STRUCTURAL CHARACTERIZATION OF THE COPPER SITE IN GALACTOSE-OXIDASE USING X-RAY-ABSORPTION SPECTROSCOPY

X-ray absorption spectroscopy has been used to characterize the local structural environment of the Cu ion in the reductively inactivated, o xidatively activated, and active+substrate oxidation state derivatives of galactose oxidase. In all three cases, the local environment of th e Cu is best modeled by a single shell of low-Z (N or O) scatterers. T his is generally consistent with the structure determined crystallogra phically, although the EXAFS bond lengths are slightly, but significan tly, shorter than those found crystallographically. The best-fit avera ge bond lengths are 1.97, 1.95, and 1.98 Angstrom for inactive, active , and active+substrate, respectively. The Cu-II ion in the active and inactive derivatives has an apparent coordination number of 4, consist ent with the equatorial ligation seen crystallographically. The Cu-I i on in the reduced+substrate derivative appears to have either a lower coordination number or a significantly more distorted local environmen t. The observed Cu-I-(N/O) bond length favors a model where the Cu bec omes 3-coordinate in the substrate-reduced complex.