Electron-Mediating Cu_A Centers in Proteins: A Comparative High Field ¹H ENDOR Study

High field (W-band, 95 GHz) pulsed electron-nuclear double resonance (ENDOR) measurements were carried out on a number of proteins that contain the mixed-valence, binuclear electron-mediating CuA center. These include nitrous oxide reductase (N2OR), the recombinant water-soluble fragment of subunit II of Thermus thermophilus cytochrome c oxidase (COX) ba3 (M160T9), its M160QT0 mutant, where the weak axial methionine ligand has been replaced by a glutamine, and the engineered "purple" azurin (purpAz). The three-dimensional (3-D) structures of these proteins, apart from the mutant, are known. The EPR spectra of all samples showed the presence of a mononuclear Cu(II) impurity with EPR characteristics of a type II copper. At W-band, the g⊥ features of this center and of CuA are well resolved, thus allowing us to obtain a clean CuA ENDOR spectrum. The latter consists of two types of ENDOR signals. The first includes the signals of the four strongly coupled cysteine β-protons, with isotropic hyperfine couplings, Aiso, in the 7−15 MHz range. The second group consists of weakly coupled protons with a primarily anisotropic character with Azz < 3 MHz. Orientation selective ENDOR spectra were collected for N2OR, M160QT0, and purpAz, and simulations of the cysteine β-protons signals provided their isotropic and anisotropic hyperfine interactions. A linear correlation with a negative slope was found between the maximum Aiso value of the β-protons and the copper hyperfine interaction. Comparison of the best-fit anisotropic hyperfine parameters with those calculated from dipolar interactions extracted from the available 3-D structures sets limit to the sulfur spin densities. Similarly, the small coupling spectral region was simulated on the basis of the 3-D structures and compared with the experimental spectra. It was found that the width of the powder patterns of the weakly coupled protons recorded at g⊥ is mainly determined by the histidine Hε1 protons. Furthermore, the splitting in the outer wings of these powder patterns indicates differences in the positions of the imidazole rings relative to the Cu2S2 core. Comparison of the spectral features of the weakly coupled protons of M160QT0 with those of the other investigated proteins shows that they are very similar to those of purpAz, where the CuA center is the most symmetric, but the copper spin density and the Hε1−Cu distances are somewhat smaller. All proteins show the presence of a proton with a significantly negative Aiso value which is assigned to an amide proton of one of the cysteines. The simulations of both strongly and weakly coupled protons, along with the known copper hyperfine couplings, were used to estimate and compare the spin density distribution in the various CuA centers. The largest sulfur spin density was found in M160T9, and the lowest was found in purpAz. In addition, using the relation between the Aiso values of the four cysteine β-protons and the H−C−S−S dihedral angles, the relative contribution of the hyperconjugation mechanism to Aiso was determined. The largest contribution was found for M160T9, and the lowest was found for purpAz. Possible correlations between the spin density distribution, structural features, and electron-transfer functionality are finally suggested.