Primary structure of a novel subunit in ba3-cytochrome oxidase from Thermus thermophilus

Subunit CcoQ is involved in the assembly of the Cbb3-type cytochrome c oxidases from Pseudomonas stutzeri ZoBell but not required for their activity

The Cbb₃-type cytochrome c oxidases (Cbb₃-CcOs), the second most abundant CcOs, catalyze the reduction of molecular oxygen to water, even at micromolar oxygen concentrations. In Pseudomonas stutzeri ZoBell, two tandemly organized cbb₃-operons encode the isoforms Cbb₃-1 and Cbb₃-2 both possessing subunits CcoN, CcoO and CcoP. However, only the cbb₃-2 operon contains an additional ccoQ gene. CcoQ consists of 62 amino acids and is predicted to possess one transmembrane spanning helix. The physiological role of CcoQ was investigated based on a CcoQ-deletion mutant and wild-type Cbb₃-2 crystals not containing subunit CcoQ. Cbb₃-2 isolated from the deletion mutant is inactive and appears as a dispersed band on blue native-PAGE gels. Surprisingly, in the absence of ccoQ, Cbb₃-1 also shows a strongly reduced activity. Our data suggest that CcoQ primarily functions as an assembly factor for Cbb₃-2 but is also required for correct assembly of Cbb₃-1. In contrast, once correctly assembled, Cbb₃-1 and Cbb₃-2 possess a full enzymatic activity even in the absence of CcoQ.

Identification and Characterization of the Novel Subunit CcoM in the cbb3₃Cytochrome c Oxidase from Pseudomonas stutzeri ZoBell

Cytochrome c oxidases (CcOs), members of the heme-copper containing oxidase (HCO) superfamily, are the terminal enzymes of aerobic respiratory chains. The cbb₃-type cytochrome c oxidases (cbb₃-CcO) form the C-family and have only the central catalytic subunit in common with the A- and B-family HCOs. In Pseudomonas stutzeri, two cbb₃ operons are organized in a tandem repeat. The atomic structure of the first cbb₃ isoform (Cbb₃-1) was determined at 3.2 Å resolution in 2010 (S. Buschmann, E. Warkentin, H. Xie, J. D. Langer, U. Ermler, and H. Michel, Science 329:327–330, 2010, http://dx.doi.org/10.1126/science.1187303). Unexpectedly, the electron density map of Cbb₃-1 revealed the presence of an additional transmembrane helix (TMH) which could not be assigned to any known protein. We now identified this TMH as the previously uncharacterized protein PstZoBell_05036, using a customized matrix-assisted laser desorption ionization (MALDI)–tandem mass spectrometry setup. The amino acid sequence matches the electron density of the unassigned TMH. Consequently, the protein was renamed CcoM. In order to identify the function of this new subunit in the cbb₃ complex, we generated and analyzed a CcoM knockout strain. The results of the biochemical and biophysical characterization indicate that CcoM may be involved in CcO complex assembly or stabilization. In addition, we found that CcoM plays a role in anaerobic respiration, as the ΔCcoM strain displayed altered growth rates under anaerobic denitrifying conditions.

The respiratory chain has recently moved into the focus for drug development against prokaryotic human pathogens, in particular, for multiresistant strains (P. Murima, J. D. McKinney, and K. Pethe, Chem Biol 21:1423–1432, 2014, http://dx.doi.org/10.1016/j.chembiol.2014.08.020). cbb₃-CcO is an essential enzyme for many different pathogenic bacterial species, e.g., Helicobacter pylori, Vibrio cholerae, and Pseudomonas aeruginosa, and represents a promising drug target. In order to develop compounds targeting these proteins, a detailed understanding of the molecular architecture and function is required. Here we identified and characterized a novel subunit, CcoM, in the cbb₃-CcO complex and thereby completed the crystal structure of the Cbb₃ oxidase from Pseudomonas stutzeri, a bacterium closely related to the human pathogen Pseudomonas aeruginosa.

