Structure and Function of CytochromebcComplexes

Abstract

Key Words oxidoreductase, respiratory chain, electron transfer, crystallography, membrane protein s Abstract The cytochrome bc complexes represent a phylogenetically diverse group of complexes of electron-transferring membrane proteins, most familiarly represented by the mitochondrial and bacterial bc 1 complexes and the chloroplast and cyanobacterial b 6 f complex. All these complexes couple electron transfer to proton translocation across a closed lipid bilayer membrane, conserving the free energy released by the oxidation-reduction process in the form of an electrochemical proton gradient across the membrane. Recent exciting developments include the application of site-directed mutagenesis to define the role of conserved residues, and the emergence over the past five years of X-ray structures for several mitochondrial complexes, and for two important domains of the b 6 f complex. CONTENTS

Engineering a Three-cysteine, One-histidine Ligand Environment into a New Hyperthermophilic Archaeal Rieske-type [2Fe-2S] Ferredoxin from Sulfolobus solfataricus

“…Thus, this archaeal protein does not contain any solvent-exposed disulfide linkage, 5 unlike the high potential Rieske proteins found as a part of aerobic respiratory chain and photosynthesis, and appears to be suitable for the ligand substitution studies. 4 See www-ssrl.slac.stanford.edu/exafspak.html. 5 The absence of any disulfide linkage in the wild-type ARF suggests that its remarkable stability must be found in a small number of highly specific alterations in the protein (e.g., charge clusters, networks of hydrogen bonds, optimization of packing, hydrophobic interactions, and charge-charge interactions on the surface of proteins) that often do not obey any obvious traffic rules in hyperthermophilic enzymes (for reviews, see Refs.…”

“…Earlier RR studies on the pH dependence of the oxidized forms of archaeal and bacterial high potential Rieske proteins (as models for components of proton-translocating respiratory complexes) (17,20,29) have been considered to be the primarily spectroscopic supporting source for the common view to assign the pK a,ox near ϳ8, which influences the E m and the visible CD spectrum, to one of the terminal histidinyl ligands (1)(2)(3)(4). Although this is chemically reasonable and likely in light of available crystal structural information (9, 11), actual experimental evidence for deprotonation of the liganding histidine imidazole groups is very weak because (i) no rigorous assignment of Fe-N stretching frequencies has been made in the RR spectra of these high potential Rieske proteins and (ii) the variations of vibrational modes with respect to those of low potential Rieske-type proteins, which do not show redox-linked ionization in the physiological pH range (1, 3), have remained poorly understood.…”

“…4 Calibration and background removal were performed according to established procedures (40,42). Theoretical amplitude and phase shift functions were calculated using the ab initio code FEFF 8.2 (43).…”

“…Proteins containing Rieske-type [2Fe-2S] clusters are widespread in nature from hyperthermophilic Archaea and Bacteria to Eukarya and play critical electron transfer roles in various pathways such as aerobic respiration, photosynthesis, and biodegradation of various alkene and aromatic compounds (1)(2)(3)(4). In contrast to regular plant-and vertebrate-type ferredoxins having complete cysteinyl ligations, the Rieske-type cluster has an asymmetric iron-sulfur core with the S ␥ atom of each of the two cysteine residues coordinated to one iron site and the N ␦ atom of each of the two histidine residues coordinated to the other iron site.…”

“…One type displays higher reduction potentials (E m ) 1 of approximately ϩ150 to ϩ490 mV and occurs in proton-translocating respiratory complexes (cytochrome bc 1 /b 6 f complexes and their archaeal homologs without c-type cytochromes), being involved in not only electron transfer but also substrate binding and oxidation at the quinol-oxidizing Q o site (2)(3)(4)(5)(12)(13)(14)(15). The other type displays lower E m values of approximately Ϫ150 to Ϫ50 mV and has been found in a diverse group of bacterial multicomponent terminal oxygenases and soluble Rieske-type ferredoxins (1, 3, 8, 9, 16 -27).…”