Spectroscopy of Mixed-Valence CuA-Type Centers: Ligand-Field Control of Ground-State Properties Related to Electron Transfer

An engineered CuA Amicyanin capable of intermolecular electron transfer reactions

The type I copper center of amicyanin was replaced with a binuclear CuA center. To create this model CuA protein, a portion of the amino acid sequence that contains three of the ligands to the native type I copper center of Paracoccus denitrificans amicyanin was replaced with the corresponding portion of sequence that provides five ligands for the CuA center of cytochrome c oxidase from P. denitrificans. UV-visible and electron paramagnetic resonance spectroscopy confirm that the engineered protein as isolated possesses the mixed-valence Cu1.5Cu1.5 (purple) CuA center. Comparison of the spectroscopic properties of this CuA amicyanin with those of the CuA centers of other natural and engineered CuA proteins suggests that the spectroscopic features may be dictated more by the protein host than the sequence of the CuA loop. Novel reactions for a simple CuA model protein are also described. In contrast to other natural and engineered CuA proteins, the fully reduced CuA amicyanin may be reoxidized by molecular oxygen to the mixed-valence state. It is also shown that CuA amicyanin can serve as an electron donor and an electron acceptor for other redox proteins. The mixed-valence form accepts electrons from cytochromes c-551i and c-550 from P. denitrificans. The fully reduced form donates electrons to native and P94F amicyanin. The function as either an electron donor or acceptor is consistent with the measured redox potential of CuA amicyanin of +273 mV. These data indicate that this CuA amicyanin will be a particularly useful model protein for structure-function studies of reactivity and the electron transfer properties of the CuA redox center.

Kinetics of intramolecular electron transfer in cytochrome bo3 from Escherichia coli

We have examined the temperature dependence of the intramolecular electron transfer (ET) between heme b and heme o(3) in CO-mixed valence cytochrome bo(3) (Cbo) from Escherichia coli. Upon photolysis of CO-mixed valence Cbo rapid ET occurs between heme o(3) and heme b with a rate constant of 2.2 x 10(5) s(-1) at room temperature. The corresponding rate of CO recombination is found to be 86 s(-1). From Eyring plots the activation energies for these two processes are found to be 3.4 kcal/mol and 6.7 kcal/mol for the ligand binding and ET reactions, respectively. Using variants of the Marcus equation the reorganization energy (lambda), electronic coupling factor (H(AB)), and the ET distance were found to be 1.4 +/- 0.2 eV, (2 +/- 1) x 10(-3) eV, and 9 +/- 1 A, respectively. These values are quite distinct from the analogous values previously obtained for bovine heart cytochrome c oxidase (CcO) (0.76 eV, 9.9 x 10(-5) eV, 13.2 A). The differences in mechanisms/pathways for heme b/heme o(3) and heme a/heme a(3) ET suggested by the Marcus parameters can be attributed to structural changes at the Cu(B) site upon change in oxidation state as well as differences in electronic coupling pathways between Heme b and heme o(3).

Quantum chemical calculations of spectroscopic properties of metalloproteins and model compounds: EPR and Mössbauer properties

Recently developed theoretical methods to predict EPR and Mössbauer parameters open the way for close interactions between theorists and experimentalists to elucidate the geometric and electronic structures of metalloenzymes and model complexes and to obtain insight into their reactive properties. Spectral calculations (g-values, hyperfine couplings, zero-field splittings, isomer shifts and quadrupole splittings) are also a means to validate theoretical approaches and therefore complement the prediction of geometries, reaction energies and transition states.

