NMR study of structure and electron transfer mechanism of Pseudomonas aeruginosa azurin

Photoinduced Electron Transfer from the Tryptophan Triplet State in Zn-Azurin

Tryptophan is one of few residues that participates in biological electron transfer reactions. Upon substitution of the native Cu²⁺ center with Zn²⁺ in the blue-copper protein azurin, a long-lived tryptophan neutral radical can be photogenerated. We report the following quantum yield values for Zn-substituted azurin in the presence of the electron acceptor Cu(II)-azurin: formation of the tryptophan neutral radical (Φ_rad), electron transfer (Φ_ET), fluorescence (Φ_fluo), and phosphorescence (Φ_phos), as well as the efficiency of proton transfer of the cation radical (Φ_PT). Increasing the concentration of the electron acceptor increased Φ_rad and Φ_ET values and decreased Φ_phos without affecting Φ_fluo. At all concentrations of the acceptor, the value of Φ_PT was nearly unity. These observations indicate that the phosphorescent triplet state is the parent state of electron transfer and that nearly all electron transfer events lead to proton loss. Similar results regarding the parent state were obtained with a different electron acceptor, [Co(NH₃)₅Cl]²⁺; however, Stern-Volmer graphs revealed that [Co(NH₃)₅Cl]²⁺ was a more effective phosphorescence quencher (K _SV = 230 000 M^-1) compared to Cu(II)-azurin (K _SV = 88 000 M^-1). Competition experiments in the presence of both [Co(NH₃)₅Cl]²⁺ and Cu(II)-azurin suggested that [Co(NH₃)₅Cl]²⁺ is the preferred electron acceptor. Implications of these results in terms of quenching mechanisms are discussed.

Electron transfer-triggered imaging of EGFR signaling activity

In vivo electron transfer processes are closely related to the activation of signaling pathways, and, thus, affect various life processes. Indeed, the signaling pathway activation of key molecules may be associated with certain diseases. For example, epidermal growth factor receptor (EGFR) activation is related to the occurrence and development of tumors. Hence, monitoring the activation of EGFR-related signaling pathways can help reveal the progression of tumor development. However, it is challenging for current detection methods to monitor the activation of specific signaling pathways in complex biochemical reactions. Here we designed a highly sensitive and specific nanoprobe that enables in vivo imaging of electronic transfer over a broad range of spatial and temporal scales. By using the ferrocene-DNA polymer “wire”, the electrons transferred in a biochemical reaction can flow to persistent luminescent nanoparticles and change their electron distribution, thereby altering the optical signal of the particles. This electron transfer-triggered imaging probe enables mapping the activation of EGFR-related signaling pathways in a temporally and spatially precise manner. By offering precise visualization of signaling activity, this approach may offer a general platform not only for understanding molecular mechanisms in various biological processes but also for promoting disease therapies and drug evaluation.

Here, the authors design a nanoprobe for in vivo imaging of electronic transfer, consisting of a ferrocene-DNA polymer to transfer electrons to luminescent nanoparticles, changing their optical signal. Using this probe, they map activation of EGFR signalling during tumour treatment.

Low Reorganization Energy for Electron Self-Exchange by a Formally Copper(III,II) Redox Couple

The rate constant for electron self-exchange (k₁₁) between LCuOH and [LCuOH]^- (L = bis-2,6-(2,6-diisopropylphenyl)carboximidopyridine) was determined using the Marcus cross relation. This work involved measurement of the rate of the cross-reaction between [Bu₄N][LCuOH] and [Fc][BAr₄^F] (Fc⁺ = ferrocenium; BAr₄^F = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate)) by stopped-flow methods at -88 °C in CH₂Cl₂ and measurement of the equilibrium constant for the redox process by UV-vis titrations under the same conditions. A value of k₁₁ = 3 × 10⁴ M^-1 s^-1 (-88 °C) led to estimation of a value 9 × 10⁶ M^-1 s^-1 at 25 °C, which is among the highest values known for copper redox couples. Further Marcus analysis enabled determination of a low reorganization energy, λ = 0.95 ± 0.17 eV, attributed to minimal structural variation between the redox partners. In addition, the reaction entropy (ΔS°) associated with the LCuOH/[LCuOH]^- self-exchange was determined from the temperature dependence of the redox potentials, and found to be dependent upon ionic strength. Comparisons to other Cu redox systems and potential new applications for the formally Cu^III,II system are discussed.

COMPARATIVE INVESTIGATION OF THE EFFECT OF TYPE OF DENSITY FUNCTIONAL IN THE DETERMINATION OF GEOMETRICAL PARAMETERS IN A Cu COMPLEX

The effects of short-range electron correlation, long-range electron exchange, local and nonlocal parts of density, higher order gradients of density, and adding some percentage of Hartree–Fock exchange to the functional on the prediction of geometrical parameters were investigated. A copper complex namely 1,2-bis(1,4,7-triaza-1-cyclononyl) ethane copper (II) with Jahn–Teller distortion in octahedral geometry was used to evaluate the performance of 50 commonly available density functionals. The standard 3-21G basis set was used for all light elements, while pseudo potential LANL2DZ was used for the copper atom. The best bond lengths and bond angles were obtained using M05-2x and OP functionals respectively. Also in order to more accurate survey the performance of B3LYP, we used this functional with two all-electron basis sets (6-31G and 3-21G) and three basis sets involving effective core potentials (LANL2DZ/3-21G, LANL2DZ, and LACVP).

