Reduction potentials and their pH dependence in site-directed-mutant forms of azurin from Pseudomonas aeruginosa

Comparison of intra- vs intermolecular long-range electron transfer in crystals of ruthenium-modified azurin

Selective metal-ion incorporation and ligand substitution are employed to control whether electrons tunnel over intra- or intermolecular separations in crystals of P. aeruginosa azurin modified with Ru-polypyridine complexes. Cu(1+)-to-Ru3+ electron transfer (ET) across a specific protein-protein interface in the crystal lattice has a time constant 5-10 times longer than ET between the same donor and acceptor within a single protein (tauET = 5 vs 0.5-1.0 micros). Slower intermolecular ET agrees well with a longer distance between redox centers across the inter-protein (18.9 A) compared to the intra-protein separation (17.0 A) and indicates that the closest donor/acceptor pair dominates crystal ET. Lowering the crystal pH accelerates inter-protein ET (tauET = 1.0 micros) but not intra-protein ET. Faster inter-protein ET likely results from a pH-induced peptide bond flip that perturbs hydrogen bonding in the path between Ru and Cu centers on adjacent molecules.

Correlation of rhombic distortion of the type 1 copper site of M98Q amicyanin with increased electron transfer reorganization energy

Mutation of the axial Met ligand of the type 1 copper site of amicyanin to Ala or Gln yielded M98A amicyanin, which exhibits typical axial type 1 ligation geometry but with a water molecule providing the axial ligand, and M98Q amicyanin, which exhibits significant rhombic distortion of the type 1 site (Carrell, C. J., Ma, J. K., Antholine, W. E., Hosler, J. P., Mathews, F. S., and Davidson, V. L. (2007) Biochemistry 46, 1900-1912). Despite the change of the axial ligand, the M98Q and M98A mutations had little effect on the redox potential of copper. The true electron transfer (ET) reactions from O-quinol methylamine dehydrogenase to oxidized native and mutant amicyanins revealed that the M98A mutation had little effect on kET, but the M98Q mutation reduced kET 45-fold. Thermodynamic analysis of the latter showed that the decrease in kET was due to an increase of 0.4 eV in the reorganization energy (lambda) associated with the ET reaction to M98Q amicyanin. No change in the experimentally determined electronic coupling or ET distance was observed, confirming that the mutation had not altered the rate-determining step for ET and that this was still a true ET reaction. The basis for the increased lambda is not the nature of the atom that provides the axial ligand because each uses an oxygen from Gln in M98Q amicyanin and from water in M98A amicyanin. Comparisons of the distance of the axial copper ligand from the equatorial plane that is formed by the other three copper ligands in isomorphous crystals of native and mutant amicyanins at atomic resolution indicate an increase in distance from 0.20 A in the native to 0.42 A in M98Q amicyanin and a slight decrease in distance for M98A amicyanin. This correlates with the rhombic distortion caused by the M98Q mutation that is clearly evident in the EPR and visible absorption spectra of the protein and suggests that the extent of rhombicity of the type 1 copper site influences the magnitude of lambda.

Modulation of the electrochemical behavior of tyrosyl radicals by the electrode surface

The ability to adsorb proteins and enzymes on electrode surfaces enhances opportunities for studying enzyme activity and redox-based catalysis. Proteins may be bound in a chosen orientation on surfaces so that specific sites within them may be preferentially studied, but to date no systematic study of a redox moiety from solvent to electrode surface to the protein milieu has been performed. We report the redox and ionization behavior of tyrosine-cysteine, using the cysteine residue to form covalent linkages with Au and self-assembled-monolayer (SAM)-modified Au surfaces and using the tyrosine for redox activity. In addition, the same redox fragment incorporated into a protein bound to a SAM is examined. We find that directly binding the dipeptide to Au causes the greatest change in properties, while binding it to the SAM causes a slight perturbation in redox potential and a significant perturbation in pK(a). When azurin with a surface-exposed tyrosine is bound to a SAM-modified electrode, the redox potential and pK(a) of the tyrosine are nearly unperturbed from the values found for the dipeptide free in solution. Finally, quantification of the voltammetric signal indicates that tyrosine oxidation in the protein triggers the additional oxidation of another nearby amino acid.

Reduction potential tuning of the blue copper center in Pseudomonas aeruginosa azurin by the axial methionine as probed by unnatural amino acids

The conserved axial ligand methionine 121 from Pseudomonas aeruginosa azurin (Az) has been replaced by isostructural unnatural amino acid analogues, oxomethionine (OxM), difluoromethionine (DFM), trifluoromethionine (TFM), selenomethionine (SeM), and norleucine (Nle) using expressed protein ligation. The replacements resulted in < 6 nm shifts in the S(Cys)-Cu charge transfer (CT) band in the electronic absorption spectra and < 8 gauss changes in the copper hyperfine coupling constants (AII) in the X-band electron paramagnetic resonance spectra, suggesting that isostructural replacement of Met resulted in minimal structural perturbation of the copper center. The slight blue shifts of the CT band follow the trend of stronger electronegativity of the ligands. This trend is supported by 19F NMR studies of the fluorinated methionine analogues. However, the order of AII differs, suggesting additional factors influencing AII. In contrast to the small changes in the UV-vis and EPR spectra, a large variation of > 227 mV in reduction potential was observed for the series of variants reported here. Additionally, a linear correlation was established between the reduction potentials and hydrophobicity of the variants. Extension of this analysis to other type 1 copper-containing proteins reveals a linear correlation between change in hydrophobicity and change in reduction potential, independent of the protein scaffold, experimental conditions, measurement techniques, and steric modifications. This analysis has also revealed for the first time high and low potential states for type 1 centers, and the difference may be attributable to destabilization of the protein fold by disruption of hydrophobic or hydrogen bonding interactions that stabilize the type 1 center.