Redox properties of an engineered purple Cu(A) azurin

Purple Cu(A) centers are a class of binuclear, mixed-valence copper complexes found in cytochrome c oxidase and nitrous oxide reductase. An engineered Cu(A) protein was formed by replacing a portion of the amino acid sequence that contains three of the ligands to the native type I copper center of Pseudomonas aeruginosa azurin with the corresponding portion of sequence from the Cu(A) center of cytochrome c oxidase from Paracoccus denitrificans [Proc. Natl. Acad. Sci. USA 93 (1996) 461]. Oxidation-reduction midpoint potential (E(m)) values of the Cu(A) azurin of +399+/-10 and +380+/-2mV, respectively, were determined by cyclic voltammetry and spectrochemical titration. An n value of one was obtained, indicating that the redox reaction is cycling between the mixed valence and the fully reduced states. Whereas the E(m) value of native azurin is pH dependent, the E(m) value of Cu(A) azurin is not, as expected for the Cu(A) center. Similarities and differences in the redox properties are discussed in terms of the known crystal structures of Cu(A) centers in cytochrome c oxidase and Cu(A) azurin.

The vibrational reorganization energy in pentacene: molecular influences on charge transport

The reorganization energy in pentacene is reported on the basis of a joint experimental and theoretical study of pentacene ionization using high-resolution gas-phase photoelectron spectroscopy, semiempirical intermediate neglect of differential overlap calculations, and first-principles correlated quantum-mechanical calculations at MP2 and density functional theory levels. The reorganization energy upon positive ionization of pentacene is determined both experimentally and theoretically to be remarkably low. This is one key element that allows one to rationalize the extremely high hole mobilities recently measured in pentacene single crystals.

Role of the coordinating histidine in altering the mixed valency of Cu(A): an electron nuclear double resonance-electron paramagnetic resonance investigation

The binuclear Cu(A) site engineered into Pseudomonas aeruginosa azurin has provided a Cu(A)-azurin with a well-defined crystal structure and a CuSSCu core having two equatorial histidine ligands, His120 and His46. The mutations His120Asn and His120Gly were made at the equatorial His120 ligand to understand the histidine-related modulation to Cu(A), notably to the valence delocalization over the CuSSCu core. For these His120 mutants Q-band electron nuclear double resonance (ENDOR) and multifrequency electron paramagnetic resonance (EPR) (X, C, and S-band), all carried out under comparable cryogenic conditions, have provided markedly different electronic measures of the mutation-induced change. Q-band ENDOR of cysteine C(beta) protons, of weakly dipolar-coupled protons, and of the remaining His46 nitrogen ligand provided hyperfine couplings that were like those of other binuclear mixed-valence Cu(A) systems and were essentially unperturbed by the mutation at His120. The ENDOR findings imply that the Cu(A) core electronic structure remains unchanged by the His120 mutation. On the other hand, multifrequency EPR indicated that the H120N and H120G mutations had changed the EPR hyperfine signature from a 7-line to a 4-line pattern, consistent with trapped-valence, Type 1 mononuclear copper. The multifrequency EPR data imply that the electron spin had become localized on one copper by the His120 mutation. To reconcile the EPR and ENDOR findings for the His120 mutants requires that either: if valence localization to one copper has occurred, the spin density on the cysteine sulfurs and the remaining histidine (His46) must remain as it was for a delocalized binuclear Cu(A) center, or if valence delocalization persists, the hyperfine coupling for one copper must markedly diminish while the overall spin distribution on the CuSSCu core is preserved.

Electronic structure and its relation to function in copper proteins

Spectroscopic and theoretical investigations of the geometric and electronic structures of mononuclear and binuclear copper sites in proteins help in understanding the contributions of these proteins to biological electron transfer. Spectroscopically calibrated density functional theory calculations, which give reasonable bonding descriptions in both ground- and excited-states, define the role of the protein in determining the geometric and electronic structure of the active site.

Mutants of the CuA site in cytochrome c oxidase of Rhodobacter sphaeroides: II. Rapid kinetic analysis of electron transfer

