Alkaline transition of phytocyanins: a comparison of stellacyanin and umecyanin

pH and solvent H/D isotope effects on the thermodynamics and kinetics of electron transfer for electrode-immobilized native and urea-unfolded stellacyanin

The thermodynamics of Cu(II) to Cu(I) reduction and the kinetics of the electron transfer (ET) process for Rhus vernicifera stellacyanin (STC) immobilized on a decane-1-thiol coated gold electrode have been measured through cyclic voltammetry at varying pH and temperature, in the presence of urea and in D(2)O. Immobilized STC undergoes a limited conformational change that mainly results in an enhanced exposure of one or both copper binding histidines to solvent which slightly stabilizes the cupric state and increases histidine basicity. The large immobilization-induced increase in the pK(a) for the acid transition (from 4.5 to 6.3) makes this electrode-SAM-protein construct an attractive candidate as a biomolecular ET switch operating near neutral pH in molecular electronics. Such a potential interest is increased by the robustness of this interface against chemical unfolding as it undergoes only moderate changes in the reduction thermodynamics and in the ET rate in the presence of up to 8 M urea. The sensitivity of these parameters to solvent H/D isotope effects testifies to the role of protein solvation as effector of the thermodynamics and kinetics of ET.

Cotton BCP genes encoding putative blue copper-binding proteins are functionally expressed in fiber development and involved in response to high-salinity and heavy metal stresses

Copper is vitally required for plants at low concentrations but extremely toxic for plants at elevated concentrations. Plants have evolved a series of mechanisms to prevent the consequences of the excess or deficit of copper. These mechanisms require copper-interacting proteins involved in copper trafficking. Blue copper-binding proteins (BCPs) are a class of copper proteins containing one blue copper-binding domain binding a single type I copper. To investigate the role of BCPs in plant development and in response to stresses, we isolated nine cDNAs encoding the putative blue copper-binding proteins (GhBCPs) from cotton (Gossypium hirsutum). Meanwhile, four corresponding genes (including GhBCP1-GhBCP4), which contain a single intron inserted in their conserved position, were isolated from cotton genome. Quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) analysis indicated that the nine GhBCP genes are differentially expressed in cotton tissues. Among them, GhBCP1 and GhBCP4 were predominantly expressed in fibers, while the transcripts of GhBCP2 and GhBCP3 were accumulated at relatively high levels in fibers. These four genes were strongly expressed in early fiber elongation, but dramatically declined with further fiber development. In addition, these GhBCP genes were upregulated in fibers by Cu(2+) , Zn(2+) , high-salinity and drought stresses, but downregulated in fibers by Al(3+) treatment. Overexpression of GhBCP1 and GhBCP4 in yeast (Schizosaccharomyces pombe) significantly increased the cell growth rate under Cu(2+) , Zn(2+) and high-salinity stresses. These results suggested that these GhBCPs may participate in the regulation of fiber development and in response to high-salinity and heavy metal stresses in cotton.

Alkaline transition of pseudoazurin Met16X mutant proteins: protein stability influenced by the substitution of Met16 in the second sphere coordination

Several blue copper proteins are known to change the active site structure at alkaline pH (alkaline transition). Spectroscopic studies of Met16Phe, Met16Tyr, Met16Trp, and Met16Val pseudoazurin variants were performed to investigate the second sphere role through alkaline transition. The visible electronic absorption and resonance Raman spectra of Met16Phe, Met16Tyr, and Met16Trp variants showed the increasing of axial component at pH approximately 11 like wild-type PAz. The visible electronic absorption and far-UV CD spectra of Met16Val demonstrated that the destabilization of the protein structure was triggered at pH>11. Resonance Raman (RR) spectra of PAz showed that the intensity-weighted averaged Cu-S(Cys) stretching frequency was shifted to higher frequency region at pH approximately 11. The higher frequency shift of Cu-S(Cys) bond is implied the stronger Cu-S(Cys) bond at alkaline transition pH approximately 11. The visible electronic absorption and far-UV CD spectra of Met16X PAz revealed that the Met16Val variant is denatured at pH>11, but Met16Phe, Met16Tyr, and Met16Trp mutant proteins are not denatured even at pH>11. These observations suggest that Met16 is important to maintain the protein structure through the possible weak interaction between methionine -SCH3 part and coordinated histidine imidazole moiety. The introduction of pi-pi interaction in the second coordination sphere may be contributed to the enhancement of protein structure stability.