Characterization and copper binding properties of human COMMD1 (MURR1)

COMMD1 (copper metabolism gene MURR1 (mouse U2af1-rs1 region1) domain) belongs to a family of multifunctional proteins that inhibit nuclear factor NF-kappaB. COMMD1 was implicated as a regulator of copper metabolism by the discovery that a deletion of exon 2 of COMMD1 causes copper toxicosis in Bedlington terriers. Here, we report the detailed characterization and specific copper binding properties of purified recombinant human COMMD1 as well as that of the exon 2 product, COMMD(61-154). By using various techniques including native-PAGE, EPR, UV-visible electronic absorption, intrinsic fluorescence spectroscopies as well as DEPC modification of histidines, we demonstrate that COMMD1 specifically binds copper as Cu(II) in 1:1 stoichiometry and does not bind other divalent metals. Moreover, the exon 2 product, COMMD(61-154), alone was able to bind Cu(II) as well as the wild type protein, with a stoichiometry of 1 mol of Cu(II) per protein monomer. The protection of DEPC modification of COMMD1 by Cu(II) implied that Cu(II) binding involves His residues. Further investigation by DEPC modification of COMMD(61-154) and subsequent MALDI MS mapping and MS/MS sequencing identified the protection of His101 and His134 residues in the presence of Cu(II). Fluorescence studies of single point mutants of the full-length protein revealed the involvement of M110 in addition to H134 in direct Cu(II) binding. Taken together, the data provide insight into the function of COMMD1 and especially COMMD(61-154), a product of exon 2 that is deleted in terriers affected by copper toxicosis, as a regulator of copper homeostasis.

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.

A Rearranging Ligand Enables Allosteric Control of Catalytic Activity in Copper-containing Nitrite Reductase

In Cu-containing nitrite reductase from Alcaligenes faecalis S-6 the axial methionine ligand of the type-1 site was replaced (M150G) to make the copper ion accessible to external ligands that might affect the enzyme's catalytic activity. The type-1 site optical spectrum of M150G (A(460)/A(600)=0.71) differs significantly from that of the native nitrite reductase (A(460)/A(600)=1.3). The midpoint potential of the type-1 site of nitrite reductase M150G (E(M)=312(+/-5)mV versus hydrogen) is higher than that of the native enzyme (E(M)=213(+/-5)mV). M150G has a lower catalytic activity (k(cat)=133(+/-6)s(-1)) than the wild-type nitrite reductase (k(cat)=416(+/-10)s(-1)). The binding of external ligands to M150G restores spectral properties, midpoint potential (E(M)<225mV), and catalytic activity (k(cat)=374(+/-28)s(-1)). Also the M150H (A(460)/A(600)=7.7, E(M)=104(+/-5)mV, k(cat)=0.099(+/-0.006)s(-1)) and M150T (A(460)/A(600)=0.085, E(M)=340(+/-5)mV, k(cat)=126(+/-2)s(-1)) variants were characterized. Crystal structures show that the ligands act as allosteric effectors by displacing Met62, which moves to bind to the Cu in the position emptied by the M150G mutation. The reconstituted type-1 site has an otherwise unaltered geometry. The observation that removal of an endogenous ligand can introduce allosteric control in a redox enzyme suggests potential for structural and functional flexibility of copper-containing redox sites.

Perturbations of the T1 copper site in the CotA laccase from Bacillus subtilis: structural, biochemical, enzymatic and stability studies

Site-directed mutagenesis has been used to replace Met502 in CotA laccase by the residues leucine and phenylalanine. X-ray structural comparison of M502L and M502F mutants with the wild-type CotA shows that the geometry of the T1 copper site is maintained as well as the overall fold of the proteins. The replacement of the weak so-called axial ligand of the T1 site leads to an increase in the redox potential by approximately 100 mV relative to that of the wild-type enzyme (E0 =455 mV). However the M502L mutant exhibits a twofold to fourfold decrease in the kcat values for the all substrates tested and the catalytic activity in M502F is even more severely compromised; 10% activity and 0.15-0.05% for the non-phenolic substrates and for the phenolic substrates tested when compared with the wild-type enzyme. T1 copper depletion is a key event in the inactivation and thus it is a determinant of the thermodynamic stability of wild-type and mutant proteins. Whilst the unfolding of the tertiary structure in the wild-type enzyme is a two-state process displaying a midpoint at a guanidinium hydrochloride concentration of 4.6 M and a free-energy exchange in water of 10 kcal/mol, the unfolding for both mutant enzymes is clearly not a two-state process. At 1.9 M guanidinium hydrochloride, half of the molecules are in an intermediate conformation, only slightly less stable than the native state (approximately 1.4 kcal/mol). The T1 copper centre clearly plays a key role, from the structural, catalytic and stability viewpoints, in the regulation of CotA laccase activity.

