Structure of the Reduced Copper Active Site in Preprocessed Galactose Oxidase: Ligand Tuning for One-Electron O2 Activation in Cofactor Biogenesis
Ryan E Cowley, Jordi Cirera, Munzarin F Qayyum, Dalia Rokhsana, Britt Hedman, Keith O Hodgson, David M Dooley, Edward I Solomon
Journal of the American Chemical Society
Galactose oxidase (GO) is a copper-dependent enzyme that accomplishes 2e(-) substrate oxidation by pairing a single copper with an unusual cysteinylated tyrosine (Cys-Tyr) redox cofactor. Previous studies have demonstrated that the post-translational biogenesis of Cys-Tyr is copper- and O2-dependent, resulting in a self-processing enzyme system. To investigate the mechanism of cofactor biogenesis in GO, the active-site structure of Cu(I)-loaded GO was determined using X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy, and density-functional theory (DFT) calculations were performed on this model. Our results show that the active-site tyrosine lowers the Cu potential to enable the thermodynamically unfavorable 1e(-) reduction of O2, and the resulting Cu(II)-O2(•-) is activated toward H atom abstraction from cysteine. The final step of biogenesis is a concerted reaction involving coordinated Tyr ring deprotonation where Cu(II) coordination enables formation of the Cys-Tyr cross-link. These spectroscopic and computational results highlight the role of the Cu(I) in enabling O2 activation by 1e(-) and the role of the resulting Cu(II) in enabling substrate activation for biogenesis.
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