Marked changes in electron transport through the blue copper protein azurin in the solid state upon deuteration

Measuring solid-state electron transport (ETp) across proteins allows studying electron transfer (ET) mechanism(s), while minimizing solvation effects on the process. ETp is, however, sensitive to any static (conformational) or dynamic (vibrational) changes in the protein. Our macroscopic measurements allow extending ETp studies to low temperatures, with the concomitant resolution of lower current densities, because of the larger electrode contact areas. Thus, earlier we reported temperature-independent ETp via the copper protein azurin (Az), from 80 K until denaturation, whereas for apo-Az ETp was temperature dependent above 180 K. Deuteration (H/D substitution) may provide mechanistic information on the question of whether the ETp involves H-bonds in the solid state. Here we report results of kinetic deuterium isotope effect (KIE) measurements on ETp through holo-Az as a function of temperature (30-340 K). Strikingly, deuteration changed ETp from temperature independent to temperature dependent above 180 K. This H/D effect is expressed in KIE values between 1.8 (340 K) and 9.1 (≤ 180 K). These values are remarkable in light of the previously reported inverse KIE on ET in Az in solution. We ascribe the difference between our KIE results and those observed in solution to the dominance of solvent effects in the latter (larger thermal expansion in H(2)O than in D(2)O), whereas in our case the KIE is primarily due to intramolecular changes, mainly in the low-frequency structural modes of the protein caused by H/D exchange. The observed high KIE values are consistent with a transport mechanism that involves through-H-bonds of the β-sheet structure of Az, likely also those in the Cu coordination sphere.

Electron transfer reactivity of type zero Pseudomonas aeruginosa azurin

Type zero copper is a hard-ligand analogue of the classical type 1 or blue site in copper proteins that function as electron transfer (ET) agents in photosynthesis and other biological processes. The EPR spectroscopic features of type zero Cu(II) are very similar to those of blue copper, although lacking the deep blue color, due to the absence of thiolate ligation. We have measured the rates of intramolecular ET from the pulse radiolytically generated C3-C26 disulfide radical anion to the Cu(II) in both type zero C112D/M121L and type 2 C112D Pseudomonas aeruginosa azurins in pH 7.0 aqueous solutions between 8 and 45 °C. We also have obtained rate/temperature (10-30 °C) profiles for ET reactions between these mutants and the wild-type azurin. Analysis of the rates and activation parameters for both intramolecular and intermolecular ET reactions indicates that the type zero copper reorganization energy falls in a range (0.9-1.1 eV) slightly above that for type 1 (0.7-0.8 eV), but substantially smaller than that for type 2 (>2 eV), consistent with XAS and EXAFS data that reveal minimal type zero site reorientation during redox cycling.

Multifrequency EPR studies of [Cu(1.5)Cu(1.5)](+) for Cu2(mu-NR2)2 and Cu2(mu-PR2)2 diamond cores

Multifrequency electron paramagnetic resonance (EPR) spectroscopy is used to explore the electronic structures of a series of dicopper complexes of the type {(LXL)Cu}(2)(+). These complexes contain two four-coordinate copper centers of highly distorted tetrahedral geometries linked by two [LXL](-) ligands featuring bridging amido or phosphido ligands and associated thioether or phosphine chelate donors. Specific chelating [LXL](-) ligands examined in this study include bis(2-tert-butylsulfanylphenyl)amide (SNS), bis(2-di-iso-butylphosphinophenyl)amide (PNP), and bis(2-di-iso-propylphosphinophenyl)phosphide (PPP). To better map the electronic coupling to copper, nitrogen, and phosphorus in these complexes, X-, S-, and Q-band EPR spectra have been obtained for each complex. The resulting EPR parameters implied by computer simulation are unusual for typical dicopper complexes and are largely consistent with previously published X-ray absorption spectroscopy and density functional theory data, where a highly covalent {Cu(2)(mu-XR(2))(2)}(+) diamond core has been assigned in which removal of an electron from the neutral {Cu(2)(mu-XR(2))(2)} can be viewed as ligand-centered to a substantial degree. To our knowledge, this is the first family of dicopper diamond core model complexes for which the compendium of X-, S-, and Q-band EPR spectra have been collected for comparison to Cu(A).

A definitive example of a geometric "entatic state" effect: electron-transfer kinetics for a copper(II/I) complex involving A quinquedentate macrocyclic trithiaether-bipyridine ligand

The quinquedentate macrocyclic ligand cyclo-6,6'-[1,9-(2,5,8-trithianonane)]-2,2'-bipyridine ([15]aneS3bpy = L), containing two pyridyl nitrogens and three thiaether sulfurs as donor atoms, has been synthesized and complexed with copper. The CuII/IL redox potential, the stabilities of the oxidized and reduced complex, and the oxidation and reduction electron-transfer kinetics of the complex reacting with a series of six counter reagents have been studied in acetonitrile at 25 degrees C, mu = 0.10 M (NaClO4). The Marcus cross relationship has been applied to the rate constants obtained for the reactions with each of the six counter reagents to permit the evaluation of the electron self-exchange rate constant, k11. The latter value has also been determined independently from NMR line-broadening experiments. The cumulative data are consistent with a value of k11 = 1 x 10(5) M(-1) s(-1), ranking this among the fastest-reacting CuII/I systems, on a par with the blue copper proteins known as cupredoxins. The resolved crystal structures show that the geometry of the CuIIL and CuIL complexes are nearly identical, both exhibiting a five-coordinate square pyramidal geometry with the central sulfur donor atom occupying the apical site. The most notable geometric difference is a puckering of an ethylene bridge between two sulfur donor atoms in the CuIL complex. Theoretical calculations suggest that the reorganizational energy is relatively small, with the transition-state geometry more closely approximating the geometry of the CuIIL ground state. The combination of a nearly constant geometry and a large self-exchange rate constant implies that this CuII/I redox system represents a true geometric "entatic state."