The function of the binuclear Cu(A) center in cytochrome c oxidase (CcO) was studied using two Rhodobacter sphaeroides CcO mutants involving direct ligands of the Cu(A) center, H260N and M263L. The rapid electron-transfer kinetics of the mutants were studied by flash photolysis of a cytochrome c derivative labeled with ruthenium trisbipyridine at lysine-55. The rate constant for intracomplex electron transfer from heme c to Cu(A) was decreased from 40000 s(-1) for wild-type CcO to 16000 s(-1) and 11000 s(-1) for the M263L and H260N mutants, respectively. The rate constant for electron transfer from Cu(A) to heme a was decreased from 90000 s(-1) for wild-type CcO to 4000 s(-1) for the M263L mutant and only 45 s(-1) for the H260N mutant. The rate constant for the reverse reaction, heme a to Cu(A), was calculated to be 66000 s(-1) for M263L and 180 s(-1) for H260N, compared to 17000 s(-1) for wild-type CcO. It was estimated that the redox potential of Cu(A) was increased by 120 mV for the M263L mutant and 90 mV for the H260N mutant, relative to the potential of heme a. Neither mutation significantly affected the binding interaction with cytochrome c. These results indicate that His-260, but not Met-263, plays a significant role in electron transfer between Cu(A) and heme a.

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.

Geometry, reduction potential, and reorganization energy of the binuclear Cu(A) site, studied by density functional theory

The dimeric Cu(A) site found in cytochrome c oxidase and nitrous oxide reductase has been studied with the density functional B3LYP method. We have optimized the structure of the realistic (Im)(S(CH(3))(2))Cu(SCH(3))(2)Cu(Im)(CH(3)CONHCH(3)) model in the fully reduced, mixed-valence, and fully oxidized states. The optimized structures are very similar to crystal structures of the protein, which shows that the protein does not strain the site significantly. Instead, inorganic model complexes of the protein site are strained by the macrocyclic connections between the ligand models. For the mixed-valence (Cu(I)+Cu(II)) state, two distinct equilibrium structures were found, one with a short Cu-Cu distance, 248 pm, similar to the protein structure, and one with a longer distance, 310 pm, similar to what is found in inorganic models. In the first state, the unpaired electron is delocalized over both copper ions, whereas in the latter, it is more localized to one of the ions. The two states are nearly degenerate. The potential energy surfaces for the Cu-Cu, Cu-S(Met), and Cu-O interactions are extremely flat. In fact, all three distances can be varied between 230 and 310 pm at an expense in energy of less than 8 kJ/mol, which explains the large variation observed in crystal structures for these interactions. Inclusion of solvation effects does not change this significantly. Therefore, we can conclude that a variation in these distances can change the reduction potential of the Cu(A) site by at most 100 mV. The model complex has a reorganization energy of 43 kJ/mol, 20 kJ/mol lower than for a monomeric blue-copper site. This lowering is caused by the delocalization of the unpaired electron in the mixed-valence state.

Recent advances in bioinorganic spectroscopy

Spectroscopic methods covering many energy regions together provide complementary insight into metalloenzyme active sites. These methods probe geometric and electronic structure and define these contributions to reactivity. Two recent advances--determination of the polarizations of electronic transitions in solution using magnetic circular dichroism, electron paramagnetic resonance and quantum chemistry, and experimental estimation of covalency using metal L-edges and ligand K-edges--are particularly important.

Structural investigations of the CuA centre of nitrous oxide reductase from Pseudomonas stutzeri by site-directed mutagenesis and X-ray absorption spectroscopy