Multilayered polyelectrolyte microcapsules: interaction with the enzyme cytochrome C oxidase

Cell-sized polyelectrolyte capsules functionalized with a redox-driven proton pump protein were assembled for the first time. The interaction of polyelectrolyte microcapsules, fabricated by electrostatic layer-by-layer assembly, with cytochrome c oxidase molecules was investigated. We found that the cytochrome c oxidase retained its functionality, that the functionalized microcapsules interacting with cytochrome c oxidase were permeable and that the permeability characteristics of the microcapsule shell depend on the shell components. This work provides a significant input towards the fabrication of an integrated device made of biological components and based on specific biomolecular functions and properties.

Single mutations that redirect internal proton transfer in the ba3 oxidase from Thermus thermophilus

The ba3-type cytochrome c oxidase from Thermus thermophilus is a membrane-bound proton pump. Results from earlier studies have shown that with the aa3-type oxidases proton uptake to the catalytic site and "pump site" occurs simultaneously. However, with ba3 oxidase the pump site is loaded before proton transfer to the catalytic site because the proton transfer to the latter is slower than that with the aa3 oxidases. In addition, the timing of formation and decay of catalytic intermediates is different in the two types of oxidases. In the present study, we have investigated two mutant ba3 CytcOs in which residues of the proton pathway leading to the catalytic site as well as the pump site were exchanged, Thr312Val and Tyr244Phe. Even though ba3 CytcO uses only a single proton pathway for transfer of the substrate and "pumped" protons, the amino-acid residue substitutions had distinctly different effects on the kinetics of proton transfer to the catalytic site and the pump site. The results indicate that the rates of these reactions can be modified independently by replacement of single residues within the proton pathway. Furthermore, the data suggest that the Thr312Val and Tyr244Phe mutations interfere with a structural rearrangement in the proton pathway that is rate limiting for proton transfer to the catalytic site.

In silico and in vitro studies to elucidate the role of Cu2+ and galanthamine as the limiting step in the amyloid beta (1-42) fibrillation process

The formation of fibrils and oligomers of amyloid beta (Aβ) with 42 amino acid residues (Aβ 1-42 ) is the most important pathophysiological event associated with Alzheimer's disease (AD). The formation of Aβ fibrils and oligomers requires a conformational change from an α-helix to a β-sheet conformation, which is encouraged by the formation of a salt bridge between Asp 23 or Glu 22 and Lys 28. Recently, Cu(2+) and various drugs used for AD treatment, such as galanthamine (Reminyl(®) ), have been reported to inhibit the formation of Aβ fibrils. However, the mechanism of this inhibition remains unclear. Therefore, the aim of this work was to explore how Cu(2+) and galanthamine prevent the formation of Aβ1-42 fibrils using molecular dynamics (MD) simulations (20 ns) and in vitro studies using fluorescence and circular dichroism (CD) spectroscopies. The MD simulations revealed that Aβ1-42 acquires a characteristic U-shape before the α-helix to β-sheet conformational change. The formation of a salt bridge between Asp 23 and Lys 28 was also observed beginning at 5 ns. However, the MD simulations of Aβ 1-42 in the presence of Cu(2+) or galanthamine demonstrated that both ligands prevent the formation of the salt bridge by either binding to Glu 22 and Asp 23 (Cu(2+) ) or to Lys 28 (galanthamine), which prevents Aβ 1-42 from adopting the U-characteristic conformation that allows the amino acids to transition to a β-sheet conformation. The docking results revealed that the conformation obtained by the MD simulation of a monomer from the 1Z0Q structure can form similar interactions to those obtained from the 2BGE structure in the oligomers. The in vitro studies demonstrated that Aβ remains in an unfolded conformation when Cu(2+) and galanthamine are used. Then, ligands that bind Asp 23 or Glu 22 and Lys 28 could therefore be used to prevent β turn formation and, consequently, the formation of Aβ fibrils.