The role of hydrogen bonding at the active site of a cupredoxin: the Phe114Pro azurin variant

The Phe114Pro mutation to the cupredoxin azurin (AZ) leads to a number of structural changes at the active site attributed to deletion of one of the hydrogen bonds to the Cys112 ligand, removal of the bulky phenyl group from the hydrophobic patch of the protein, and steric interactions made by the introduced Pro. The remaining hydrogen bond between the coordinating thiolate and the backbone amide of Asn47 is strengthened. At the type-1 copper site, the Cu(II)-O(Gly45) axial interaction decreases, while the metal moves out of the plane formed by the equatorial His46, Cys112, and His117 ligands, shortening the bond to the axially coordinating Met121. The resulting distorted tetrahedral geometry is distinct from the trigonal bipyramidal arrangement in the wild-type (WT) protein. The unique position of the main S(Cys) --> Cu(II) ligand-to-metal charge-transfer transition in AZ (628 nm) has shifted in the Phe114Pro variant to a value that is more typical for cupredoxins (599 nm). This probably occurs because of the removal of the Phe114-Cys112 hydrogen bond. The Phe114Pro mutation results in a 90 mV decrease in the reduction potential of AZ, and removal of the second hydrogen bond to the Cys ligand seems to be the major cause of this change. The C-terminal His117 ligand does not protonate in the reduced Phe114Pro AZ variant, which suggests that none of the structural features altered by the mutation are responsible for the absence of this effect in the WT protein. Upon reduction, the copper displaces further from the equatorial ligand plane and the Cu-S(Met121) bond length decreases. These changes are larger than those seen in the WT protein and contribute to the order of magnitude decrease in the intrinsic electron-transfer capabilities of the Phe114Pro variant.

Blue copper model complexes with distorted tetragonal geometry acting as effective electron-transfer mediators in dye-sensitized solar cells

The electron self-exchange rate constants of blue copper model complexes, [(-)-sparteine-N,N'](maleonitriledithiolato-S,S')copper ([Cu(SP)(mmt)])(0/)(-), bis(2,9-dimethy-1,10-phenanthroline)copper ([Cu(dmp)(2)](2+/+)), and bis(1,10-phenanthroline)copper ([Cu(phen)(2)](2+/+)) have been determined from the rate constants of electron transfer from a homologous series of ferrocene derivatives to the copper(II) complexes in light of the Marcus theory of electron transfer. The resulting electron self-exchange rate constant increases in the order: [Cu(phen)(2)](2+/+) < [Cu(SP)(mmt)](0/)(-) < [Cu(dmp)(2)](2+/+), in agreement with the order of the smaller structural change between the copper(II) and copper(I) complexes due to the distorted tetragonal geometry. The dye-sensitized solar cells (DSSC) were constructed using the copper complexes as redox couples to compare the photoelectrochemical responses with those using the conventional I(3)(-)/I(-) couple. The light energy conversion efficiency (eta) values under illumination of simulated solar light irradiation (100 mW/cm(2)) of DSSCs using [Cu(phen)(2)](2+/+), [Cu(dmp)(2)](2+/+), and [Cu(SP)(mmt)](0/)(-) were recorded as 0.1%, 1.4%, and 1.3%, respectively. The maximum eta value (2.2%) was obtained for a DSSC using the [Cu(dmp)(2)](2+/+) redox couple under the light irradiation of 20 mW/cm(2) intensity, where a higher open-circuit voltage of the cell was attained as compared to that of the conventional I(3)(-)/I(-) couple.

Reduction Potential Tuning at a Type 1 Copper Site Does Not Compromise Electron Transfer Reactivity

Type 1 (T1) copper sites promote biological electron transfer (ET) and typically possess a weakly coordinated thioether sulfur from an axial Met [Cu(II)-Sdelta approximately 2.6 to 3.3 A] along with the conserved His2Cys equatorial ligands. A strong axial bond [Cu(II)-Oepsilon1 approximately 2.2 A] is sometimes provided by a Gln (as in the stellacyanins), and the axial ligand can be absent (a Val, Leu or Phe in the axial position) as in ceruloplasmin, Fet3p, fungal laccases and some plantacyanins (PLTs). Cucumber basic protein (CBP) is a PLT which has a relatively short Cu(II)-S(Met89) axial bond (2.6 A). The Met89Gln variant of CBP has an electron self-exchange (ESE) rate constant (k(ese), a measure of intrinsic ET reactivity) approximately 7 times lower than that of the wild-type protein. The Met89Val mutation to CBP results in a 2-fold increase in k(ese). As the axial interaction decreases from strong Oepsilon1 of Gln to relatively weak Sdelta of Met to no ligand (Val), ESE reactivity is therefore enhanced by approximately 1 order of magnitude while the reduction potential increases by approximately 350 mV. The variable coordination position at this ubiquitous ET site provides a mechanism for tuning the driving force to optimize ET with the correct partner without significantly compromising intrinsic reactivity. The enhanced reactivity of a three-coordinate T1 copper site will facilitate intramolecular ET in fungal laccases and Fet3p.

Loop-contraction mutagenesis of type 1 copper sites

The shortest known type 1 copper binding loop (that of amicyanin, Ami) has been introduced into three different cupredoxin beta-barrel scaffolds. All of the loop-contraction variants possess copper centers with authentic type 1 properties and are redox active. The Cu(II) and Co(II) sites experience only small structural alterations upon loop contraction with the largest changes in the azurin variant (AzAmi), which can be ascribed to the removal of a hydrogen bond to the coordinating thiolate sulfur of the Cys ligand. In all cases, loop contraction leads to an increase in the pK(a) of the His ligand found on the loop in the reduced proteins, and in the pseudoazurin (Paz) and plastocyanin (Pc) variants the values are almost identical to that of Ami ( approximately 6.7). Thus, in Paz, Pc, and Ami, the length of this loop tunes the pK(a) of the His ligand. In the AzAmi variant, the pK(a) is 5.5, which is considerably higher than the estimated value for Az (<2), and other controlling factors, along with loop length, are involved. The reduction potentials of the loop-contraction variants are all lower than those of the wild-type proteins by approximately 30-60 mV, and thus this property of a type 1 copper site is fine-tuned by the C-terminal loop. The electron self-exchange rate constant of Paz is significantly diminished by the introduction of a shorter loop. However, in PcAmi only a 2-fold decrease is observed and in AzAmi there is no effect, and thus in these two cupredoxins loop contraction does not significantly influence electron-transfer reactivity. Loop contraction provides an active site environment in all of the cupredoxins which is preferable for Cu(II), whereas previous loop elongation experiments always favored the cuprous site. Thus, the ligand-containing loop plays an important role in tuning the entatic nature of a type 1 copper center.

