The catalytically relevant, oxidized state of the active site [Cu(II)-Y center dot-C] of galactose oxidase (GO) is composed of antiferromagnetically coupled Cu(II) and a post-translationally generated Tyr-Cys radical cofactor [Y center dot-C]. The thioether bond of the Tyr-Cys cross-link has been shown experimentally to affect the stability, the reduction potential, and the catalytic efficiency of the GO active site. However, the origin of these structural and energetic effects on the GO active site has not yet been investigated in detail. Here we present copper and sulfur K-edge X-ray absorption data and a systematic computational approach for evaluating the role of the Tyr-Cys cross-link in GO. The sulfur contribution of the Tyr-Cys cross-link to the redox active orbital is estimated from sulfur K-edge X-ray absorption spectra of oxidized GO to be about 24 +/- 3%, compared to the values from computational models of apo-GO (15%) and bob-GO (22%). The results for the apo-GO computational models are in good agreement with the previously reported value for apo-GO (20 +/- 3% from EPR). Surprisingly, the Tyr-Cys cross-link has only a minimal effect on the inner sphere, coordination geometry of the Cu site in the holo-protein. Its effect on the electronic structure is more striking as it facilitates the delocalization of the redox active orbital onto the thioether sulfur derived from Cys, thereby reducing the spin coupling between the [Y center dot-C] radical and the Cu(II) center (752 cm(-1)) relative to the unsubstituted [Y center dot] radical and the Cu(II) center (2210 cm(-1)). Energetically, the Tyr-Cys cross-link lowers the reduction potential by about 75 mV (calculated) allowing a more facile oxidation of the holo active site versus the site without the cross-link. Overall, the Tyr-Cys cross-link confers unique ground state properties on the GO active site that tunes its function in a remarkably nuanced fashion.