Interactions between the Cytochrome b, Cytochrome c1, and Fe−S Protein Subunits at the Ubihydroquinone Oxidation Site of the bc1 Complex of Rhodobacter capsulatus (original) (raw)

Structural basis of functions of the mitochondrial cytochrome bc1 complex

Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1998

The crystal structure of the cytochrome bcl complex (ubiquinol-cytochrome c reductase) from bovine heart submitochondria was determined at 2.9 ~ resolution. The bcl complex in crystal exists as a closely interacting dimer, suggesting that the dimer is a functional unit. Over half of the mass of the complex, including subunits core 1 and core 2, are on the matrix side of the membrane, while most of the cytochrome b subunit is located within the membrane. There are 13 transmembrane helices in each monomer, eight of them belonging to cytochrome b. Two large cavities are made of the transmembrane helices D, C, F and H in one monomer and helices D' and E' from the other monomer of cytochrome b, and the transmembrane helices of cl, iron-sulfur protein (ISP), and subunits 10 and 11. These cavities provide entrances for ubiquinone or inhibitor and connect the Qi pocket of one monomer and the Qo pocket of the other monomer. Ubiquinol made at the Q~ site of one monomer can proceed to the nearby Qo site of the other monomer without having to leave the bcl complex. The soluble parts of cytochrome cl and ISE including their redox prosthetic groups, are located on the cytoplasmic side of the membrane. The distances between the four redox centers in the complex have been determined, and the binding sites for several electron transfer inhibitors have been located. Structural analysis of the protein/inhibitor complexes revealed that the extramembrane domain of the Rieske iron-sulfur protein may undergo substantial movement during the catalytic cycle of the complex. The Rieske protein movement and the larger than expected distance between FeS and cytochrome c 1 heme suggest that electron transfer reaction between FeS and cytochrome c 1 may involve movements or conformational changes in the soluble domain of iron-sulfur protein. The inhibitory function of E-13-methoxyacrylatestilbene and myxothiazol may result from the increase of mobility in ISP, whereas the function of stigmatellin and 5-undecyl-6-hydroxy-4,7-dioxobenzothiazole may result from the immobilization of ISE © 1998 Elsevier Science B.V.

Physicochemical Aspects of the Movement of the Rieske Iron Sulfur Protein during Quinol Oxidation by the bc 1 Complex from Mitochondria and Photosynthetic Bacteria †

Biochemistry, 1999

Crystallographic structures for the mitochondrial ubihydroquinone:cytochrome c oxidoreductase (bc 1 complex) from different sources, and with different inhibitors in cocrystals, have revealed that the extrinsic domain of the iron sulfur subunit is not fixed [Zhang677-684], but moves between reaction domains on cytochrome c 1 and cytochrome b subunits. We have suggested that the movement is necessary for quinol oxidation at the Q o site of the complex. In this paper, we show that the electron-transfer reactions of the high-potential chain of the complex, including oxidation of the iron sulfur protein by cytochrome c 1 and the reactions by which oxidizing equivalents become available at the Q o site, are rapid compared to the rate-determining step. Activation energies of partial reactions that contribute to movement of the iron sulfur protein have been measured and shown to be lower than the high activation barrier associated with quinol oxidation. We conclude that the movement is not the source of the activation barrier. We estimate the occupancies of different positions for the iron sulfur protein from the crystallographic electron densities and discuss the parameters determining the binding of the iron sulfur protein in different configurations. The low activation barrier is consistent with a movement between these locations through a constrained diffusion. Apart from ligation in enzymesubstrate or inhibitor complexes, the binding forces in the native structure are likely to be eRT, suggesting that the mobile head can explore the reaction interfaces through stochastic processes within the time scale indicated by kinetic measurements.