Spectroscopic characterization of a high-potential lipo-cupredoxin found in Streptomyces coelicolor

For many streptomycetes, a distinct dependence on the "bioavailability" of copper ions for their morphological development has been reported. Analysis of the Streptomyces coelicolor genome reveals a number of gene products encoding for putative copper-binding proteins. One of these appears as an unusual copper-binding protein with a lipoprotein signal sequence and a cupredoxin-like domain harboring a putative Type-1 copper-binding motif. Cloning of this gene from S. coelicolor and subsequent heterologous expression in Escherichia coli has allowed for a thorough spectroscopic interrogation of this putative copper-binding protein. Optical and electron paramagnetic resonance spectroscopies have confirmed the presence of a "classic" Type-1 copper site with the axial ligand to the copper a methionine. Paramagnetic NMR spectroscopy on both the native Cu(II) form and Co(II)-substituted protein has yielded active-site structural information, which on comparison with that of other cupredoxin active sites reveals metal-ligand interactions most similar to the "classic" Type-1 copper site found in the amicyanin family of cupredoxins. Despite this high structural similarity, the Cu(II)/(I) midpoint potential of the S. coelicolor protein is an unprecedented +605 mV vs normal hydrogen electrode at neutral pH (amicyanin approximately +250 mV), with no active-site protonation of the N-terminal His ligand observed. Suggestions for the physiological role/function of this high-potential cupredoxin are discussed.

Thermodynamics of the alkaline transition in phytocyanins

The thermodynamics of the alkaline transition which influences the spectral and redox properties of the type 1 copper center in phytocyanins has been determined spectroscopically. The proteins investigated include Rhus vernicifera stellacyanin, cucumber basic protein and its Met89Gln variant, and umecyanin, the stellacyanin from horseradish roots, along with its Gln95Met variant. The changes in reaction enthalpy and entropy within the protein series show partial compensatory behavior. Thus, the reaction free energy change (hence the pK (a) value) is rather variable. This indicates that species-dependent differences in reaction thermodynamics, although containing an important contribution from changes in the hydrogen-bonding network of water molecules in the hydration sphere of the protein (which feature enthalpy-entropy compensation), are to a large extent protein-based. The data for axial ligand variants are consistent with the hypothesis of a copper-binding His as the deprotonating residue responsible for this transition.

Ultrafast Pump−Probe Study of Excited-State Charge-Transfer Dynamics in Umecyanin from Horseradish Root

We have applied femtosecond pump-probe spectroscopy to investigate the excited-state dynamics of umecyanin from horseradish roots, by exciting its 600-nm ligand-to-metal charge-transfer band with a 15-fs pulse and probing over a broad range in the visible region. The decay of the pump-induced ground-state bleaching is modulated by clearly visible oscillations and occurs exponentially with a time constant depending on the observed spectral component of the transmission difference signal, ranging from 270 fs up to 700 fs. The slower decaying process characterizes the spectral component corresponding to the metal-to-ligand charge-transfer transition. The excited-state decay rate is significantly lower than in other blue copper proteins, probably because of the larger energy gap between ligand- and metal-based orbitals in umecyanin. Wavelength dependence of the recovery times could be due to either the excitation of several transitions or the occurrence of intramolecular vibrational relaxation within the excited state. We also find evidence of a hot ground-state absorption, at 700 nm, persisting for several picoseconds. The vibrational coherence induced by the ultrashort pump pulse allows vibrational activity to be observed, mainly in the ground state, as expected in a system with fast excited-state decay. However, we find evidence of a rapidly damped oscillation, which we assign to the excited state. Finally, the Fourier transform of the oscillatory component of the signal presents additional bands in the low-frequency region which are assigned to collective motions of the protein.