Mimicking Protein−Protein Electron Transfer: Voltammetry of Pseudomonas aeruginosa Azurin and the Thermus thermophilus CuA Domain at ω-Derivatized Self-Assembled-Monolayer Gold Electrodes

Well-defined voltammetric responses of redox proteins with acidic-to-neutral pI values have been obtained on pure alkanethiol as well as on mixed self-assembled-monolayer (SAM) omega-derivatized alkanethiol/gold bead electrodes. Both azurin (P. aeruginosa) (pI = 5.6) and subunit II (Cu(A) domain) of ba(3)-type cytochrome c oxidase (T. thermophilus) (pI = 6.0) exhibit optimal voltammetric responses on 1:1 mixtures of [H(3)C(CH(2))(n)()SH + HO(CH(2))(n)()SH] SAMs. The electron transfer (ET) rate vs distance behavior of azurin and Cu(A) is independent of the omega-derivatized alkanethiol SAM headgroups. Strikingly, only wild-type azurin and mutants containing Trp48 give voltammetric responses: based on modeling, we suggest that electronic coupling with the SAM headgroup (H(3)C- and/or HO-) occurs at the Asn47 side chain carbonyl oxygen and that an Asn47-Cys112 hydrogen bond promotes intramolecular ET to the copper. Inspection of models also indicates that the Cu(A) domain of ba(3)-type cytochrome c oxidase is coupled to the SAM headgroup (H(3)C- and/or HO-) near the main chain carbonyl oxygen of Cys153 and that Phe88 (analogous to Trp143 in subunit II of cytochrome c oxidase from R. sphaeroides) is not involved in the dominant tunneling pathway. Our work suggests that hydrogen bonds from hydroxyl or other proton-donor groups to carbonyl oxygens potentially can facilitate intermolecular ET between physiological redox partners.

Amido-bridged Cu2N2 diamond cores that minimize structural reorganization and facilitate reversible redox behavior between a Cu1Cu1 and a class III delocalized Cu1.5Cu1.5 species

A novel Cu(2)N(2) diamond core structure supported by an [SNS](-) ligand (1) ([SNS](-) = bis(2-tert-butylsulfanylphenyl)amido) has been prepared. This dicopper system exhibits a fully reversible one-electron redox process between a reduced Cu(1)Cu(1) complex, [[SNS][Cu]](2) (2), and a class III delocalized Cu(1.5)Cu(1.5) state, [[[SNS][Cu]](2)][B(3,5-(CF(3))(2)C(6)H(3))(4)] (3). Structural snapshots of both redox forms have been obtained to reveal remarkably little overall structural reorganization. The Cu...Cu bond distance nonetheless undergoes an appreciable compression (approximately 0.13 A) upon oxidation, providing a Cu...Cu distance of 2.4724(4) A in the mixed-valence state that is virtually identical to the Cu...Cu distance observed in the reduced form of the Cu(A) site of thiolate-bridged cytochrome c oxidase. Despite the low structural reorganization evident between 2 and 3, the [SNS](-) ligand is quite flexible. For example, square-planar geometries can prevail for divalent copper ions supported by [SNS](-) as evident from the crystal structure of [SNS]CuCl (4). Physical characterization for the mixed valence complex 3 includes electrochemical, magnetic (SQUID), EPR, and optical data. The complex has also been examined by density functional methods. An attempt was made to measure the rate of electron self-exchange k(s) between the Cu(1)Cu(1) and the Cu(1.5)Cu(1.5) complexes 2 and 3 by NMR line-broadening analysis in dichloromethane solution. While the system is certainly in the fast-exchange regime, the exchange process is too fast to be accurately measured by this technique. The value for k(s) can be bracketed with a conservative lower boundary of > or =10(7) M(-1) s(-1), a value that appears to be larger than other low molecular weight copper model complexes for which similar data is available. The unusually large magnitude of k(s) likely reflects the minimal structural reorganization that accompanies Cu(1)Cu(1) <--> Cu(1.5)Cu(1.5) interchange.

Two azurins with unusual redox and spectroscopic properties isolated from the Pseudomonas chlororaphis strains DSM 50083T and DSM 50135

Two azurins (Az624 and Az626) were isolated from the soluble extract of two strains of Pseudomonas chlororaphis, DSM 50083(T) and DSM 50135, respectively, grown under microaerobic conditions with nitrate as final electron acceptor. The azurins, purified to electrophoretic homogeneity in three chromatographic steps, exhibit several peculiar properties. They have high reduction potentials and lower pI than most azurins described in the literature. As previously observed for Pseudomonas aeruginosa azurin, their reduction potentials are pH-dependent, but the pK values of their oxidized forms are lower, which suggests that deeper structural changes are associated with the oxidation process of these novel azurins. A hitherto undescribed pH-dependence of the diffusion coefficient was observed in Az624, that could be caused either by conformational changes, or by the formation of supramolecular aggregates associated with a protonation process. Both azurins exhibit axial X-band electron paramagnetic resonance spectra in frozen solution showing a typical hyperfine with the copper nucleus (I=3/2) and a well-resolved superhyperfine structure with two equivalent 14N nucleus (I=1), which is not usually observed for azurins from other species.

Electron-transfer kinetics of copper(II/I) tripodal ligand complexes

The electron-transfer kinetics for each of three copper(II/I) tripodal ligand complexes reacting with multiple reducing and oxidizing counter reagents have been examined in aqueous solution at 25 degrees C, mu = 0.10 M. For all of the ligands studied, an amine nitrogen serves as the bridgehead atom. Two of the ligands (PMMEA and PEMEA) contain two thioether sulfurs and one pyridyl nitrogen as donor atoms on the appended legs while the third ligand (BPEMEA) has two pyridyl nitrogens and one thioether sulfur. Very limited kinetic studies were also conducted on two additional closely related tripodal ligand complexes. The results are compared to our previous kinetic study on a Cu(II/I) system involving a tripodal ligand (TMMEA) with thioether sulfur donor atoms on all three legs. In all systems, the Cu(II/I) electron self-exchange rate constants (k(11)) are surprisingly small, ranging approximately 0.03-50 M(-)(1) s(-)(1). The results are consistent with earlier studies reported by Yandell involving the reduction of Cu(II) complexes with four similar tripodal ligand systems, and it is concluded that the dominant reaction pathway involves a metastable Cu(II)L intermediate species (designated as pathway B). Since crystal structures suggest that the ligand reorganization accompanying electron transfer is relatively small compared to our earlier studies on macrocyclic ligand complexes of Cu(II/I), it is unclear why the k(11) values for the tripodal ligand systems are of such small magnitude.