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.

Reduction thermodynamics of the T1 Cu site in plant and fungal laccases

The thermodynamic parameters for reduction of the type-1 (T1) copper site in Rhus vernicifera and Trametes versicolor laccases and for the derivative of the former protein from which the type-2 copper has been selectively removed (T2D) have been determined with UV-vis spectroelectrochemistry. In all cases, the enthalpic term turns out to be the main determinant of the Eo' of the T1 site. Also the difference between the reduction potentials of the two laccases is enthalpy-based and reflects differences in the coordination features of the T1 sites and their protein environment. The T1 sites in native R. vernicifera laccase and its T2D derivative show the same Eo', as a result of compensatory differences in the reduction thermodynamics. This suggests that removal of the type-2 (T2) copper results in modification of the reduction-induced solvent reorganization effects, with no influence in the structure of the multicopper protein site. This conclusion is supported by NMR data recorded on the native, the T2D, and Hg-substituted T1 derivatives of R. vernicifera laccase, which show that the T1 and T2/T3 sites are largely noninteracting.

Ligand and loop variations at type 1 copper sites: influence on structure and reactivity

Type 1 (T1) copper sites promote biological electron transfer and are found in the cupredoxins and a number of copper-containing enzymes including the multi-copper oxidases. A T1 copper site usually has a distorted tetrahedral geometry with strong ligands provided by the thiolate sulfur of a Cys and the imidazole nitrogens of two His residues. The active site structure is typically completed by a weak axial Met ligand (a second weak axial interaction is found in azurin resulting in a trigonal bipyramidal geometry). The axial Met is not conserved and Gln, Phe, Leu and Val are also found in this position. Three of the four ligands at a T1 copper site are situated on a single C-terminal loop whose length and structure varies. Studies are discussed which investigate both the influence of physiologically relevant axial ligand alterations, and also of mutations to the length and structure of the ligand-containing loop, on the properties of T1 copper sites.

Axial methionine has much less influence on reduction potentials in a CuA center than in a blue copper center

The role of the highly conserved axial methionine of the purple CuA center in an engineered CuA azurin on modulating the reduction potentials of the copper center was investigated by a systematic replacement of the methionine with glutamate, aspartate, and leucine. In contrast to the same substitutions in the structurally related blue copper azurin, much smaller changes in reduction potential were observed in the CuA azurin upon replacing the methionine ligand with negatively charged Glu (-8 mV) and Asp (-5 mV) and more hydrophobic Leu (+16 mV). These findings are important in understanding the different roles of the two cupredoxins. The diamond core Cu2S2(Cys) structure of the CuA is much more resistant to variations of axial ligand interactions than the distorted tetrahedral structure of the blue copper protein. This difference may translate into a much wider range of reduction potentials (>1000 mV) for blue copper proteins that transfer electrons to a variety of partners in many different biological systems and a much narrower range of reduction potentials (<40 mV) for CuA proteins where a small difference in reduction potentials between the CuA and its redox partners is required.

Direct Tyrosine Oxidation Using the MLCT Excited States of Rhenium Polypyridyl Complexes

Rhenium(I) polypyridyl complexes have been designed for the intramolecular photogeneration of tyrosyl radical. Tyrosine (Y) and phenylalanine (F) have each been separately appended to a conventional Re(I)(bpy)(CO)(3)CN framework via an amide linkage to the bipyridine (bpy) ligand. Comparative time-resolved emission quenching and transient absorption spectra of Re(bpy-Y)(CO)(3)CN and Re(bpy-F)(CO)(3)CN show that Y is oxidized only upon its deprotonation at pH 12. In an effort to redirect electron transport so that it is more compatible with intramolecular Y oxidation, polypyridyl Re(I) complexes have been prepared with the amide bond functionality located on a pendant phosphine ligand. A [Re(phen)(PP-Bn)(CO)(2)](PF(6)) (PP = bis(diphenylphosphino)ethylene) complex has been synthesized and crystallographically characterized. Electrochemistry and phosphorescence measurements of this complex indicate a modest excited-state potential for tyrosine oxidation, similar to that for the (bpy)Re(I)(CO)(3)CN framework. The excited-state oxidation potential can be increased by introducing a monodentate phosphine to the Re(I)(NN)(CO)(3)(+) framework (NN = polypyridyl). In this case, Y is oxidized at all pHs when appended to the triphenylphosphine (P) of [Re(phen)(P-Y)(CO(3))](PF(6)). Analysis of the pH dependence of the rate constant for tyrosyl radical generation is consistent with a proton-coupled electron transfer (PCET) quenching mechanism.

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.

