Formation of Monofluorinated Radical Cofactor in Galactose Oxidase through Copper-Mediated C–F Bond Scission
Jiasong Li, Ian Davis, Wendell P. Griffith, Aimin Liu
Abstract
Galactose oxidase (GAO) contains a Cu(II)-ligand radical cofactor. The cofactor, which is autocatalytically generated through the oxidation of the copper, consists of a cysteine-tyrosine radical (Cys-Tyr•) as a copper ligand. The formation of the cross-linked thioether bond is accompanied by a C–H bond scission on Tyr272 with few details known thus far. Here, we report the genetic incorporation of 3,5-dichlorotyrosine (Cl2-Tyr) and 3,5-difluorotyrosine (F2-Tyr) to replace Tyr272 in the GAOV previously optimized for expression through directed evolution. The proteins with an unnatural tyrosine residue are catalytically competent. We determined the high-resolution crystal structures of the GAOV, Cl2-Tyr272, and F2-Tyr272 incorporated variants at 1.48, 1.23, and 1.80 Å resolution, respectively. The structural data showed only one halogen remained in the cofactor, indicating that an oxidative carbon-chlorine/fluorine bond scission has occurred during the autocatalytic process of cofactor biogenesis. Using hydroxyurea as a radical scavenger, the spin-coupled hidden Cu(II) was observed by EPR spectroscopy. Thus, the structurally defined catalytic center with genetic unnatural tyrosine substitution is in the radical containing form as in the wild-type, i.e., Cu(II)-(Cl-Tyr•-Cys) or Cu(II)-(F-Tyr•-Cys). These findings illustrate a previously unobserved C–F/C–Cl bond cleavage in biology mediated by a mononuclear copper center.