Introduction of a pi-pi interaction at the active site of a cupredoxin: characterization of the Met16Phe Pseudoazurin mutant

The Met16Phe mutant of the type 1 copper protein pseudoazurin (PACu), in which a phenyl ring is introduced close to the imidazole moiety of the His81 ligand, has been characterized. NMR studies indicate that the introduced phenyl ring is parallel to the imidazole group of His81. The mutation has a subtle effect on the position of the two S(Cys)-->Cu(II) ligand-to-metal charge transfer bands in the visible spectrum of PACu(II) and a more significant influence on their intensities resulting in a A(459)/A(598) ratio of 0.31 for Met16Phe as compared to a A(453)/A(594) ratio of 0.43 for wild-type PACu(II) at pH 8. The electron paramagnetic resonance spectrum of the Met16Phe variant is more axial than that of the wild-type protein, and the resonance Raman spectrum of the mutant exhibits subtle differences. A C(gamma)H proton of Met86 exhibits a much smaller hyperfine shift in the paramagnetic (1)H NMR spectrum of Met16Phe PACu(II) as compared to its position in the wild-type protein, which indicates a weaker axial Cu-S(Met86) interaction in the mutant. The Met16Phe mutation results in an approximately 60 mV increase in the reduction potential of PACu. The pK(a) value of the ligand His81 decreases from 4.9 in wild-type PACu(I) to 4.5 in Met16Phe PACu(I) indicating that the pi-pi contact with Phe16 stabilizes the Cu-N(His81) interaction. The Met16Phe variant of PACu has a self-exchange rate constant at pH 7.6 (25 degrees C) of 9.8 x 10(3) M(-)(1) s(-)(1) as compared to the considerably smaller value of 3.7 x 10(3) M(-)(1) s(-)(1) for the wild-type protein under identical conditions. The enhanced electron transfer reactivity of Met16Phe PACu is a consequence of a lower reorganization energy due to additional active site rigidity caused by the pi-pi interaction between His81 and the introduced phenyl ring.

Active-site structure and electron-transfer reactivity of plastocyanins

The active-site structures of Cu(II) plastocyanins (PCu's) from a higher plant (parsley), a seedless vascular plant (fern, Dryopteris crassirhizoma), a green alga (Ulva pertusa), and cyanobacteria (Anabaena variabilis and Synechococcus) have been investigated by paramagnetic (1)H NMR spectroscopy. In all cases the spectra are similar, indicating that the structures of the cupric sites, and the spin density distributions onto the ligands, do not differ greatly between the proteins. The active-site structure of PCu has remained unaltered during the evolutionary process. The electron transfer (et) reactivity of these PCu's is compared utilizing the electron self-exchange (ESE) reaction. At moderate ionic strength (0.10 M) the ESE rate constant is dictated by the distribution of charged amino acid residues on the surface of the PCu's. Most higher plant and the seedless vascular plant PCu's, which have a large number of acidic residues close to the hydrophobic patch surrounding the exposed His87 ligand (the proposed recognition patch for the self-exchange process), have ESE rate constants of approximately 10(3) M(-)(1) s(-)(1). Removal of some of these acidic residues, as in the parsley and green algal PCu's, results in more favorable protein-protein association and an ESE rate constant of approximately 10(4) M(-)(1) s(-)(1). Complete removal of the acidic patch, as in the cyanobacterial PCu's, leads to ESE rate constants of approximately 10(5)-10(6) M(-)(1) s(-)(1). The ESE rate constants of the PCu's with an acidic patch also tend toward approximately 10(5)-10(6) M(-)(1) s(-)(1) at higher ionic strength, thus indicating that once the influence of charged residues has been minimized the et capabilities of the PCu's are comparable. The cytochromes and Fe-S proteins, two other classes of redox metalloproteins, also possess ESE rate constants of approximately 10(5)-10(6) M(-)(1) s(-)(1) at high ionic strength. The effect of the protonation of the His87 ligand in PCu(I) on the ESE reactivity has been investigated. When the influence of the acidic patch is minimized, the ESE rate constant decreases at high [H(+)].

Unusual properties of plastocyanin from the fern Dryopteris crassirhizoma

The effect of pH on the (1)H NMR spectrum, reduction potential, and self-exchange rate constant of the novel plastocyanin (PCu) from the fern plant Dryopteris crassirhizoma has been studied. The results are compared with those for the higher-plant PCu from parsley. In the (1)H NMR spectrum of D. crassirhizoma PCu(I), there is no sign that either of the His ligands is protonated at pH* down to 5.4. The reduction potentials of D. crassirhizoma and parsley PCu are 382 and 379 mV, respectively, at pH 7.4. When the pH value is decreased, the reduction potential of parsley PCu is seen to increase quite dramatically, consistent with protonation at His87 in PCu(I). A pK(a) of 5.8 is obtained from the electrochemistry data, consistent with a value of 5.6 determined by NMR. The reduction potential of D. crassirhizoma PCu exhibits a much less pronounced dependence on pH. The self-exchange rate constant of D. crassirhizomaPCu(I) is 3.4 x 10(3) M(-1) s(-1) at pH* 7.9. This is the smallest self-exchange rate constant reported to date for a PCu and can be rationalized by considering the altered distribution of charged residues on the surface of the D. crassirhizoma protein compared to the charge distributions of other higher-plant PCus. The self-exchange rate constant increases to 9 x 10(3) M(-1) s(-1) at pH* 5.4, consistent with enhanced protein-protein association at lower pH*, and the absence of His87 protonation in D. crassirhizoma PCu(I) in the accessible pH range.