Structural studies of two mutants of amicyanin from Paracoccus denitrificans that stabilize the reduced state of the copper

Mutation of Pro94 to phenylalanine or alanine significantly alters the redox properties of the type I copper center of amicyanin. Each mutation increases the redox midpoint potential (E(m)) value by at least 140 mV and shifts the pK(a) for the pH dependence of the E(m) value to a more acidic value. Atomic resolution (0.99-1.1 A) structures of both the P94F and P94A amicyanin have been determined in the oxidized and reduced states. In each amicyanin mutant, an electron-withdrawing hydrogen bond to the copper-coordinating thiolate sulfur of Cys92 is introduced by movement of the amide nitrogens of Phe94 and Ala94 much closer to the thiolate sulfur than in wild-type amicyanin. This is the likely explanation for the much more positive E(m) values which result from each of these mutations. The observed decrease in the pK(a) value for the pH dependence of the E(m) value that is seen in the mutants seems to be correlated with steric hindrance to the rotation of the His95 copper ligand which results from the mutations. In wild-type amicyanin the His95 side chain undergoes a redox and pH-dependent conformational change which accounts for the pH dependence of the E(m) value of amicyanin. The reduced P94A amicyanin exhibits two alternate conformations with the positions of the copper 1.4 A apart. In one of these conformations, a water molecule appears to have replaced Met98 as a copper ligand. The relevance of these structures to the electron transfer properties of P94F and P94A amicyanin are also discussed.

Ferrous Binding to the Multicopper OxidasesSaccharomyces cerevisiaeFet3p and Human Ceruloplasmin: Contributions to Ferroxidase Activity

The multicopper oxidases are a family of enzymes that couple the reduction of O(2) to H(2)O with the oxidation of a range of substrates. Saccharomyces cerevisiae Fet3p and human ceruloplasmin (hCp) are members of this family that exhibit ferroxidase activity. Their high specificity for Fe(II) has been attributed to the existence of a binding site for iron. In this study, mutations at the E185 and Y354 residues, which are putative ligands for iron in Fet3p, have been generated and characterized. The effects of these mutations on the electronic structure of the T1 Cu site have been assessed, and the reactivities of this site toward 1,4-hydroquinone (a weak binding substrate) and Fe(II) have been evaluated and interpreted in terms of the semiclassical Marcus theory for electron transfer. The electronic and geometric structure of the Fe(II) substrate bound to Fet3p and hCp has been studied for the first time, using variable-temperature variable field magnetic circular dichroism (VTVH MCD) spectroscopy. The iron binding sites in Fet3p and hCp appear to be very similar in nature, and their contributions to the ferroxidase activity of these proteins have been analyzed. It is found that these iron binding sites play a major role in tuning the reduction potential of iron to provide a large driving force for the ferroxidase reaction, while still supporting the delivery of the Fe(III) product to the acceptor protein. Finally, the analysis of possible electron-transfer (ET) pathways from the protein-bound Fe(II) to the T1 Cu site indicates that the E185 residue not only plays a role in iron binding, but also provides the dominant ET pathway to the T1 Cu site.

Determination of reduction potential of an engineered CuA azurin by cyclic voltammetry and spectrochemical titrations

The reduction potentials of an engineered CuA azurin in its native and thermally denatured states have been determined using cyclic voltammetry and spectrochemical titrations. Using a 4,4'-dipyridyl disulfide modified gold electrode, the reduction potentials of native and thermally denatured CuA azurin are the same within the experimental error (422 +/- 5 and 425 +/- 5 mV vs. NHE, respectively, in 50 mM ammonium acetate buffer, pH 5.1, 300 mM NaCl, 25 degrees C), indicating that the potential is that of a nonnative state. In contrast, using a didodecyldimethylammonium bromide (DDAB) film-pyrolytic graphite edge (PGE) electrode, the reduction potentials of native and thermally denatured CuA azurin have been determined to be 271 +/- 7 mV (50 mM ammonium acetate buffer, pH 5.1, 4 degrees C) and 420 +/- 1 mV (50 mM ammonium acetate buffer, pH 5.1, 25 degrees C), respectively. Spectroscopic redox titration using [Ru(NH3)5Py]2+ resulted in a reduction potential (254+/-4 mV) (50 mM ammonium acetate buffer, pH 5.1, 4 degrees C) similar to the value obtained using the DDAB film-PGE electrochemical method. Complete reoxidation of [Ru(NH3)5Py]2+-reduced CuA azurin is also consistent with the conclusion that this spectrochemical titration method using [Ru(NH3)5Py]2+ measures the reduction potential of native CuA azurin.

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.

