion of residues 309 -326 significantly lowers the enzyme activity (46). The ef1-helix may play an important role in lipid binding, as features of several potential lipid molecules are visible in the electron density (Figs.…”