Further perspectives on the charge transfer transitions of blue copper proteins and the ligand moieties in stellacyanin

Amino acid sequence of a basic blue protein from cucumber seedlings

The amino acid sequence of a type 1 copper protein, the 96-residue basic blue protein from cucumber seedlings, has been determined by Edman degradation of the intact molecule and of fragments produced by cleavage with cyanogen bromide and with trypsin. The cucumber basic blue protein shows a marked sequence homology with stellacyanin, and to a smaller degree with plastocyanin and azurin. The known copper ligands of plastocyanin and azurin (corresponding to histidine-37, cysteine-84, histidine-87, and methionine-92 in plastocyanin) are present in the cucumber basic blue protein. However, the latter also contains a half-cystine residue analogous to the suggested fourth ligand of stellacyanin, where methionine is absent.

A detailed resonance Raman spectrum of Nickel(II)-substituted Pseudomonas aeruginosa azurin

Nickel(II) and cobalt(II) derivatives of the blue copper protein Pseudomonas aeruginosa azurin have been studied by resonance Raman (RR) spectroscopy at liquid-nitrogen temperatures. Vibrational assignments for the observed RR bands of Ni(II)-azurin have been made through a study of (62)Ni-substituted azurin. A comparison of Ni(II)-azurin RR spectra with those of the wild type (Cu-containing) protein showed Ni(II)-S(Cys) stretching vibrations, nu(Ni-S)(Cys), at substantially lower frequencies (approximately 360 versus approximately 400 cm(-1), respectively), indicating that the Ni(II)-S(Cys) bond is much weaker than the corresponding Cu(II)-S(Cys) bond. Resonance enhanced predominantly nu(Ni-N)(His) modes indicate that the metal-N(His) bond distances in the Ni(II) derivative are the same as those in native azurin. The vibrational data also confirm a tetrahedral disposition of ligands about the metal in Ni(II)-azurin found in the protein crystallographic structures. As expected, excitation profile measurements on Ni(II)-azurin show that the nu(Ni-S)(Cys) assignable modes give maxima at the 440-nm absorption band, which confirms a S(Cys) --> Ni(II) charge-transfer origin of the 440-nm electronic transition in Ni(II)-substituted azurin.

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.

Low temperature optical absorption spectroscopy: an approach to the study of stereodynamic properties of hemeproteins

In this short review we show how suitable analysis of the temperature dependence of the optical absorption spectra of metalloproteins can give insight into their stereodynamic properties in the region of the chromophore. To this end, the theory of coupling between an intense allowed electronic transition of a chromophore and Franck-Condon active vibrations of the nearby atoms is applied to the Soret band of hemeproteins to obtain an analytical expression suitable for fitting the spectral profile at various temperatures. The reported approach enables one to separate the various contributions to the overall bandwidth together with the parameters that characterize the vibrational coupling. The thermal behavior of these quantities gives information on the dynamic properties of the active site and on their dependence upon protein structure and ligation state. The Soret band of hemeproteins appears to be coupled to high frequency vibrational modes of the heme group (as already shown by resonance Raman spectroscopy) and to a "bath" of low frequency modes most likely deriving from the bulk of the protein. For the deoxy derivatives inhomogeneous broadening arising from conformational heterogeneity appears to contribute substantially to the linewidth. The data indicate the onset, at temperatures near 180 K, of large scale anharmonic motions that can be attributed to jumping among different conformational substates of the protein.

Gene synthesis, expression, and mutagenesis of the blue copper proteins azurin and plastocyanin

Genes for the blue copper proteins Populus nigra var. italica plastocyanin and Pseudomonas aeruginosa azurin have been constructed by a stepwise procedure. The leader sequence for azurin has been placed before the genes directing plastocyanin and azurin transport to the periplasmic space when the genes are expressed in Escherichia coli. Site-saturation mutagenesis has been used to alter two copper-binding residues of azurin (Met-121 and His-46) and Met-92 of plastocyanin. While the plastocyanin mutants do not appear to bind copper, the azurin variants all bind copper and show characteristic type I blue copper centers. In particular, the electronic spectra reflect the dominance of the charge transfer interaction between copper and the thiolate of Cys-112, being relatively insensitive to changes in Met-121 or His-46. In contrast, removal of Met-121 appreciably alters the EPR spectra of the mutants, although, to a first order, the spectra of all mutants are themselves similar, suggesting a more distorted geometry around copper in the mutants than in the wild type.