The hyperthermophilic bacterium Aquifex aeolicus: from respiratory pathways to extremely resistant enzymes and biotechnological applications

Aquifex aeolicus isolated from a shallow submarine hydrothermal system belongs to the order Aquificales which constitute an important component of the microbial communities at elevated temperatures. This hyperthermophilic chemolithoautotrophic bacterium, which utilizes molecular hydrogen, molecular oxygen, and inorganic sulfur compounds to flourish, uses the reductive TCA cycle for CO(2) fixation. In this review, the intricate energy metabolism of A. aeolicus is described. As the chemistry of sulfur is complex and multiple sulfur species can be generated, A. aeolicus possesses a multitude of different enzymes related to the energy sulfur metabolism. It contains also membrane-embedded [NiFe] hydrogenases as well as oxidases enzymes involved in hydrogen and oxygen utilization. We have focused on some of these proteins that have been extensively studied and characterized as super-resistant enzymes with outstanding properties. We discuss the potential use of hydrogenases in an attractive H(2)/O(2) biofuel cell in replacement of chemical catalysts. Using complete genomic sequence and biochemical data, we present here a global view of the energy-generating mechanisms of A. aeolicus including sulfur compounds reduction and oxidation pathways as well as hydrogen and oxygen utilization.

Structural insights into electron transfer in caa3-type cytochrome oxidase

Cytochrome c oxidase is a member of the heme copper oxidase superfamily (HCO)¹. HCOs function as the terminal enzymes in the respiratory chain of mitochondria and aerobic prokaryotes, coupling molecular oxygen reduction to transmembrane proton pumping. Integral to the enzyme’s function is the transfer of electrons from cytochrome c to the oxidase via a transient association of the two proteins. Electron entry and exit are proposed to occur from the same site on cytochrome c^2–4. Here we report the crystal structure of the caa₃-type cytochrome oxidase from Thermus thermophilus, which has a covalently tethered cytochrome c domain. Crystals were grown in a bicontinuous mesophase using a synthetic short-chain monoacylglycerol as the hosting lipid. From the electron density map, at 2.36 Å resolution, a novel integral membrane subunit and a native glycoglycerophospholipid embedded in the complex were identified. Contrary to previous electron transfer mechanisms observed for soluble cytochrome c, the structure reveals the architecture of the electron transfer complex for the fused cupredoxin/cytochrome c domain which implicates different sites on cytochrome c for electron entry and exit. Support for an alternative to the classical proton gate characteristic of this HCO class is presented.

Bioenergetics at extreme temperature: Thermus thermophilus ba(3)- and caa(3)-type cytochrome c oxidases

Seven years into the completion of the genome sequencing projects of the thermophilic bacterium Thermus thermophilus strains HB8 and HB27, many questions remain on its bioenergetic mechanisms. A key fact that is occasionally overlooked is that oxygen has a very limited solubility in water at high temperatures. The HB8 strain is a facultative anaerobe whereas its relative HB27 is strictly aerobic. This has been attributed to the absence of nitrate respiration genes from the HB27 genome that are carried on a mobilizable but highly-unstable plasmid. In T. thermophilus, the nitrate respiration complements the primary aerobic respiration. It is widely known that many organisms encode multiple biochemically-redundant components of the respiratory complexes. In this minireview, the presence of the two cytochrome c oxidases (CcO) in T. thermophilus, the ba(3)- and caa(3)-types, is outlined along with functional considerations. We argue for the distinct evolutionary histories of these two CcO including their respective genetic and molecular organizations, with the caa(3)-oxidase subunits having been initially 'fused'. Coupled with sequence analysis, the ba(3)-oxidase crystal structure has provided evolutionary and functional information; for example, its subunit I is more closely related to archaeal sequences than bacterial and the substrate-enzyme interaction is hydrophobic as the elevated growth temperature weakens the electrostatic interactions common in mesophiles. Discussion on the role of cofactors in intra- and intermolecular electron transfer and proton pumping mechanism is also included.