Probing the role of axial methionine in the blue copper center of azurin with unnatural amino acids

Expressed protein ligation was used to replace the axial methionine of the blue copper protein azurin from Pseudomonas aeruginosa with unnatural amino acids. The highly conserved methionine121 residue was replaced with the isostructural amino acids norleucine (Nle) and selenomethionine (SeM). The UV-visible absorption, X- and Q-band EPR, and Cu EXAFS spectra of the variants are slightly perturbed from WT. All variants have a predominant S(Cys) to Cu(II) charge transfer band around 625 nm and narrow EPR hyperfine splittings. The Se EXAFS of the M121SeM variant is also reported. In contrast to the small spectral changes, the reduction potentials of M121SeM, M121Leu, and M121Nle are 25, 135, and 140 mV, respectively, higher than that of WT azurin. The use of unnatural amino acids allowed deconvolution of different factors affecting the reduction potentials of the blue copper center. A careful analysis of the WT azurin and its variants obtained in this work showed the large reduction potential variation was linearly correlated with the hydrophobicity of the axial ligand side chains. Therefore, hydrophobicity is the dominant factor in tuning the reduction potentials of blue copper centers by axial ligands.

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.

Crystal structure of the double azurin mutant Cys3Ser/Ser100Pro from Pseudomonas aeruginosa at 1.8 A resolution: its folding-unfolding energy and unfolding kinetics

Azurin is a cupredoxin, which functions as an electron carrier. Its fold is dominated by a beta-sheet structure. In the present study, azurin serves as a model system to investigate the importance of a conserved disulphide bond for protein stability and folding/unfolding. For this purpose, we have examined two azurin mutants, the single mutant Cys3Ser, which disrupts azurin's conserved disulphide bond, and the double mutant Cys3Ser/Ser100Pro, which contains an additional mutation at a site distant from the conserved disulphide. The crystal structure of the azurin double mutant has been determined to 1.8 A resolution(2), with a crystallographic R-factor of 17.5% (R(free)=20.8%). A comparison with the wild-type structure reveals that structural differences are limited to the sites of the mutations. Also, the rates of folding and unfolding as determined by CD and fluorescence spectroscopy are almost unchanged. The main difference to wild-type azurin is a destabilisation by approximately 20 kJ x mol(-1), constituting half the total folding energy of the wild-type protein. Thus, the disulphide bond constitutes a vital component in giving azurin its stable fold.

Biochemical and crystallographic studies of the Met144Ala, Asp92Asn and His254Phe mutants of the nitrite reductase from Alcaligenes xylosoxidans provide insight into the enzyme mechanism

Dissimilatory nitrite reductase catalyses the reduction of nitrite (NO(2)(-)) to nitric oxide (NO). Copper-containing nitrite reductases contain both type 1 and type 2 Cu sites. Electron transfer from redox partners is presumed to be mediated via the type 1 Cu site and used at the catalytic type 2 Cu centre along with the substrate nitrite. At the type 2 Cu site, Asp92 has been identified as a key residue in substrate utilisation, since it hydrogen bonds to the water molecule at the nitrite binding site. We have also suggested that protons enter the catalytic site via Asp92, through a water network that is mediated by His254. The role of these residues has been investigated in the blue copper nitrite reductase from Alcaligenes xylosoxidans (NCIMB 11015) by a combination of point mutation, enzymatic activity measurement and structure determination.In addition, it has been suggested that the enzyme operates via an ordered mechanism where an electron is transferred to the type 2 Cu site largely when the second substrate nitrite is bound and that this is controlled via the lowering of the redox potential of the type 2 site when it is loaded with nitrite. Thus, a small perturbation of the type 1 Cu site should result in a significant effect on the activity of the enzyme. For this reason a mutation of Met144, which is the weakest ligand of the type 1 Cu, is investigated. The structures of H254F, D92N and M144A have been determined to 1.85 A, 1.9 A and 2.2 A resolution, respectively. The D92N and H254F mutants have negligible or no activity, while the M144A mutant has 30 % activity of the native enzyme. Structural and spectroscopic data show that the loss of activity in H254F is due to the catalytic site being occupied by Zn while the loss/reduction of activity in D92N/M144A are due to structural reasons. The D92N mutation results in the loss of the Asp92 hydrogen bond to the Cu-ligated water. Therefore, the ligand is no longer able to perform proton abstraction. Even though the loss of activity in H254F is due to lack of catalytic Cu, the mutation does cause the disruption of the water network, confirming its key role in proton channel. The structure of the H254F mutant is the first case where full occupancy Zn at the type 2 Cu site is observed, but despite the previously noted similarity of this site to the carbonic anhydrase catalytic site, no carbonic anhydrase activity is observed. The H254F and D92N mutant structures provide, for the first time, observation of surface Zn sites which may act as a Zn sink and prevent binding of Zn at the catalytic Cu site in the native enzyme.

Crystal structure of the soluble domain of the major anaerobically induced outer membrane protein (AniA) from pathogenic Neisseria: a new class of copper-containing nitrite reductases

