The fully oxidized state of cytochrome c peroxidase (CcP), called ES, contains two oxidizing equivalents, one as an oxyferryl heme and the other as an organic radical on an amino acid residue. The unusual electron paramagnetic resonance spectrum of ES has been shown to be due to a weak, distributed exchange coupling between the two paramagnetic redox centers (Houseman, A.

No abstract

Although the cytochrome c peroxidase/H2O2 reaction product, compound ES, has been a long-standing subject of research, only recently has its broad EPR signal been proven to arise from a radical at Trp-191. Despite this advance, no model has satisfactorily explained the anomalous breadth and shape of this signal, which is conventionally interpreted as having axial symmetry with g parallel approximately 2.04 > g perpendicular approximately 2.01, contrary to expectations for a planar pi radical. Furthermore, these g values exhibit marked temperature and preparation dependencies as well as an unexplained high-field "tail" extending from the g = 2.01 peak. We have reexamined the EPR and ENDOR spectra of compound ES at 35 GHz, as well as those of compound ES in the mutant D235E. This mutation significantly alters the line shape of the Trp-191 free radical. We present a comprehensive model that successfully accounts for the properties of this unusual protein free radical. We show that the EPR spectra of both proteins can be described in terms of a weak exchange interaction between the S = 1 oxyferryl (Fe = O)2+ moiety and a radical on Trp-191; a distribution in protein conformation leads to a distribution in the coupling, which ranges from ferromagnetic to antiferromagnetic. We also derive, for the first time, explicit expressions for frozen-solution and single-crystal spectra of such spin-coupled systems and show that the model accounts for all the data that previously led to apparent anomalies in the interpretation of the frozen-solution and single-crystal [Hori, H., & Yonetani, T. (1985) J. Biol. Chem. 260, 349-355] EPR properties. Finally, we have used the CW EPR and pulsed-EPR saturation-recovery methodology to address reports that the broad signal from the spin-coupled Trp-191 radical is accompanied by a minority (approximately 10%), narrow signal that is associated with a radical site other than Trp-191. We find no evidence for such a species and discuss the earlier reports in light of our model.

The complexes [VO(H(2)O)ada] (1), [VO(H(2)O)Hheida] (2), and [VO(H(2)O)aeida] (3) (H(2)ada, N-(carbamoylmethyl)iminodiacetic acid; H(3)heida, N-(2-hydroxyethyl)iminodiacetic acid; H(2)aeida, N-(2-aminoethyl)iminodiacetic acid) were synthesized and crystallographically characterized. Crystallographic parameters for 1.2H(2)O: monoclinic, space group P2(1)/c (No. 14), a = 7.327(2) Å, b = 23.386(7) Å, c = 7.258(3) Å, alpha = 90 degrees, beta = 110.95(2) degrees, gamma = 90 degrees, V = 1204.6(7) Å(3), Z = 4, R1 = 0.0353, and wR(2)() = 0.0848. Crystallographic parameters for 2.H(2)O: orthorhombic, space group Pbca (No. 61), a = 10.512(2) Å, b = 11.727(2) Å, c = 16.719(5) Å, alpha = 90 degrees, beta = 90 degrees, gamma = 90 degrees, V = 2060.6(8) Å(3), Z = 8, R1 = 0.0297, and wR(2)() = 0.0758. Crystallographic parameters for 3: monoclinic, space group P2(1)/c (No. 14), a = 6.785(1) Å, b = 9.714(2) Å, c = 14.959(2) Å, alpha = 90 degrees, beta = 95.12(1) degrees, gamma = 90 degrees, V = 982.2(3) Å(3), Z = 4, R1 = 0.0298, and wR(2)() = 0.0762. In each structure, the tetradentate ligand is disposed so that the tertiary nitrogen is bound trans to the vanadyl oxo, and the rest of the donors occupy equatorial coordination positions. In solution, the structural integrity of these compounds is maintained as observed by UV/visible and EPR spectroscopies, and axial ligation by nitrogen is inferred on the basis of ESEEM spectroscopy. The implications of this study with respect to understanding the coordination environment of VO(2+) in the reduced, inactive form of vanadium bromoperoxidase (VBrPO) are discussed, and it is proposed that significant changes in the coordination environment of vanadium in VBrPO occur upon its reduction, which may provide a plausible explanation for its irreversible inactivation.