| Abstract: | Cu K-edge X-ray absorption spectra have been recorded for the enzyme tyrosinase from Neurospora crassa, in its oxy, resting (met-aquo), and inhibitor-bound (met-mimosine) forms. The K-edges proper resemble those of oxy- and met-hemocyanin, and confirm the presence of CuII. The forbidden 1s----3d transition is noticeably stronger for the 1-mimosine-bound enzyme, implying some distortion of the tetragonal Cu coordination group on inhibitor binding. The extended fine structure (EXAFS) beyond the K-edge has been analyzed. The first shell scattering is consistent with the presence of two N- and two O-ligand atoms, at 2.0 and 1.9 A, for all three forms of the enzyme; there is no evidence for heavy atom (S) scattering in the first shell. As in analogous hemocyanin derivatives, the outer shell scattering contains contributions from distant atoms of imidazole ligands, as well as from an addition scattering atom, at 3.4-3.6 A. For oxy-tyrosinase the additional scatterer is unambiguously a heavy atom (Cu), although a larger Debye-Waller factor suggests a somewhat less rigid binuclear site than in oxy-hemocyanin. |
