High-resolution proton nuclear magnetic resonance studies of the nickel(II) derivative of azurin

The Role of Metal Ions in the Electron Transport through Azurin-Based Junctions

We studied the coherent electron transport through metal–protein–metal junctions based on a blue copper azurin, in which the copper ion was replaced by three different metal ions (Co, Ni and Zn). Our results show that neither the protein structure nor the transmission at the Fermi level change significantly upon metal replacement. The discrepancy with previous experimental observations suggests that the transport mechanism taking place in these types of junctions is probably not fully coherent.

Biomolecular NMR: Past and future

The editors of this special volume suggested this topic, presumably because of the perspective lent by our combined >90-year association with biomolecular NMR. What follows is our personal experience with the evolution of the field, which we hope will illustrate the trajectory of change over the years. As for the future, one can confidently predict that it will involve unexpected advances. Our narrative is colored by our experience in using the NMR Facility for Biomedical Studies at Carnegie-Mellon University (Pittsburgh) and in developing similar facilities at Purdue (1977-1984) and the University of Wisconsin-Madison (1984-). We have enjoyed developing NMR technology and making it available to collaborators and users of these facilities. Our group's association with the Biological Magnetic Resonance data Bank (BMRB) and with the Worldwide Protein Data Bank (wwPDB) has also been rewarding. Of course, many groups contributed to the early growth and development of biomolecular NMR, and our brief personal account certainly omits many important milestones.

Azurin as a protein scaffold for a low-coordinate nonheme iron site with a small-molecule binding pocket

The apoprotein of Pseudomonas aeruginosa azurin binds iron(II) to give a 1:1 complex, which has been characterized by electronic absorption, Mössbauer, and NMR spectroscopies, as well as X-ray crystallography and quantum-chemical computations. Despite potential competition by water and other coordinating residues, iron(II) binds tightly to the low-coordinate site. The iron(II) complex does not react with chemical redox agents to undergo oxidation or reduction. Spectroscopically calibrated quantum-chemical computations show that the complex has high-spin iron(II) in a pseudotetrahedral coordination environment, which features interactions with side chains of two histidines and a cysteine as well as the C═O of Gly45. In the (5)A(1) ground state, the d(z(2)) orbital is doubly occupied. Mutation of Met121 to Ala leaves the metal site in a similar environment but creates a pocket for reversible binding of small anions to the iron(II) center. Specifically, azide forms a high-spin iron(II) complex and cyanide forms a low-spin iron(II) complex.

Palladium(II) and platinum(II) bind strongly to an engineered blue copper protein

Studies of palladium(II) and platinum(II) binding to well-characterized proteins contribute to understanding the influence of these metals in the environment and body. The well-characterized apoprotein of azurin has a soft-metal binding site that may be exposed to solvent by mutation of a coordinating His-117 residue to glycine (H117G). Palladium(II) and platinum(II) form strong 1:1 adducts with the apo form of H117G azurin. A combination of UV-vis, circular dichroism, and inductively coupled plasma mass spectrometry techniques suggests that the metal binds specifically at His-46 and Cys-112 of the protein.

Structural and Spectroscopic Characterization of First-Row Transition Metal(II) Substituted Blue Copper Model Complexes with Hydrotris(pyrazolyl)borate

[CuL(SC(6)F(5))] (1) (L = hydrotris(3,5-diisopropyl-1-pyrazolyl)borate anion) has been reported as a good model for blue copper proteins [Kitajima, N.; Fujisawa, K.; Tanaka, M.; Moro-oka, Y. J. Am. Chem. Soc. 1992, 114, 9232-9233]. To obtain more structural and spectroscopic insight, the first-row transition metal(II) substituted complexes of Cu(II) (1) to Mn(II) (2), Fe(II) (3), Co(II) (4), Ni(II) (5), and Zn(II) (6) were synthesized and their crystal structures were determined. These model complexes have a distorted tetrahedral geometry arising from the tripodal ligand L. The d value, which is defined by the distance from the N(2)S basal plane to the metal(II) ion, and the bond angles such as N-M-N and S-M-N are good indicators of these structural distortions. The obtained complexes were characterized by UV-vis absorption, EPR, NMR, far-IR, and FT-Raman spectroscopies and electrochemical and magnetic properties. In UV-vis absorption spectra, the sulfur-to-metal(II) CT bands and the d-d transition bands are observed for 1 and 3-5. For 1, the strong sulfur to Cu(II) CT band at 663 nm, which is one of the unique properties of blue copper proteins, is observed. The CT energies of the Fe(II) (3), Co(II) (4), and Ni(II) (5) complexes are shifted to higher energy (308 and 355 nm for 3, 311 and 340 nm for 4, 357 and 434 nm for 5) and are almost the same as the corresponding Co(II)- and Ni(II)-substituted blue copper proteins. In the far-IR spectra, three far-IR absorption bands for 2-6 at ca. 400, ca. 350, and ca. 310 cm(-1) are also observed similar to those for 1. Other properties are consistent with their distorted tetrahedral geometries.