Heme-copper terminal oxidase using both cytochrome c and ubiquinol as electron donors

The cytochrome c oxidase Cox2 has been purified from native membranes of the hyperthermophilic eubacterium Aquifex aeolicus. It is a cytochrome ba(3) oxidase belonging to the family B of the heme-copper containing terminal oxidases. It consists of three subunits, subunit I (CoxA2, 63.9 kDa), subunit II (CoxB2, 16.8 kDa), and an additional subunit IIa of 5.2 kDa. Surprisingly it is able to oxidize both reduced cytochrome c and ubiquinol in a cyanide sensitive manner. Cox2 is part of a respiratory chain supercomplex. This supercomplex contains the fully assembled cytochrome bc(1) complex and Cox2. Although direct ubiquinol oxidation by Cox2 conserves less energy than ubiquinol oxidation by the cytochrome bc(1) complex followed by cytochrome c oxidation by a cytochrome c oxidase, ubiquinol oxidation by Cox2 is of advantage when all ubiquinone would be completely reduced to ubiquinol, e.g., by the sulfidequinone oxidoreductase, because the cytochrome bc(1) complex requires the presence of ubiquinone to function according to the Q-cycle mechanism. In the case that all ubiquinone has been reduced to ubiquinol its reoxidation by Cox2 will enable the cytochrome bc(1) complex to resume working.

SpyA is a membrane-bound ADP-ribosyltransferase of Streptococcus pyogenes which modifies a streptococcal peptide, SpyB

All sequenced genomes of Streptococcus pyogenes (Group A Streptococcus, GAS) encode a protein, SpyA, with homology to C3-like ADP-ribosyltransferase toxins. SpyA is a novel virulence factor which plays a role in pathogenesis in a mouse model of soft-tissue infection. In this study we demonstrate that SpyA is a surface-exposed membrane protein which is anchored to the streptococcal membrane by an N-terminal transmembrane sequence. We identified a small gene upstream of spyA, designated spyB, which encodes a peptide of 35 amino acids, and is co-transcribed with spyA. Expression of spyBA is strongly influenced by translational coupling: mutational inactivation of spyB translation completely abolishes translation of spyA. spyB expression increases with increasing cell density and reaches its maximum at late exponential growth phase. The SpyB N-terminus is predicted to fold into an amphipathic α-helix, a structural motif that targets a protein to the cytoplasmic membrane. Consistent with the prediction, we found that a SpyB fusion with peptide affinity tags is located in the streptococcal membrane. An ADP-ribosylation assay with recombinant SpyA demonstrated that SpyA modifies SpyB. Thus, our study suggests that ADP-ribosylation of SpyB may be an important function of SpyA.

The elusive third subunit IIa of the bacterial B-type oxidases: the enzyme from the hyperthermophile Aquifex aeolicus

The reduction of molecular oxygen to water is catalyzed by complicated membrane-bound metallo-enzymes containing variable numbers of subunits, called cytochrome c oxidases or quinol oxidases. We previously described the cytochrome c oxidase II from the hyperthermophilic bacterium Aquifex aeolicus as a ba₃-type two-subunit (subunits I and II) enzyme and showed that it is included in a supercomplex involved in the sulfide-oxygen respiration pathway. It belongs to the B-family of the heme-copper oxidases, enzymes that are far less studied than the ones from family A. Here, we describe the presence in this enzyme of an additional transmembrane helix “subunit IIa”, which is composed of 41 amino acid residues with a measured molecular mass of 5105 Da. Moreover, we show that subunit II, as expected, is in fact longer than the originally annotated protein (from the genome) and contains a transmembrane domain. Using Aquifex aeolicus genomic sequence analyses, N-terminal sequencing, peptide mass fingerprinting and mass spectrometry analysis on entire subunits, we conclude that the B-type enzyme from this bacterium is a three-subunit complex. It is composed of subunit I (encoded by coxA₂) of 59000 Da, subunit II (encoded by coxB₂) of 16700 Da and subunit IIa which contain 12, 1 and 1 transmembrane helices respectively. A structural model indicates that the structural organization of the complex strongly resembles that of the ba₃ cytochrome c oxidase from the bacterium Thermus thermophilus, the IIa helical subunit being structurally the lacking N-terminal transmembrane helix of subunit II present in the A-type oxidases. Analysis of the genomic context of genes encoding oxidases indicates that this third subunit is present in many of the bacterial oxidases from B-family, enzymes that have been described as two-subunit complexes.