Optical absorption spectra of azurin and stellacyanin in glycerol/water and ethylene glycol/water solutions in the temperature range 290-20 K

We have measured the optical absorption spectra of azurin and stellacyanin in the wavelength range 1100-350 nm and in the temperature interval 290-20 K. Samples are protein aqueous solutions containing 65% (v/v) glycerol or ethylene glycol as cryoprotectants and remain homogeneous and transparent throughout the whole temperature range investigated. Spectra are deconvoluted into Gaussian components and the temperature dependence of the zeroth, first and second moments of the various bands is analyzed, within the harmonic Franck-Condon approximation, to obtain information on the stereodynamic properties of the active sites of these proteins. Sizable differences of the integrated intensities of all the bands with temperature are observed and are attributed to variations of the metal-ligand relative positions (i.e., deformations of the active site) that occur as the temperature is lowered. The mean effective frequency of the nuclear vibrations coupled to all the observed bands is about 150 cm(-1) for both proteins in both solvents used; this indicates that the electronic transitions from which the optical spectrum originates are substantially coupled with low-frequency vibrational modes, likely ligand-metal-ligand deformations. The relevance of the stereodynamic properties of azurin and stellacyanin, investigated in this work, to their functional behavior is also suggested.

EXAFS of the type-1 copper site of rusticyanin

Extended X-ray absorption fine structure (EXAFS) spectra at the Cu K-edge have been recorded of the oxidized and reduced form at pH 3.5 of rusticyanin, the type-1 or 'blue'-copper protein from Thiobacillus ferrooxidans. The EXAFS of oxidized rusticyanin is well simulated with models assuming a ligand set of 2 N(His) and 1 S(Cys) at 1.99 and 2.16 A, respectively. Upon reduction, the average Cu-N ligand distance increases by approx. 0.08A. For both redox states studied, the fit by the simulation is significantly improved by including a contribution of an additional sulfur ligand at approx. 2.8 A. From comparison with structural data of other blue-copper proteins, it is concluded that the copper coordination environment is relatively rigid, which may be a clue to its high redox potential.

Studies of thermally induced denaturation of azurin and azurin derivatives by differential scanning calorimetry: evidence for copper selectivity

Azurin, a blue copper protein from Pseudomonas aeruginosa, and several derivatives of azurin have been studied by differential scanning calorimetry. Two well-separated, irreversible transitions are observed in a scan of apoazurin under a variety of conditions, and they are assigned to distinct steps in the denaturation process. No specific structural component can be assigned to the lower temperature transition, but a "flap" structure which is found near the metal binding site may be involved. Circular dichroic spectra suggest that melting of the beta-sheet structure, the main structural motif in the native protein, occurs during the second transition. With the exceptions of the Ni(II) and p-(hydroxymercuri)benzoate derivatives, the transitions are superposed in the metalated forms, and the enthalpies of denaturation are more endothermic. By comparison with other first-row divalent transition ions and especially Zn(II), the Cu(II) derivative exhibits the most endothermic denaturation process. Along with other data, this suggests that the binding energy is greater for Cu(II). It is postulated that the selectivity for copper over zinc arises because of the irregular binding geometry offered by the folded protein. Denaturation of the Hg(II) derivative is even more endothermic, confirming that the type 1 binding site has a very great affinity for Hg(II). Finally, when substoichiometric amounts of Hg(II) are added to the apoprotein, there is evidence that a novel mercury-bridged dimer of azurin forms.

A new assignment of the disulfide linkage in stellacyanin

Which cysteine of apostellacyanine reacts with 4-vinylpyridine in 6 M guanidine-HC1 depends upon the pH. We infer that a disulfide switch occurs at higher pH and that in the native protein the disulfide bridge occurs between Cys-59 and Cys-93. Thus Cys-87, which is homologous to the cysteine ligands of azurin and plastocyanin, is available to bind copper.