Optical Spectroscopic Investigation of the Alkaline Transition in Umecyanin from Horseradish Root†

Umecyanin (UMC) from horseradish root belongs to the stellacyanin subclass of the phytocyanins, a family of plant cupredoxins. The protein possesses the typical type-1 His(2)Cys equatorial ligand set at its mononuclear copper site but has an axial Gln ligand in place of the usual weakly coordinated Met of the plantacyanins, uclacyanins, and most other cupredoxins. UMC exhibits, like other phytocyanins, altered visible, EPR, and paramagnetic (1)H NMR spectra at elevated pH values and also a modified reduction potential. This alkaline transition occurs with a pK(a) of approximately 10 [Dennison, C., Lawler, A. T. (2001) Biochemistry 40, 3158-3166]. In this study, we investigate the alkaline transition by complementary optical spectroscopic techniques. The contemporary use of absorption, fluorescence, dynamic light scattering, and resonance Raman spectroscopy allows us to demonstrate that the alkaline transition induces a reorganization of the protein and that the overall size of UMC increases, but protein aggregation does not occur. The transition does not have a dramatic influence on the active-site environment of UMC, but there are subtle alterations in the Cu site geometry. Direct evidence for the strengthening of a Cu-N(His) bond is presented, which is in agreement with the hypothesis that the deprotonation of the N(epsilon2)H moiety of one of the His ligands is the cause of the alkaline transition. A weakening of the Cu-S(Cys) bond is also observed which, along with a weakened axial interaction, must be due to the enhanced Cu-N(His) interaction.

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.

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.

Crystal structures of oxidized and reduced stellacyanin from horseradish roots

Umecyanin (UMC) is a type 1 copper-containing protein which originates from horseradish roots and belongs to the stellacyanin subclass of the phytocyanins, a ubiquitous family of plant cupredoxins. The crystal structures of Cu(II) and Cu(I) UMC have been determined at 1.9 and 1.8 A, respectively. The protein has an overall fold similar to those of other phytocyanins. At the active site the cupric ion is coordinated by the N(delta1) atoms of His44 and His90, the S(gamma) of Cys85, and the O(epsilon)(1) of Gln95 in a distorted tetrahedral geometry. Both His ligands are solvent exposed and are surrounded by nonpolar and polar side chains on the protein surface. Thus, UMC does not possess a distinct hydrophobic patch close to the active site in contrast to almost all other cupredoxins. UMC has a large surface acidic patch situated approximately 10-30 A from the active site. The structure of Cu(I) UMC is the first determined for a reduced phytocyanin and demonstrates that the coordination environment of the cuprous ion is more trigonal pyramidal. This subtle change in geometry is primarily due to the Cu-N(delta1)(His44) and Cu-O(epsilon1)(Gln95) bond lengths increasing from 2.0 and 2.3 A in Cu(II) UMC to 2.2 and 2.5 A, respectively, in the reduced form, as a consequence of slight rotations of the His44 and Gln95 side chains. The limited structural changes upon redox interconversion at the active site of this stellacyanin are analogous to those observed in a typical type 1 copper site with an axial Met ligand and along with its surface features suggest a role for UMC in interprotein electron transfer.