Effect of histidine 6 protonation on the active site structure and electron-transfer capabilities of pseudoazurin from Achromobacter cycloclastes

The paramagnetic (1)H NMR spectrum of Cu(II) pseudoazurin [PACu(II)] contains eight directly observed hyperfine-shifted resonances which we have assigned using saturation transfer experiments on a 1:1 mixture of PACu(I) and PACu(II). The spectrum exhibits a number of similarities to those of other cupredoxins, but differences are found concerning the Cu-S(Met) interaction. The spectrum is dependent on pH* in the range 8.5-4.5 (pK(a)* 6.4), and a conformational change involving movement of the copper ion away from the Met toward the equatorial ligands, as a consequence of protonation of the surface His6 residue, is identified. Corresponding changes are also seen in the UV/vis spectrum. The protonation/deprotonation equilibrium of His6 influences the reduction potential of the protein in the same pH range. The self-exchange rate constant of PACu at pH* 6.0 (25 degrees C) is considerably smaller (1.1 x 10(3) M(-1) s(-1)) than the value obtained at pH* 7.6 (3.7 x 10(3) M(-1) s(-1)). The effect on the self-exchange reactivity is mainly due to an alteration in the reorganization energy of the copper site brought about by the structural change resulting from His6 protonation.

Direct evidence for a geometrically constrained "entatic state" effect on copper(II/I) electron-transfer kinetics as manifested in metastable intermediates

The absolute magnitude of an "entatic" (constrained) state effect has never been quantitatively demonstrated. In the current study, we have examined the electron-transfer kinetics for five closely related copper(II/I) complexes formed with all possible diastereomers of [14]aneS(4) (1,4,8,11-tetrathiacyclotetradecane) in which both ethylene bridges have been replaced by cis- or trans-1,2-cyclohexane. The crystal structures of all five Cu(II) complexes and a representative Cu(I) complex have been established by X-ray diffraction. For each complex, the cross-reaction rate constants have been determined with six different oxidants and reductants in aqueous solution at 25 degrees C, mu = 0.10 M. The value of the electron self-exchange rate constant (k(11)) has then been calculated from each cross reaction rate constant using the Marcus cross relation. All five Cu(II/I) systems show evidence of a dual-pathway square scheme mechanism for which the two individual k(11) values have been evaluated. In combination with similar values previously determined for the parent complex, Cu(II/I)([14]aneS(4)), and corresponding complexes with the two related monocyclohexanediyl derivatives, we now have evaluated a total of 16 self-exchange rate constants which span nearly 6 orders of magnitude for these 8 closely related Cu(II/I) systems. Application of the stability constants for the formation of the corresponding 16 metastable intermediates--as previously determined by rapid-scan cyclic voltammetry--makes it possible to calculate the specific electron self-exchange rate constants representing the reaction of each of the strained intermediate species exchanging electrons with their stable redox partners--the first time that calculations of this type have been possible. All but three of these 16 specific self-exchange rate constants fall within--or very close to--the range of 10(5)-10(6) M(-1) s(-1), values which are characteristic of the most labile Cu(II/I) systems previously reported, including the blue copper proteins. The results of the current investigation provide the first unequivocal demonstration of the efficacy of the entatic state concept as applied to Cu(II/I) systems.

Effects of dimerization on protein electron transfer

In order to investigate the relationship between the rate of protein-protein electron transfer and the structure of the association complex, a dimer of the blue copper protein azurin was constructed and its electron exchange properties were determined. For this purpose, a site for covalent cross-linking was engineered by replacing the surface-exposed asparagine 42 with a cysteine. This mutation enabled the formation of disulfide-linked homo-dimers of azurin. Based on NMR line-broadening experiments, the electron self-exchange (e.s.e.) rate constant for this dimer was determined to be 4.2(+/-0.7) x 10(5)M(-1)s(-1), which is a seven-fold decrease relative to wild-type azurin. This difference is ascribed to a less accessible hydrophobic patch in the dimer. To discriminate between intramolecular electron transfer within a dimer and intermolecular electron transfer between two dimers, the e.s.e. rate constant of (Cu-Cu)-N42C dimers was compared with that of (Zn-Cu)- and (Ag-Cu)-N42C dimers. As Zn and Ag are redox inactive, the intramolecular electron transfer reaction in these latter dimers can be eliminated. The e.s.e. rate constants of the three dimers are the same and an upper limit for the intramolecular electron transfer rate of 10 s(-1) could be determined. This rate is compatible with a Cu-Cu distance of 18 A or more, which is larger than the Cu - Cu distance of 15 A observed in the wild-type crystal structure that shows two monomers that face each other with opposing hydrophobic patches. Modelling of the dimer shows that the Cu-Cu distance should be in the range of 17 A < rCu-Cu < 28 A, which is in agreement with the experimental findings. For efficient electron transfer, it appears crucial that the two molecules interact in the proper orientation. Direct cross-linking may disturb the formation of such an optimal electron transfer complex.

Effect of pH on the self-exchange reactivity of the plant plastocyanin from parsley

The self-exchange rate constant (25 degrees C) for parsley plastocyanin is 5.0 x 10(4) M-1 s-1 at pH* 7.5 (I = 0.10 M). This value is quite large for a higher plant plastocyanin and can be attributed to a diminished upper acidic patch in this protein. The self-exchange rate constant is almost independent of pH* in the range 7.5-5.6, with a value (25 degrees C) of 5.6 x 10(4) M-1 s-1 at pH* 5.6 (I = 0.10 M). At this pH*, the ligand His87 is protonated in approximately 50% of the reduced protein molecules (pKa* 5.6), and this would be expected to hinder electron transfer between the two oxidation states. However, this effect is counterbalanced by the enhanced association of two parsley plastocyanins at lower pH* due to the partial protonation of the acidic patch.