Nitrous oxide reductase is the terminal component of a respiratory chain that utilizes N2O in lieu of oxygen. It is a homodimer carrying in each subunit the electron transfer site, CuA, and the substrate-reducing catalytic centre, CuZ. Spectroscopic data have provided robust evidence for CuA as a binuclear, mixed-valence metal site. To provide further structural information on the CuA centre of N2O reductase, site directed mutagenesis and Cu K-edge X-ray absorption spectroscopic investigation have been undertaken. Candidate amino acids as ligands for the CuA centre of the enzyme from Pseudomonas stutzeri ATCC14405 were substituted by evolutionary conserved residues or amino acids similar to the wild-type residues. The mutations identified the amino acids His583, Cys618, Cys622 and Met629 as ligands of Cu1, and Cys618, Cys622 and His626 as the minimal set of ligands for Cu2 of the CuA centre. Other amino acid substitutions indicated His494 as a likely ligand of CuZ, and an indirect role for Asp580, compatible with a docking function for the electron donor. Cu binding and spectroscopic properties of recombinant N2O reductase proteins point at intersubunit or interdomain interaction of CuA and CuZ. Cu K-edge X-ray absorption spectra have been recorded to investigate the local environment of the Cu centres in N2O reductase. Cu K-edge Extended X-ray Absorption Fine Structure (EXAFS) for binuclear Cu chemical systems show clear evidence for Cu backscattering at approximately 2.5 A. The Cu K-edge EXAFS of the CuA centre of N2O reductase is very similar to that of the CuA centre of cytochrome c oxidase and the optimum simulation of the experimental data involves backscattering from a histidine group with Cu-N of 1.92 A, two sulfur atoms at 2.24 A and a Cu atom at 2. 43 A, and allows for the presence of a further light atom (oxygen or nitrogen) at 2.05 A. The interpretation of the CuA EXAFS is in line with ligands assigned by site-directed mutagenesis. By a difference spectrum approach, using the Cu K-edge EXAFS of the holoenzyme and that of the CuA-only form, histidine was identified as a major contributor to the backscattering. A structural model for the CuA centre of N2O reductase has been generated on the basis of the atomic coordinates for the homologous domain of cytochrome c oxidase and incorporating our current results and previous spectroscopic data.

Ligand replacement study at the His120 site of purple CuA azurin

The CuA center is a dinuclear Cu2S2(Cys) electron transfer center found in cytochrome c oxidase and nitrous oxide reductase. In a previous investigation of the equatorial histidine ligands' effect on the reduction potential, electron transfer and spectroscopic properties of the CuA center, His120 in the engineered CuA azurin was mutated to Asn, Asp, and Ala. The identical absorption and EPR spectra of these mutants indicate that a common ligand is bound to the copper center. To identify this replacement ligand, the His120Gly CuA azurin mutant was constructed and purified. Absorption and X-band EPR spectra show that His120Gly is similar to the other His120X (X = Asn, Asp, Ala) mutant proteins. Titrations with chloride, imidazole, and azide suggest that the replacement ligand is not exchangeable with exogenous ligands. The possibility of an internal amino acid acting as the replacement ligand for His120 in the His120X mutant proteins was investigated by analyzing the CuA azurin crystal structure and then converting the likely internal ligand, Asn 119, to Asp, Ser, or Ala in the His120Gly mutant. The double mutants H120G/Asn 119X (X = Asp, Ser, or Ala) displayed UV-Vis absorption and EPR spectra that are identical to His120Gly and the other His120X mutants, indicating that Asn119 is not the internal ligand replacing His120 in the His120X mutant proteins. These results demonstrate the remarkable stability of the dinuclear His120 mutants of CuA azurin.

Paramagnetic NMR studies of blue and purple copper proteins

1H- and 13C-NMR spectroscopy is applied to investigate the CU(A) and type 1 active sites of copper proteins in solution. The analysis of hyperfine shifted 1H resonances allows the comparison of the electron spin density delocalization in the CU(A) site of the wild-type soluble domains of various cytochrome c oxidases (Thermus thermophilus, Paracoccus denitrificans, and Paracoccus versutus) and genetically engineered constructs (soluble domain of quinol oxidase from Escherichia coli and Thiobacillus versutus amicyanin). Comparable spin densities are found on the two terminal His ligands for the wild-type constructs as opposed to the engineered proteins where the spin is more unevenly distributed on the two His residues. A reevaluation of the Cys H(beta) chemical shifts that is in agreement with the data published for both the P. denitrificans and the P. versutus Cu(A) soluble domains confirms the thermal accessibility of the 2B(3u) electronic excited state and indicates the existence of slightly different spin densities on the two bridging Cys ligands. The 13C-NMR spectrum of isotopically enriched oxidized azurin from Pseudomonas aeruginosa reveals six fast relaxing signals, which can be partially identified by 1- and 2-dimensional (1-D, 2-D) direct detection techniques combined with 3-D triple resonance experiments. The observed contact shifts suggest the presence of direct spin density transfer and spin polarization mechanisms for the delocalization of the unpaired electron.