Mutation analysis of the interaction of B-type cytochrome c oxidase with its natural substrate cytochrome c-551

Heme-copper oxidases in the respiratory chain are classified into three subfamilies: A-, B- and C-types. Cytochrome bo(3)-type cytochrome c oxidase from thermophilic Bacillus is a B-type oxidase that is thought to interact with cytochrome c through hydrophobic interactions. This is in contrast to A-type oxidases, which bind cytochrome c molecules primarily through electrostatic forces between acidic residues in the oxidase subunit II and basic residues within cytochromes. In order to investigate the substrate-binding site in cytochrome bo(3), eight acidic residues in subunit II were mutated to corresponding neutral residues and enzymatic activity was measured using cytochrome c-551 from closely related Bacillus PS3. The mutation of E116, located at the interface to subunit I, decreased the k(cat) value most prominently without affecting the K(m) value, indicating that the residue is important for electron transfer. The mutation of D99, located close to the Cu(A) site, largely affected both values, suggesting that it is important for both electron transfer and substrate binding. The mutation of D49 and E84 did not affect enzyme kinetic parameters, but the mutation of E64, E66 and E68 lowered the affinity of cytochrome bo(3) for cytochrome c-551 without affecting the k(cat) value. These three residues are located at the front of the hydrophilic globular domain and distant from the Cu(A) site, suggesting that these amino acids compose an acidic patch for a second substrate-binding site. This is the first report on site-directed mutagenesis experiments of a B-type heme-copper oxidase.

Diversity of the heme-copper superfamily in archaea: insights from genomics and structural modeling

Recent advances in DNA sequencing technologies have provided unprecedented access into the diversity of the microbial world. Herein we use the comparative genomic analysis of microbial genomes and environmental metagenomes coupled with structural modelling to explore the diversity of aerobic respiration in Archaea. We focus on the heme-copper oxidoreductase superfamily which is responsible for catalyzing the terminal reaction in aerobic respiration-the reduction of molecular oxygen to water. Sequence analyses demonstrate that there are at least eight heme-copper oxygen reductase families: A-, B-, C-, D-, E-, F-, G-, and H-families. Interestingly, five of these oxygen reductase families (D-, E-, F-, G-, and H-families) are currently found exclusively in Archaea. We review the structural properties of all eight families focusing on the members found within Archaea. Structural modelling coupled with sequence analysis suggests that many of the oxygen reductases identified from thermophilic Archaea have modified proton channel properties compared to the currently studied mesophilic bacterial oxygen reductases. These structural differences may be due to adaptation to the specific environments in which these enzymes function. We conclude with a brief analysis of the phylogenetic distribution and evolution of Archaeal heme-copper oxygen reductases.

A homologous expression system for obtaining engineered cytochrome ba3 from Thermus thermophilus HB8