X-Ray Structure of Rhodobacter Capsulatus Cytochrome bc 1 : Comparison with its Mitochondrial and Chloroplast Counterparts

Photosynthesis Research, 2004

Ubihydroquinone: cytochrome (cyt) c oxidoreductase, or cyt bc 1 , is a widespread, membrane integral enzyme that plays a crucial role during photosynthesis and respiration. It is one of the major contributors of the electrochemical proton gradient, which is subsequently used for ATP synthesis. The simplest form of the cyt bc 1 is found in bacteria, and it contains only the three ubiquitously conserved catalytic subunits: the FeS protein, cyt b and cyt c 1. Here we present a preliminary X-ray structure of Rhodobacter capsulatus cyt bc 1 at 3.8 Å, and compare it to the available structures of its homologues from mitochondria and chloroplast. Using the bacterial enzyme structure, we highlight the structural similarities and differences that are found among the three catalytic subunits between the the different members of this family of enzymes. In addition, we discuss the locations of currently known critical mutations, and their implications in terms of the cyt bc 1 catalysis.

Cloning and Dna Sequencing of the FBC Operon Encoding the Cytochrome BC1 Complex from Rhodobacter Sphaeroides : Characterization of FBC Deletion Mutan TS and Complementation by a Site-Specific Mutational Variant

European Journal of Biochemistry, 1990

The ubiquinol : cytochrome-c oxidoreductase (cytochrome bcl complex) is a central component of the mitochondrial respiratory chain as well as the respiratory and/or photosynthetic systems of numerous prokaryotic organisms. In Rhodobacter sphaeroides, the bel complex has a dual function. When the cells are grown photosynthetically, the bel complex is present in the intracytoplasmic membrane and is a critical component of the cyclic electron transport system. When the cells are grown in the dark in the presence of oxygen, the same bcl complex is a necessary component of the cytochrome-c2-dependent respiratory chain. The fact that the bel complex from R. sphaeroides has been extensively studied, plus the ability to manipulate this organism genetically, makes this an ideal system for using site-directed mutagenesis to address questions relating to the structure and function of the bel complex.

Q-cycle bypass reactions at the Qo site of the cytochrome bc1 (and related) complexes

Methods in enzymology, 2004

The Q-Cycle and Its Bypass Reactions The cytochrome (cyt) bc 1 complex plays a central role in chemiosmotic energy conversion in mitochondria and many bacteria, oxidizing quinol (QH 2)-ubihydroquinone (UQH 2) in the case of mitochondria-and reducing a soluble electron carrier (cyt c in mitochondria), while acting as a proton ''shuttle'' or translocator to store energy in an electrochemical proton gradient, or proton motive force (pmf). 1,2 The pmf in turn drives the synthesis of ATP at the F 0-F 1-ATP synthase. The structurally analogous cyt b 6 f complex of chloroplasts plays a similar role-but oxidizes plastohydroquinone (PQH 2)-during oxygenic photosynthesis. The cyt bc 1 /b 6 f enzymes are dimeric integral membrane complexes composed of as few as three subunits in some prokaryotes and up to eleven subunits in mitochondria. The proteins house four essential redox-active, metal centers in two distinct chains. The ''low potential chain'' consists of two b-type hemes, cyt b H and cyt b L with relatively higher and lower redox potentials, housed in a single cyt b protein. The ''high potential chain'' consists of a ''Rieske'' iron-sulfur complex (2Fe2S) in the Rieske iron-sulfur protein (ISP) and another relatively high potential carrier, cyt c 1 in the cyt bc 1 complex, cyt f in the cyt b 6 f complex, and an assortment of other carriers in complexes from distantly related bacteria. 3,4 The cyt bc 1 and b 6 f complexes also possess two quinone/quinol binding sites. One of these, the Q o site is located on the p-side of the membrane (the intermembrane space in mitochondria), at the interface between cyt b L and the ISP and acts during normal turnover to oxidize QH 2 to quinone (Q). The other site, termed the Q i site, is located close to cyt b H , towards the n-side of the membrane, and acts to reduce Q to QH 2. Biochemical studies and X-ray crystal structures of mitochondrial cyt bc 1 complexes show that the Q o site possesses proximal and distal Q o site