A tyrosine-derived free radical in apogalactose oxidase

• Published in: The Journal of biological chemistry 1990-06, Vol.265 (17), p.9610-9613
• Main Authors: WHITTAKER, M. M, WHITTAKER, J. W
• Format: Article
• Language: English
• Subjects: Analytical, structural and metabolic biochemistry; Apoenzymes - metabolism; Apoproteins - metabolism; Biological and medical sciences; Electron Spin Resonance Spectroscopy; Enzymes and enzyme inhibitors; Free Radicals; Fundamental and applied biological sciences. Psychology; Galactose Oxidase - metabolism; Kinetics; Mitosporic Fungi - enzymology; Oxidation-Reduction; Oxidoreductases; Spectrophotometry; Tyrosine
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1016/S0021-9258(19)38711-3

Oxidation of apogalactose oxidase with ferricyanide leads to the formation of a stable free radical exhibiting distinctive optical absorption and EPR spectral features. The radical is associated with absorption in both near-UV and near-IR spectral regions, and its EPR spectrum is characteristic of an aromatic free radical with gav = 2.005. Reconstitution of both the apoenzyme and the free radical-containing form with copper substantially restores both the absorption spectra and the catalytic activity of the active enzyme, indicating that the preparation of the radical species does not significantly damage the protein. The absence of a free radical EPR signal in reconstituted and activated galactose oxidase containing nearly stoichiometric copper suggests the radical is an active site species relating to the free radical-coupled copper site previously proposed for this enzyme. Isotopic labeling experiments demonstrate that the radical derives from a tyrosine residue. The distinctive spectra associated with this radical indicate an environment which is different from that associated with the tyrosyl phenoxyl sites in other free radical enzymes.