Synthesis and Dimer Cleavage Reactions of the N(2)S Thiolate Bridged Dimer [(mmp-dach)(2)Ni(2)]Cl(2)

The facile synthesis of a monoderivatized N(2)S ligand based on the cyclic diamine diazacycloheptane provides a tridentate donor which coordinates nickel(II), yielding a dithiolate bridged nickel dimer with a hinge angle of 135.7 degrees and acute N-Ni-N bite angles of ca. 80 degrees. Dimer splitting occurs on reaction with iodoacetamide, producing an S-alkylated octahedral complex. Cyanide also cleaves the dimer, resulting in a cyanide-bound N(2)S square planar monomer with two nucleophilic sites, thiolate-S and cyanide-N. Molecular structures of the dimeric cation, [(mmp-dach)(2)Ni(2)]Cl(2), the iodoacetamide derivative, [(dtp-dach)Ni(MeCN)(2)]Cl(2), and an S-oxygenate, [(smp-dach)Ni(CN)], are reported.

MCD C-Term Signs, Saturation Behavior, and Determination of Band Polarizations in Randomly Oriented Systems with Spin S >/= (1)/(2). Applications to S = (1)/(2) and S = (5)/(2)

The magnetic circular dichroism (MCD) properties of a spin-allowed transition from an orbitally nondegenerate ground state manifold A to an orbitally nondegenerate excited state manifold J in the presence of spin-orbit coupling (SOC) are derived for any S >/= (1)/(2). Three physically distinct mechanisms are identified that lead to MCD intensity and depend on SOC between excited states which leads to a sum rule and SOC between the ground state and other excited states that leads to deviations from the sum rule. The model is valid for any symmetry of the magnetic coupling tensors and arbitrary transition polarizations. The S = (1)/(2) case is analytically solved, and the determination of linear polarizations from MCD saturation magnetization data is discussed. For all mechanisms the MCD intensity is proportional to the spin-expectation values of the ground state sublevels which are conveniently generated from a spin-Hamiltonian (SH). For Kramers systems with large zero-field splittings (ZFSs) this allows the contribution from each Kramers doublet to the total MCD intensity to be related through their effective g-values, therefore significantly reducing the number of parameters required to analyze experimental data. The behavior of high-spin systems is discussed in the limits of weak, intermediate, and strong ZFS relative to the Zeeman energy. The model remains valid in the important case of intermediate ZFS where the ground state sublevels may cross as a function of applied magnetic field and there are significant off-axis contributions to the MCD intensity due to a change of the electron spin quantization axis. The model permits calculation of MCD C-term signs from molecular wave functions, and explicit expressions are derived in terms of MOs for S = (1)/(2) and S = (5)/(2). Two examples from the literature are analyzed to demonstrate how the C-term signs can be evaluated by a graphical method that gives insight into their physical origin.

Enhanced rate of intramolecular electron transfer in an engineered purple CuA azurin

The recent expression of an azurin mutant where the blue type 1 copper site is replaced by the purple CuA site of Paracoccus denitrificans cytochrome c oxidase has yielded an optimal system for examining the unique electron mediation properties of the binuclear CuA center, because both type 1 and CuA centers are placed in the same location in the protein while all other structural elements remain the same. Long-range electron transfer is induced between the disulfide radical anion, produced pulse radiolytically, and the oxidized binuclear CuA center in the purple azurin mutant. The rate constant of this intramolecular process, kET = 650 +/- 60 s-1 at 298 K and pH 5.1, is almost 3-fold faster than for the same process in the wild-type single blue copper azurin from Pseudomonas aeruginosa (250 +/- 20 s-1), in spite of a smaller driving force (0.69 eV for purple CuA azurin vs. 0.76 eV for blue copper azurin). The reorganization energy of the CuA center is calculated to be 0.4 eV, which is only 50% of that found for the wild-type azurin. These results represent a direct comparison of electron transfer properties of the blue and purple CuA sites in the same protein framework and provide support for the notion that the binuclear purple CuA center is a more efficient electron transfer agent than the blue single copper center because reactivity of the former involves a lower reorganization energy.