The major anaerobically induced outer membrane protein (AniA) from pathogenic Neisseria gonorrhoeae is essential for cell growth under oxygen limiting conditions in the presence of nitrite and is protective against killing by human sera. A phylogenic analysis indicates that AniA is a member of a new class of copper-containing nitrite reductases. Expression of the soluble domain of AniA yields a protein capable of reducing nitrite with specific activity of 160 units/mg, approximately 50 % of that measured for the nitrite reductase from the strong soil denitrifier Alcaligenes faecalis S-6. The crystal structure of the soluble domain of AniA was solved by molecular replacement and sixfold averaging to a resolution of 2.4 A. The nitrite soaked AniA crystal structure refined to 1.95 A reveals a bidentate mode of substrate binding to the type II copper. Despite low sequence identity (approximately 30 %), the core cupredoxin fold of AniA is similar to that found in copper-containing nitrite reductases from soil bacteria. The main structural differences are localized to two attenuated surface loops that map to deletions in the sequence alignment. In soil nitrite reductases, one of these surface loops is positioned near the type I copper site and contributes residues to the docking surface for proteaceous electron donors. In AniA, the attenuation of this loop results in a restructured hydrophobic binding surface that may be required to interact with a lipid anchored azurin. The second attenuated loop is positioned on the opposite side of AniA and may facilitate a more intimate interaction with the lipid membrane. A unique combination of structural effectors surrounding the type I copper site of sAnia contribute to a unusual visible absorption spectra with components observed previously in either green or blue type I copper sites.

Electron tunneling in single crystals of Pseudomonas aeruginosa azurins

Rates of reduction of Os(III), Ru(III), and Re(I) by Cu(I) in His83-modified Pseudomonas aeruginosa azurins (M-Cu distance approximately 17 A) have been measured in single crystals, where protein conformation and surface solvation are precisely defined by high-resolution X-ray structure determinations: 1.7(8) x 10(6) s(-1) (298 K), 1.8(8) x 10(6) s(-1) (140 K), [Ru(bpy)2(im)(3+)-]; 3.0(15) x 10(6) s(-1) (298 K), [Ru(tpy)(bpy)(3+)-]; 3.0(15) x 10(6) s(-1) (298 K), [Ru(tpy)(phen)(3+)-]; 9.0(50) x 10(2) s(-1) (298 K), [Os(bpy)2(im)(3+)-]; 4.4(20) x 10(6) s(-1) (298 K), [Re(CO)3(phen)(+)] (bpy = 2,2'-bipyridine; im = imidazole; tpy = 2,2':6',2' '-terpyridine; phen = 1,10-phenanthroline). The time constants for electron tunneling in crystals are roughly the same as those measured in solution, indicating very similar protein structures in the two states. High-resolution structures of the oxidized (1.5 A) and reduced (1.4 A) states of Ru(II)(tpy)(phen)(His83)Az establish that very small changes in copper coordination accompany reduction but reveal a shorter axial interaction between copper and the Gly45 peptide carbonyl oxygen [2.6 A for Cu(II)] than had been recognized previously. Although Ru(bpy)2(im)(His83)Az is less solvated in the crystal, the reorganization energy for Cu(I) --> Ru(III) electron transfer falls in the range (0.6-0.8 eV) determined experimentally for the reaction in solution. Our work suggests that outer-sphere protein reorganization is the dominant activation component required for electron tunneling.

Spectroscopy and reactivity of the type 1 copper site in Fet3p from Saccharomyces cerevisiae: correlation of structure with reactivity in the multicopper oxidases

Fet3p is a multicopper oxidase recently isolated from the yeast, Saccharomyces cerevisiae. Fet3p is functionally homologous to ceruloplasmin (Cp) in that both are ferroxidases. However, by sequence homology Fet3p is more similar to fungal laccase, and both contain a type 1 Cu site that lacks the axial methionine ligand present in the functional type 1 sites of Cp. To determine the contribution of the electronic structure of the type 1 Cu site of Fet3p to the ferroxidase mechanism, we have examined the absorption, circular dichroism, magnetic circular dichroism, electron paramagnetic resonance, and resonance Raman spectra of wild-type Fet3p and type 1 and type 2 Cu-depleted mutants. The spectroscopic features of the type 1 Cu site of Fet3p are nearly identical to those of fungal laccase, indicating a very similar three-coordinate geometry. We have also examined the reactivity of the type 1 Cu site by means of redox titrations and stopped-flow kinetics. From poised potential redox titrations, the E degrees of the type 1 Cu site is 427 mV, which is low for a three-coordinate type 1 Cu site. The kinetics of reduction of the type 1 Cu sites of four different multicopper oxidases with two different substrates were compared. The type 1 site of a plant laccase (Rhus vernicifera) is reduced moderately slowly by both Fe(II) and a bulky organic substrate, 1,4-hydroquinone (with 6 equiv of substrate, k(obs) = 0.029 and 0.013 s(-)(1), respectively). On the other hand, the type 1 site of a fungal laccase (Coprinus cinereus) is reduced very rapidly by both substrates (k(obs) > 23 s(-)(1)). In contrast, both Fet3p and Cp are rapidly reduced by Fe(II) (k(obs) > 23 s(-)(1)), but only very slowly by 1,4-hydroquinone (10- and 100-fold more slowly than plant laccase, respectively). Semiclassical theory is used to analyze the origin of these differences in reactivity in terms of type 1 Cu site accessibility to specific substrates.

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.

Crystal structure of the disulfide bond-deficient azurin mutant C3A/C26A: how important is the S-S bond for folding and stability?

