The Copper Sites of Dopamine β-hydroxylase: An X-ray Absorption Spectroscopic Study

X-ray absorption edge and extended X-ray absorption fine structure (EXAFS) spectra are reported for the Cu(I) and Cu(II) forms of bovine dopamine 0-hydroxylase (DBH; EC 1.14.17.1) and for the Cu(I) form of DBH bound either to tyramine substrate or to a multisubstrate inhibitor [Kruse, L. I., DeWolf, W. E., Jr., Chambers, P. A., & Goodhart, P. J. (1986) Biochemistry 25, 7271–7278]. A significant change in the structure of the copper sites occurs upon ascorbate-mediated reduction of Cu(II) DBH to the Cu(I) form. While the average Cu(II) site most likely consists of a square-planar array of four (N,0)-containing ligands at 1.98 Å, the average Cu(I) site shows a reduction in (N,O) coordination number (from ∼4 to ∼2) and the addition of a S-containing ligand at 2.30 Å. No change in the average Cu(I) ligand environment accompanies binding of tyramine substrate, whereas binding of a multisubstrate inhibitor, l-(3,5-di-fluoro-4-hydroxybenzyl)-lH-imidazole-2(3H)-thione, causes an increase in the Cu-S coordination, consistent with inhibitor binding to the Cu(I) site through the S atom. Although excellent signal-to-noise ratio in the EXAFS spectra of ascorbate-reduced DBH facilitated analysis of outer-shell scattering for a Cu…Cu interaction, the presence of a binuclear site could not be proven or disproven due to interference from Cu…C scattering involving the carbons of imidazole ligands.