The Met99Gln mutant of amicyanin from Paracoccus versutus

The axial copper ligand methionine has been replaced by a glutamine in the cupredoxin amicyanin from Paracoccus versutus. Dynamic and structural characteristics of the mutant have been studied in detail using UV/Vis, EPR, NMR, cyclic voltammetry, and isomorphous metal replacement. M99Q amicyanin is a blue copper protein with significant spectral and structural similarities to the other cupredoxins umecyanin, stellacyanin, and M121Q azurin. In addition, the functional properties of M99Q amicyanin, as reflected in the electron self-exchange rate constant and midpoint potential (165 mV), have been assessed and compared to values for M121Q azurin. For the latter protein, the published midpoint potential was corrected to the much lower value of 147 mV at pH 7, I = 0.1 M. These values are very similar to the midpoint potential of stellacyanin, which naturally possesses an axial glutamine ligand and has the lowest reduction potential for a naturally occurring cupredoxin. A remarkable feature of M99Q amicyanin, in the reduced state, is the relatively high pK(a) value of 7.1 for its His96 ligand.

Electron transfer properties of iron-sulfur proteins

The details of most electron transfer reactions involving iron-sulfur proteins have remained undisclosed because of the lack of experimental methods suitable to measure precisely the relevant rates. Nuclear magnetic resonance (NMR) provides a powerful means to overcome these problems, at least with selected proteins. A combination of NMR studies and site-directed mutagenesis experiments has been instrumental in defining both the site of interaction and the main trends of the intracomplex electron transfer in the case of rubredoxin electron self-exchange. Analysis of the NMR data obtained for mixtures of different redox levels of several 2[4Fe-4S] ferredoxins provided both first-order, for intramolecular, and second-order, for intermolecular, rate constants. Their dependence as a function of structural changes gave insight into the mechanism of electron transfer in this type of protein. Contrary to some expectations, the high-spin [4Fe-4Se]+ clusters assembled in isopotential ferredoxins do not change the intramolecular electron transfer rate as compared to low-spin [4Fe-4S]+ homologs. In combination with activity measurements, the kinetic data have been used to model the electron transfer competent complexes between Clostridium pasteurianum ferredoxin and the main enzymes acting as redox partners in vivo.

Electron-transfer properties of Pseudomonas aeruginosa [Lys44, Glu64]azurin

In the hydrophobic patch of azurin from Pseudomonas aeruginosa, an electric dipole was created by changing Met44 into Lys and Met64 into Glu. The effect of this dipole on the electron-transfer properties of azurin was investigated. From a spectroscopic characterization (NMR, EPR and ultraviolet-visible) it was found that both the copper site and the overall structure of the [Lys44, Glu64]azurin were not disturbed by the two mutations. A small perturbation of the active site at high pH, similar to that observed for [Lys44]azurin, occurs in the double mutant. At neutral pH the electron-self-exchange rate constant of the double mutant shows a decrease of three orders of magnitude compared with the wild-type value. The possible reasons for this decrease are discussed. Electron transfer with the proposed physiological redox partners cytochrome c551 and nitrite reductase have been investigated and the data analyzed in the Marcus framework. From this analysis it is confirmed that the hydrophobic patch of azurin is the interaction site with both partners, and that cytochrome c551 uses its hydrophobic patch and nitrite reductase a negatively charged surface area for the electron transfer.

Redox reactivity of the type 1 copper protein amicyanin from Thiobacillus versutus with its physiological partner cytochrome C550 and inter-protein cross-reaction studies

Reduction potentials Eo' for the T. versutus amicyanin couple, AmCuII/I, were determined at pH values in the range 4.4-9.0 by direct measurement using cyclic voltammetry, and from rate constants for the reactions AmCu1 + [Co(terpy)2]3+ and [Co(terpy)2]2+ + AmCuII, using an Eo' for the [Co(terpy)2]2+/3+ couple of 260 mV. At pH > 7.5 the value obtained is 236 mV, which increases with decreasing pH in keeping with proton inactivation of AmCuI. Together with previously determined Eo' values for the T. versutus cytochrome C550 FeIII/FeII couple, it is concluded that the physiologically relevant reaction AmCuI + cyt C550FeIII (kf) is thermodynamically favourable at pH > 6.25, but that the back reaction cyt C550FeII + AmCuII (kb) is favourable at pH < 6.25. Values of kf (25 degrees C) at pH > 6.25 were determined directly by the stopped-flow method, I = 0.100 M (NaCl). At pH < 6.25 kf values were obtained indirectly from the measured kb and equilibrium constants from delta Eo'. The combined kf variations with pH give an acid dissociation pKa for AmCuIH+ of 6.6. In further studies (25 degrees C) rate constants/M-1 S-1 (pH 6.0-8.6) were determined for the cross-reactions of AmCuI with P. aeruginosa azurin AzCuII, and AmCuI with P. aeruginosa cyt C550FeIII, and are 11.0 x 10(5) and 6.4 x 10(5) M-1 S-1 respectively at pH 8.6. Using the Marcus equations corresponding electron self-exchange rate constants (kese/M-1 S-1) of 1.3 x 10(5) and 0.6 x 10(5) M-1 S-1 were calculated for the exchange of AmCuII with unprotonated AmCuI, in good agreement with the value 1.2 x 10(5) M-1 S-1 determined by NMR at pH 8.6. Information was also obtained as to the effect of pH on these kese values.

Nuclear-magnetic-resonance determination of the electron self-exchange rate constant of Clostridium pasteurianum rubredoxin

The iron ion of rubredoxins efficiently exchanges one electron between the Fe(II) and Fe(III) oxidation states in mixtures of oxidized and reduced protein. The conditions under which the relaxation properties of the NMR signals can provide information about this exchange process have been worked out. The rate constant for the rubredoxin electron self-exchange ranges between 1.5 x 10(5) M-1 s-1 at 12 degrees C and 3 x 10(5) M-1 s-1 at 30 degrees C with an activation energy of the order of 24-30 kJ mol-1 in 50 mM potassium phosphate, pH 7. The increase of the electron self-exchange rate constant with ionic strength suggests that neutralizing electrostatic repulsion between the active sites of two molecules further accelerates the already fast electron exchange.