Azurin has a beta-barrel fold comprising eight beta-strands and one alpha helix. A disulfide bond between residues 3 and 26 connects the N-termini of beta strands beta1 and beta3. Three mutant proteins lacking the disulfide bond were constructed, C3A/C26A, C3A/C26I and a putative salt bridge (SB) in the C3A/S25R/C26A/K27R mutant. All three mutants exhibit spectroscopic properties similar to the wild-type protein. Furthermore, the crystal structure of the C3A/C26A mutant was determined at 2.0 A resolution and, in comparison to the wild-type protein, the only differences are found in the immediate proximity of the mutation. The mutants lose the 628 nm charge-transfer band at a temperature 10-22 degrees C lower than the wild-type protein. The folding of the zinc loaded C3A/C26A mutant was studied by guanidine hydrochloride (GdnHCl) induced denaturation monitored both by fluorescence and CD spectroscopy. The midpoint in the folding equilibrium, at 1.3 M GdnHCl, was observed using both CD and fluorescence spectroscopy. The free energy of folding determined from CD is -24.9 kJ.mol-1, a destabilization of approximately 20 kJ.mol-1 compared to the wild-type Zn2+-protein carrying an intact disulfide bond, indicating that the disulfide bond is important for giving azurin its stable structure. The C3A/C26I mutant is more stable and the SB mutant is less stable than C3A/C26A, both in terms of folding energy and thermal denaturation. The folding intermediate of the wild-type Zn2+-azurin is not observed for the disulfide-deficient C3A/C26A mutant. The rate of unfolding for the C3A/C26A mutant is similar to that of the wild-type protein, suggesting that the site of the mutation is not involved in an early unfolding reaction.

Role of the axial ligand in type 1 Cu centers studied by point mutations of met148 in rusticyanin

Type 1 Cu centers in cupredoxins, nitrite reductases, and multi-copper oxidases utilize the same trigonal core ligation to His-Cys-His, with a weak axial ligand generally provided by a Met sulfur. In azurin, an additional axial ligand, a carbonyl oxygen from a Gly, is present. The importance of these axial ligands and in particular the Met has been debated extensively in terms of their role in fine-tuning the redox potential, spectroscopic properties, and rack-induced or entatic state properties of the copper sites. Extensive site-directed mutagenesis of the Met ligand has been carried out in azurin, but the presence of an additional carbonyl oxygen axial ligand has made it difficult to interpret the effects of these substitutions. Here, the axial methionine ligand (Met148) in rusticyanin is replaced with Leu, Gln, Lys, and Glu to examine the effect on the redox potential, acid stability, and copper site geometry. The midpoint redox potential varies from 363 (Met148Lys) to 798 mV (Met148Leu). The acid stability of the oxidized proteins is reduced except for the Met148Gln mutant. The Gln mutant remains blue at all pH values between 2.8 and 8, and has a redox potential of 563 mV at pH 3.2. The optical and rhombic EPR properties of this mutant closely resemble those of stellacyanin, which has the lowest redox potential among single-type 1 copper proteins (185 mV). The Met148Lys mutant exhibits type 2 Cu EPR and optical spectra in this pH range. The Met148Glu mutant exhibits a type 2 Cu EPR spectrum above pH 3 and a mixture of type 1 and type 2 Cu spectra at lower pH. The Met148Leu mutant exhibits the highest redox potential ( approximately 800 mV at pH 3.2) which is similar to the values in fungal laccase and in the type 1 Cu site of ceruloplasmin where this axial ligand is also a Leu.

Human ceruloplasmin. Intramolecular electron transfer kinetics and equilibration

Pulse radiolytic reduction of disulfide bridges in ceruloplasmin yielding RSSR(-) radicals induces a cascade of intramolecular electron transfer (ET) processes. Based on the three-dimensional structure of ceruloplasmin identification of individual kinetically active disulfide groups and type 1 (T1) copper centers, the following is proposed. The first T1 copper(II) ion to be reduced in ceruloplasmin is the blue copper center of domain 6 (T1A) by ET from RSSR(-) of domain 5. The rate constant is 28 +/- 2 s(-1) at 279 K and pH 7.0. T1A is in close covalent contact with the type 3 copper pair and indeed electron equilibration between T1A and the trinuclear copper center in the domain 1-6 interface takes place with a rate constant of 2.9 +/- 0.6 s(-1). The equilibrium constant is 0.17. Following reduction of T1A Cu(II), another ET process takes place between RSSR(-) and T1B copper(II) of domain 4 with a rate constant of 3.9 +/- 0.8. No reoxidation of T1B Cu(I) could be resolved. It appears that the third T1 center (T1C of domain 2) is not participating in intramolecular ET, as it seems to be in a reduced state in the resting enzyme.