Investigating the Cause of the Alkaline Transition of Phytocyanins

The phytocyanins are a family of plant cupredoxins that have been subdivided into the stellacyanins, plantacyanins, and uclacyanins. All of these proteins possess the typical type 1 His(2)Cys equatorial ligand set at their mononuclear copper sites, but the stellacyanins have an axial Gln ligand in place of the weakly coordinated Met of the plantacyanins, uclacyanins, and most other cupredoxins. The stellacyanins exhibit altered visible, EPR, and paramagnetic (1)H NMR spectra at elevated pH values and also modified reduction potentials. This alkaline transition occurs with a pK(a) of approximately 10 [Dennison, C., Lawler, A. T. (2001) Biochemistry 40, 3158-3166]. In this study we demonstrate that the alkaline transition has a similar influence on the visible, EPR, and paramagnetic NMR spectra of cucumber basic protein (CBP), which is a plantacyanin. The mutation of the axial Gln95 ligand into a Met in umecyanin (UMC), the stellacyanin from horseradish roots, and the axial Met89 into a Gln in CBP have very limited, yet similar, influence on the pK(a) for the alkaline transition as judged from alterations in visible spectra. The complete removal of the axial ligand in the Met89Val variant of CBP results in a slightly larger decrease in the pK(a) for this effect, but similar spectral alterations are still observed at elevated pH. Thus, the axial Gln ligand is not the cause of the alkaline transition in Cu(II) stellacyanins, and alterations in the active site structures of the phytocyanins have a limited effect on this feature. The conserved Lys residue found adjacent to the axial ligand in the sequences of all phytocyanins, and implicated as the trigger for the alkaline transition, has been mutated to an Arg in UMC. The influence of increasing pH on the spectroscopic properties of Lys96Arg UMC is almost identical to those of the wild type protein, and thus, this residue is not responsible for the alkaline transition. However, a positively charged residue in this position seems to be important for the correct folding of UMC. Other possible triggers for the effects seen in the phytocyanins at elevated pH are discussed along with the relevance of the alkaline transition.

Characterization of Arabidopsis thaliana stellacyanin: A comparison with umecyanin

The cupredoxin domain of a putative type 1 blue copper protein (BCB) from Arabidopsis thaliana was overexpressed and purified. A recursive polymerase chain reaction method was used to synthesize an artificial coding region for the cupredoxin domain of horseradish stellacyanin (commonly known as umecyanin), prior to overexpression and purification. The recombinant proteins were refolded from inclusion bodies and reconstituted with copper, and their in vitro characteristics were studied. Recombinant umecyanin, which is nonglycosylated, has identical spectroscopic and redox properties to the native protein. The UV/Vis and EPR spectra of recombinant BCB and umecyanin demonstrate that they have comparable axial type 1 copper binding sites. Paramagnetic (1)H NMR spectroscopy highlights the similarity between the active site architectures of BCB and umecyanin. The reduction potential of recombinant BCB is 252 mV, compared to 293 mV for recombinant umecyanin. Identical pK(a) values of 9.7 are obtained for the alkaline transitions in both proteins. This study demonstrates that BCB is the A. thaliana stellacyanin and the results form the biochemical basis for a discussion of BCB function in the model vascular plant.

The Active-Site Structure of Umecyanin, the Stellacyanin from Horseradish Roots

The type 1 copper sites of cupredoxins typically have a His(2)Cys equatorial ligand set with a weakly interacting axial Met, giving a distorted tetrahedral geometry. Natural variations to this coordination environment are known, and we have utilized paramagnetic (1)H NMR spectroscopy to study the active-site structure of umecyanin (UMC), a stellacyanin with an axial Gln ligand. The assigned spectra of the Cu(II) UMC and its Ni(II) derivative [Ni(II) UMC] demonstrate that this protein has the typical His(2)Cys equatorial coordination observed in other structurally characterized cupredoxins. The NMR spectrum of the Cu(II) protein does not exhibit any paramagnetically shifted resonances from the axial ligand, showing that this residue does not contribute to the singly occupied molecular orbital (SOMO) in Cu(II) UMC. The assigned paramagnetic (1)H NMR spectrum of Ni(II) UMC demonstrates that the axial Gln ligand coordinates in a monodentate fashion via its side-chain amide oxygen atom. The alkaline transition, a feature common to stellacyanins, influences all of the ligating residues but does not alter the coordination mode of the axial Gln ligand in UMC. The structural features which result in Cu(II) UMC possessing a classic type 1 site as compared to the perturbed type 1 center observed for other stellacyanins do not have a significant influence on the paramagnetic (1)H NMR spectra of the Cu(II) or Ni(II) proteins.

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.