X-ray Absorption Studies of the Cu-Dependent Phenylalanine Hydroxylase from Chromobacterium violaceum. Comparison of the Copper Coordination in Oxidized and Dithionite-Reduced Enzymes

The coordination chemistry of the Cu sites of phenylalanine hydroxylase (PAH) from Chromobacterium violaceum has been studied by X-ray absorption spectroscopy (XAS). The EXAFS of the Cu(II) form of the enzyme resembles that of other non-blue copper proteins such as plasma amine oxidases and dopamines-hydroxylase and is characteristic of a mixed N/O coordination shell containing histidine ligation. Detailed simulations of the raw EXAFS data have been carried out using full curved-wave restrained refinement methodologies which allow imidazole ligands to be treated as structural units. The results suggest a Cu(II) coordination of two histidines and two additional O/N-donor groups. A reasonable fit to both data sets can be obtained by assuming that the non-imidazole first-shell donor atoms are derived from solvent (H₂O or OH^-). The EXAFS of the reduced enzyme shows major differences. The amplitude of the first shell in the Fourier transform is only 50% of that of the oxidized enzyme, indicative of a substantial reduction in coordination number. In addition, the first shell of the transform is split into two components. Simulations of the reduced data can be obtained by either two histidines at a long distance of 2.08 Å and an O ligand at a short distance of 1.88 Å or two histidines at a short distance of 1.90 Å and one second-row scatterer such as S or CI at 2.20 Å. Comparison of absorption edge data on the reduced enzyme with data from Cu(I) bis-and tris(1, 2-dimethylimidazole) complexes suggests a pseudo-three-coordinate structure.