Investigation of the anomalous spectroscopic features of the copper sites in chicken ceruloplasmin: comparison to human ceruloplasmin

Chicken ceruloplasmin has been previously reported to display a number of key differences relative to human ceruloplasmin: a lower copper content and a lack of a type 2 copper signal by electron paramagnetic resonance (EPR) spectroscopy. We have studied the copper sites of chicken ceruloplasmin in order to probe the origin of these differences, focusing on two forms of the enzyme: "resting" (as isolated by a fast, one-step procedure) and "peroxide-oxidized". From X-ray absorption, EPR, and UV/visible absorption spectroscopies, we have shown that all of the copper sites are oxidized in peroxide-oxidized chicken ceruloplasmin and that none of the type 1 copper sites display the EPR features typical for type 1 copper sites that lack an axial methionine. In the resting form, the type 2 copper center is reduced. Upon oxidation, it does not appear in the EPR spectrum at 77 K, but it can be observed by using magnetic susceptibility, EPR at approximately 8 K, and magnetic circular dichroism spectroscopy. It displays unusually fast relaxation, indicative of coupling with the adjacent type 3 copper pair of the trinuclear copper cluster. From reductive titrations, we have found that the reduction potential of the type 2 center is higher than those of the other copper sites, thus explaining why it is reduced in the resting form. These results provide new insight into the nature of the additional type 1 copper sites and the redox distribution among copper sites in the different ceruloplasmins relative to other multicopper oxidases.

Targeted mutations in a Trametes villosa laccase. Axial perturbations of the T1 copper

Trametes villosa laccase was mutated on a tetrapeptide segment near the type 1 site. The mutations F463M and F463L were at the position corresponding to the type 1 copper axial methionine (M517) ligand in Zucchini ascorbate oxidase. The mutations E460S and A461E were near the T1 copper site. The mutated Trametes laccases were expressed in an Aspergillus oryzae host and characterized. The E460S mutation failed to produce a transformant with meaningful expression. The F463L and A461E mutations did not significantly alter the molecular and enzymological properties of the laccase. In contrast, the F463M mutation resulted in a type 1 copper site with an EPR signal intermediate between that of the wild type laccase and plastocyanin, an altered UV-visible spectrum, and a decreased redox potential (by 0.1 V). In oxidizing phenolic substrate, the mutation led to a more basic optimal pH as well as an increase in kcat and Km. These effects are attributed to a significant perturbation of the T1 copper center caused by the coordination of the axial methionine (M463) ligand.

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.

Uclacyanins, stellacyanins, and plantacyanins are distinct subfamilies of phytocyanins: plant-specific mononuclear blue copper proteins

The cDNAs encoding plantacyanin from spinach were isolated and characterized. In addition, four new cDNA sequences from Arabidopsis ESTs were identified that encode polypeptides resembling phytocyanins, plant-specific proteins constituting a distinct family of mononuclear blue copper proteins. One of them encodes plantacyanin from Arabidopsis, while three others, designated as uclacyanin 1, 2, and 3, encode protein precursors that are closely related to precursors of stellacyanins and a blue copper protein from pea pods. Comparative analyses with known phytocyanins allow further classification of these proteins into three distinct subfamilies designated as uclacyanins, stellacyanins, and plantacyanins. This specification is based on (1) their spectroscopic properties, (2) their glycosylation state, (3) the domain organization of their precursors, and (4) their copper-binding amino acids. The recombinant copper binding domain of Arabidopsis uclacyanin 1 was expressed, purified, and shown to bind a copper atom in a fashion known as "blue" or type 1. The mutant of cucumber stellacyanin in which the glutamine axial ligand was substituted by a methionine (Q99M) was purified and shown to possess spectroscopic properties similar to uclacyanin 1 rather than to plantacyanins. Its redox potential was determined by cyclic voltammetry to be +420 mV, a value that is significantly higher than that determined for the wild-type protein (+260 mV). The available structural data suggest that stellacyanins (and possibly other phytocyanins) might not be diffusible electron-transfer proteins participating in long-range electron-transfer processes. Conceivably, they are involved in redox reactions occurring during primary defense responses in plants and/or in lignin formation.

Spectroscopic and magnetic studies of human ceruloplasmin: identification of a redox-inactive reduced Type 1 copper site

Ceruloplasmin is unique among the multicopper oxidases in that in addition to the usual copper stoichiometry of one Type 1 copper site and a Type 2/Type 3 trinuclear copper cluster, it contains two other Type 1 sites. This assignment of copper sites, based on copper quantitation, sequence alignment, and crystallography, is difficult to reconcile with the observed spectroscopy. Furthermore, some chemical or spectroscopic differences in ceruloplasmin have been reported depending on the method of purification. We have studied the resting (as isolated by a fast, one-step procedure) and peroxide-oxidized forms of human ceruloplasmin. Using a combination of X-ray absorption spectroscopy, a chemical assay, magnetic susceptibility, electron paramagnetic resonance spectroscopy, and absorption spectroscopy, we have determined that peroxide-oxidized ceruloplasmin contains one permanently reduced Type 1 site. This site is shown to have a reduction potential of approximately 1.0 V. Thus, one of the additional Type 1 sites in ceruloplasmin cannot be catalytically relevant in the form of the enzyme studied. Furthermore, the resting form of the enzyme contains an additional reducing equivalent, which is distributed among the remaining five copper sites as expected from their relative potentials. This may indicate that the resting form of ceruloplasmin in plasma under aerobic conditions is a four-electron oxidized form, which is consistent with its function in the four-electron reduction of dioxygen to water.