Cytochrome ba3 is an integral membrane protein that serves as a terminal oxidase of the respiratory chain in some prokaryotes. We have cloned the complete cba operon of Thermus thermophilus HB8 in an Escherichia coli/T. thermophilus shuttle vector. The ba3-encoding operon, cba, was eliminated from the chromosome of T. thermophilus strain MT111 using the pyrE system of Yamagishi and co-workers. Expression of functional cytochrome ba3 occurred in cells grown at reduced dioxygen levels. A hepta-histidine tag was placed at the N-terminus of subunit I, and a purification method for this form of the enzyme was developed. Growth conditions were investigated for moderate sized cultures (2L) with typical yields of approximately 2 mg of highly pure enzyme per liter of culture medium. The physical properties and enzymatic activities of these recombinant enzymes were compared with those of native enzyme. Recombinant enzyme lacking the histidine tag is spectrally identical to wild-type enzyme. Histidine-tagged cytochrome ba3 shows minor differences from wild-type, and it appears be somewhat less active as a cytochrome c552 oxidase. Exemplary mutants were also produced and compared to native protein. Tyrosine I-237, previously found to be covalently bonded to I-His-233, was changed to phenylalanine (I-Y237F) and to histidine (I-Y237H) in the hepta-histidine tagged cytochrome ba3. The Y to F mutant is devoid of enzyme activity whereas the Y to H mutant possesses approximately 5% wild-type oxidase activity; their properties are compared with those of wild-type enzyme. The above versions of the histidine-tagged enzyme have been crystallized, and our analysis of a 2.3 angstrom resolution electron-density map will be discussed elsewhere.

Theoretical identification of proton channels in the quinol oxidase aa3 from Acidianus ambivalens

Heme-copper oxidases are membrane proteins found in the respiratory chain of aerobic organisms. They are the terminal electron acceptors coupling the translocation of protons across the membrane with the reduction of oxygen to water. Because the catalytic process occurs in the heme cofactors positioned well inside the protein matrix, proton channels must exist. However, due to the high structural divergence among this kind of proteins, the proton channels previously described are not necessarily conserved. In this work we modeled the structure of the quinol oxidase from Acidianus ambivalens using comparative modeling techniques for identifying proton channels. Additionally, given the high importance that water molecules may have in this process, we have developed a methodology, within the context of comparative modeling, to identify high water probability zones and to deconvolute them into chains of ordered water molecules. From our results, and from the existent information from other proteins from the same superfamily, we were able to suggest three possible proton channels: one K-, one D-, and one Q-spatial homologous proton channels. This methodology can be applied to other systems where water molecules are important for their biological function.

Separation methods in the analysis of protein membrane complexes

The separation of membrane protein complexes can be divided into two categories. One category, which is operated on a relatively large scale, aims to purify the membrane protein complex from membrane fractions while retaining its native form, mainly to characterize its nature. The other category aims to analyze the constituents of the membrane protein complex, usually on a small scale. Both of these face the difficulty of isolating the membrane protein complex without interference originating from the hydrophobic nature of membrane proteins or from the close association with membrane lipids. To overcome this difficulty, many methods have been employed. Crystallized membrane protein complexes are the most successful example of the former category. In these purification methods, special efforts are made in the steps prior to the column chromatography to enrich the target membrane protein complexes. Although there are specific aspects for each complex, the most popular method for isolating these membrane protein complexes is anion-exchange column chromatography, especially using weak anion-exchange columns. Another remarkable trend is metal affinity column chromatography, which purifies the membrane protein complex as an intact complex in one step. Such protein complexes contain subunit proteins which are genetically engineered so as to include multiple-histidine tags at carboxyl- or amino-termini. The key to these successes for multi-subunit complex isolation is the idea of keeping the expression at its physiological level, rather than overexpression. On the other hand, affinity purification using the Fv fragment, in which a Strep tag is genetically introduced, is ideal because this method does not introduce any change to the target protein. These purification methods supported by affinity interaction can be applied to minor membrane protein complexes in the membrane system. Isoelectric focusing (IEF) and blue native (BN) electrophoresis have also been employed to prepare membrane protein complexes. Generally, a combination of two or more chromatographic and/or electrophoretic methods is conducted to separate membrane protein complexes. IEF or BN electrophoresis followed by 2nd dimension electrophoresis serve as useful tools for analytical demand. However, some problems still exist in the 2D electrophoresis using IEF. To resolve such problems, many attempts have been made, e.g. introduction of new chaotropes, surfactants, reductants or supporting matrices. This review will focus in particular on two topics: the preparative methods that achieved purification of membrane protein complexes in the native (intact) form, and the analytical methods oriented to resolve the membrane proteins. The characteristics of these purification and analytical methods will be discussed along with plausible future developments taking into account the nature of membrane protein complexes.