Metal coordination of azurin in the unfolded state

1H NMR data applied to the paramagnetic cobalt(II) derivative of azurin from Pseudomonas aeruginosa have made it possible to show that the metal ion is bound to the protein in the unfolded state. The relaxation data as well as the low magnetic anisotropy of the metal ion indicate that the cobalt ion is tetrahedral in the unfolded form. The cobalt ligands have been identified as the residues Gly45, His46, Cys112 and His117. Met121 is not coordinated in the unfolded state. In this state, the metal ion is not constrained to adopt a bipyramidal geometry, as imposed by the protein when it is folded. This is clear confirmation of the rack-induced bonding mechanism previously proposed for the metal ion in azurin.

The crystal structure of nickel(II)-azurin

The nickel(II)-azurin metalloderivative has been crystallized and its structure solved at 0.205-nm resolution by X-ray diffraction. The overall structure is not modified by the metal exchange and the only differences with regard to the native copper(II)-azurin occur in the metal site region. These variations affect principally the axial ligands. Nickel co-ordinates more strongly to the carbonyl oxygen of Gly45 while its distance to the Met121 S4 enlarges up to 0.330 nm. The resulting metal center structure is intermediate between those of the Cu(II) and Zn(II) azurins, and can be described as distorted tetrahedral. However, the existence of contact interaction between Met121 and the nickel ion is still possible as has been shown by paramagnetic 1H-NMR studies in solution.

Teaching high-resolution nuclear magnetic resonance to graduate students in biophysics

Teaching modern methods of high resolution, multi-dimensional nuclear magnetic resonance (NMR) spectroscopy in the limited time usually available is a challenge because of the breadth and complexity of the subject. Here we present two outlines for lectures on basic principles and biophysical applications of NMR, either in one lecture or in six to twelve lectures. We advocate emphasizing the versatility of NMR, and its numerous applications to biophysical questions. An annotated list of references is provided.

Crystal structure analysis of oxidized Pseudomonas aeruginosa azurin at pH 5.5 and pH 9.0. A pH-induced conformational transition involves a peptide bond flip

The X-ray crystal structure of recombinant wild-type azurin from Pseudomonas aeruginosa was determined by difference Fourier techniques using phases derived from the structure of the mutant His35Leu. Two data sets were collected from a single crystal of oxidized azurin soaked in mother liquor buffered at pH 5.5 and pH 9.0, respectively. Both data sets extend to 1.93 A resolution. The two pH forms were refined independently to crystallographic R-factors of 17.6% (pH 5.5) and 17.5% (pH 9.0). The conformational transition previously attributed to the protonation/deprotonation of residue His35 (pKa(red) = 7.3, pKa(ox) = 6.2), which lies in a crevice of the protein close to the copper binding site, involves a concomitant Pro36-Gly37 main-chain peptide bond flip. At the lower pH, the protonated imidazole N delta 1 of His35 forms a strong hydrogen bond with the carbonyl oxygen from Pro36, while at alkaline pH the deprotonated N delta 1 acts as an acceptor of a weak hydrogen bond from HN Gly37. The structure of the remainder of the azurin molecule, including the copper binding site, is not significantly affected by this transition.

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.

1H NMR studies of electron exchange rate of Pseudomonas aeruginosa azurin

T1 values of the His-35 C-2 proton resonance of reduced Pseudomonas aeruginosa azurin were determined at 25 degrees C and pH values 4.5, 7.3, and 9.0 in the presence of different fractional amounts of oxidized azurin. The C-2 proton of His-35 undergoes very rapid spin relaxation in oxidized azurin because of its close proximity to the paramagnetic copper. In the presence of oxidized protein, the T1 values of this proton in reduced azurin depend on the lifetime of the reduced protein. From the T1 data, the electron self-exchange rate constant for azurin was calculated to be 1.4 X 10(4) M-1 X s-1, 4.3 X 10(3) M-1 X s-1, and 6.0 X 10(3) M-1 X s-1 at pH values 4.5, 7.3, and 9, respectively. At pH 7.3, the C-2 proton of His-35 is in slow exchange between the imidazole and imidazolium forms and gives rise to two separate resonances at 9.39 and 8.00 ppm. By using these two resonances, the electron self-exchange rate constants were determined separately for the two species of azurin for which the His-35 residue is in the imidazole or the imidazolium forms; results showed that both species participate in self-exchange of electrons with equal efficiency.