X-ray absorption spectroscopic study of the active copper sites in dopamine β-hydroxylase

X-ray absorption spectroscopy has been used to investigate the local environment of the copper sites in bovine dopamine β-hydroxylase, the enzyme that catalyzes the conversion of dopamine to norepinephrine in the adrenal medulla and noradrenergic nerve cells. The marked similarity of the x-ray absorption edge features of the oxidized and ascorbate-reduced forms of the enzyme with those of the corresponding Cu(imidazole)₄ complexes suggests that the ligation in both cases is very similar. Furthermore, this similarity is found for the extended x-ray absorption fine structure data, and analysis shows only nitrogen (or oxygen) ligation for both enzyme forms. Thus, four nitrogen atoms provide the best fit to the data at an average distance of 1.97 ± 0.02 Å for the oxidized enzyme and four nitrogen atoms at 2.05 ± 0.02 Å for the ascorbate-reduced form. The present data analysis also indicates that there is little change in the average copper ligand environment upon reduction of the enzyme-bound copper from Cu(II) to the Cu(I). The data for the oxidized form of the enzyme are in agreement with previous spin-echo EPR experiments that show three to four imidazole nitrogen ligands for each copper (McCracken, J., Desai, P.R., Papadopoulos, N.J., Villafranca, J.J., and Peisach, J. (1988) Biochemistry 27, 4133-4137). In addition, the data do not indicate the presence of any heavy atom (sulfur or chlorine) ligation to the ascorbate-reduced form of the enzyme as reported by Scott et al. (Scott, R.A., Sullivan, R.J., DeWolf, W.E., Jr., Dolle, R.E., and Kruse, L.I. (1988) Biochemistry 27, 5411-5417).