A 16-residue peptide fragment of macrophage migration inhibitory factor, MIF-(50-65), exhibits redox activity and has MIF-like biological functions

Macrophage migration inhibitory factor (MIF) is a cytokine that participates in the host inflammatory response. A Cys-Xaa-Xaa-Cys (CXXC)-based thiol-protein oxidoreductase activity of MIF is associated with certain biological functions. Peptides spanning the CXXC region of thiol-protein oxidoreductases retain some biochemical properties of the full-length protein. We report on the characterization of CXXC-spanning MIF-(50-65) and its serine variant, C57S/C60S-MIF-(50-65). Following disulfide-mediated cyclization, MIF-(50-65) adapted a beta-turn conformation comparable with that of beta-turn-containing cyclo-57,60-[Asp57,Dap60]MIF-(50-65). MIF-(50-65) had a redox potential E'0 of -0.258 V and formed mixed disulfides with glutathione and cysteine. MIF-(50-65) but not C57S/C60S-MIF-(50-65) had oxidoreductase activity in vitro. Intriguingly, MIF-(50-65) exhibited MIF-like cellular activities. The peptide but not its variant had glucocorticoid overriding and proliferation-enhancing activity and stimulated ERK1/2 phosphorylation. MIF-(50-65) and its variant bound to the MIF-binding protein JAB1 and enhanced cellular levels of p27Kip1. As the peptide and its variant were endocytosed at similar efficiency, sequence 50-65 appears sufficient for the JAB1-related effects of MIF, whereas other activities require CXXC. Cyclo-57,60-[Asp57,Dap60]MIF-(50-65) activated ERK1/2, indicating that CXXC-dependent disulfide and beta-turn formation is associated with an activity-inducing conformation. We conclude that CXXC and sequence 50-65 are critical for the activities of MIF. MIF-(50-65) is a surprisingly short sequence with MIF-like functions that could be an excellent molecular template for MIF therapeutics.

Axial ligand modulation of the electronic structures of binuclear copper sites: analysis of paramagnetic 1H NMR spectra of Met160Gln Cu(A)

Cu(A) is an electron-transfer copper center present in heme-copper oxidases and N2O reductases. The center is a binuclear unit, with two cysteine ligands bridging the metal ions and two terminal histidine residues. A Met residue and a peptide carbonyl group are located on opposite sides of the Cu2S2 plane; these weaker ligands are fully conserved in all known Cu(A) sites. The Met160Gln mutant of the soluble subunit II of Thermus thermophilus ba3 oxidase has been studied by NMR spectroscopy. In its oxidized form, the binuclear copper is a fully delocalized mixed-valence pair, as are all natural Cu(A) centers. The faster nuclear relaxation in this mutant suggests that a low-lying excited state has shifted to higher energies compared to that of the wild-type protein. The introduction of the Gln residue alters the coordination mode of His114 but does not affect His157, thereby confirming the proposal that the axial ligand-to-copper distances influence the copper-His interactions (Robinson, H.; Ang, M. C.; Gao, Y. G.; Hay, M. T.; Lu, Y.; Wang, A. H. Biochemistry 1999, 38, 5677). Changes in the hyperfine coupling constants of the Cys beta-CH2 groups are attributed to minor geometrical changes that affect the Cu-S-C(beta)-H(beta) dihedral angles. These changes, in addition, shift the thermally accessible excited states, thus influencing the spectral position of the Cys beta-CH2 resonances. The Cu-Cys bonds are not substantially altered by the Cu-Gln160 interaction, in contrast to the situation found in the evolutionarily related blue copper proteins. It is possible that regulatory subunits in the mitochondrial oxidases fix the relative positions of thermally accessible Cu(A) excited states by tuning axial ligand interactions.