Analysis of the paramagnetic copper(II) site of amicyanin by 1H NMR spectroscopy

Application of the tailored pulse sequences like super-WEFT allows the direct observation of the hyperfine-shifted signals of the paramagnetic Cu(II) forms of blue copper proteins in solution. The signals can be assigned by applying 2D NMR techniques, like EXSY, to solutions containing a mixture of reduced and oxidized species. The Fermi contact shift is separated from the pseudocontact shift on the basis of the known g-tensor anisotropy of the Cu(II) state, allowing the determination of a number of hyperfine-splitting constants between protons on the Cu ligands and the unpaired electron. These results are used to quantify the spin density distribution over the Cu ligands. In amicyanin about 50%-60% of the unpaired electron density is found on the ligands. It appears possible to quantify the Cu-S(Met) interaction on the basis of the NMR results. Application of the technique to the wild type forms of amicyanin and azurin and to two active site mutants of amicyanin (His96Asp and a plastocyanin-amicyanin loop exchange mutant) shows that the Cu-S(Met) interaction parallels the rhombicity and axial distortion of the Cu site.

Spectroscopic and electrochemical studies on active-site transitions of the type 1 copper protein pseudoazurin from Achromobacter cycloclastes

The single type 1 copper protein pseudoazurin from Achromobacter cycloclastes gives reversible electrochemical behavior at a (4-pyridyl)disulfide-modified gold electrode. Measurements carried out at 25.0 degrees C indicate a midpoint reduction potential of E 1/2 = 260 mV versus normal hydrogen electrode at pH 7.0 and a peak-to-peak separation of delta Ep = 59 mV. The diffusion coefficient and heterogeneous electron transfer rate constant are estimated to be 2.23 x 10(-6) cm2 s-1 and 3.7 x 10(-2) cm s-1, respectively. Also, controlled potential electrolysis indicates a 1-electron transfer process and a formal reduction potential of 259 mV versus normal hydrogen electrode for the Cu(II)/Cu(I) couple. The heterogeneous electron transfer rate constant determined at the (4-pyridyl)disulfide-modified gold electrode at pH 4.6 is 6.7 x 10(-3) cm s-1, consistent with a slower process at the positively charged electrode surface. At pH 11.3, UV-visible, EPR, and resonance Raman spectra indicate a conversion of the distorted tetrahedral copper geometry to a trigonal structure. The trigonal form has elongated axial bonding and an axial EPR spectrum. At pH 11.3, the reduction potential is further decreased, and Cu-S bands in resonance Raman spectra at 330-460 cm-1 are shifted to higher energy (approximately 10 cm-1), consistent with a stronger Cu-S bond.

1H-NMR study of a cobalt-substituted blue copper protein: Pseudomonas aeruginosa Co(II)-azurin

Substitution of copper by cobalt in blue copper proteins gives a paramagnetic metalloderivative suitable for paramagnetic NMR studies. A thorough analysis of the 1H-NMR spectrum of Pseudomonas aeruginosa Co(II)-azurin is presented here. All the observable contact-shifted signals as well as many other paramagnetic signals from protons placed up to about 1.0 nm around the metal center, including some residues belonging to functionally important parts of the protein like the hydrophobic patch and the His35 region, have been assigned. The results obtained permit the detection and study of structural variations like those originated by the His35 ionization, and allow us to draw a feasible picture of the metal coordination site. Contact-shifted signals correspond to the same five residues which are found in the coordination sphere of the native Cu(II)-azurin, i.e. His46, His117, Cys112, Met121 and Gly45. Among them, the histidine residues present a pattern of resonances typical for histidines coordinated to cobalt in other cobalt protein derivatives, and the cysteine signals clearly indicate a strong interaction with the paramagnetic Co(II) ion. In contrast, the Met121 signals indicate a weak but still existent contact interaction with the metal center. On the other hand, the very weak copper ligand, Gly45, appears here as clearly coordinated to cobalt. Results are consistent with a distorted tetrahedral metal site with the cobalt deviated from the N2S plane towards the Gly45 O axial position and weakly interacting with the Met121 sulfur.

The introduction of a negative charge into the hydrophobic patch of Pseudomonas aeruginosa azurin affects the electron self-exchange rate and the electrochemistry

By changing Met64 into a glutamate by means of site-directed mutagenesis a negative charge was introduced into the hydrophobic patch of azurin from Pseudomonas aeruginosa. The three-dimensional structure of the protein and the structure of the metal site in particular, appear unaffected by the mutation. The observed change of the midpoint potential of the mutant of 28 mV is ascribed to the deprotonation of Glu64. The electron-self-exchange rate constant equals that of the wild-type protein at pH 4.5 but decreases by almost two orders of magnitude at high pH. Electron transfer is inhibited only when both of the reacting azurin molecules have an ionized glutamate at position 64 in their hydrophobic patch. Electron transfer at a graphite electrode is slowed down by the presence of the negative charge in the hydrophobic patch. The results demonstrate once again that the Cu-ligand His117 in the hydrophobic patch is the likely entry and exit point for electrons. This observation holds both for the (homogeneous) electron-self-exchange reaction in solution as well as for the (heterogenous) reaction at an electrode.

Effect of lysine ionization on the structure and electrochemical behaviour of the Met44-->Lys mutant of the blue-copper protein azurin from Pseudomonas aeruginosa

The structural and spectrochemical effects of the replacement of Met44 in the hydrophobic surface patch of azurin from Pseudomonas aeruginosa by a lysine residue were studied as a function of the ionization state of the lysine. In the pH range 5-8, the optical absorption, resonance Raman, EPR and electron spin-echo envelope modulation spectroscopic properties of wild-type and Met44-->Lys (M44K) azurin are very similar, indicating that the Cu-site geometry has been maintained. At higher pH, the deprotonation of Lys44 in M44K azurin (pKa 9-10) is accompanied by changes in the optical-absorption maxima (614 nm and 450 nm instead of 625 nm and 470 nm) and in the EPR gII value (2.298 instead of 2.241), indicative of a change in the bonding interactions of Cu at high pH. The strong pH dependence of the electron self-exchange rate of M44K azurin supports the assignment of Lys44 as the ionizable group and demonstrates the importance of the hydrophobic patch for electron transfer. The pH dependence of the midpoint potentials of wild-type and M44K azurin can be accounted for by the ionizations of His35 and His83 and by the additional electrostatic effect of the mutation.