The function of the stacking tryptophan, W290, a second coordination sphere residue in galactose oxidase has been investigated via steady-state kinetics measurements, absorption, CD and EPR spectroscopy, and x -ray crystallography of the W290F, W290G, and W290H variants. Enzymatic turnover is significantly lower in the W290 variants. The K m for D-galactose for W290H is similar to wild type, whereas the Km is greatly elevated in W290G and W290F, suggesting a role for W290 in substrate binding/positioning via the -NH group of the indole ring. Hydrogen bonding between W290 and azide in the wild type-azide crystal structure are consistent with this function. W290 modulates the properties and reactivity of the redox-active tyrosine radical; the Y272 tyrosyl radical in both the W290G and W290H variants have elevated redox potentials and are highly unstable compared to the radical in W290F, which has similar properties to the wild type tyrosyl radical. W290 restricts the accessibility of the Y272 radical site to solvent. Crystal structures show that Y272 is significantly more solvent exposed in W290G variant but that W290F limits solvent access comparable to the wild-type indole side chain. Spectroscopic studies indicate that the Cu(II) ground states in the semi-reduced W290 variants are very similar to that of the wild-type protein. In addition, the electronic structures of W290X-azide complexes the variants are also closely similar to the wild type electronic structure. Azide binding and azide-mediated proton uptake by Y495 are perturbed in the variants, indicating that tryptophan also modulates the function of the catalytic base (Y495) in the wild-type enzyme. Thus, W290 plays multiple critical roles in enzyme catalysis, affecting substrate binding, the tyrosyl radical redox potential and stability, and the axial tyrosine function.Over the past twenty years, there has been a growing appreciation for the catalytic utility of protein-derived free radical cofactors in enzymes (1-3). Free radical chemistry is harnessed to catalyze bond activation and molecular rearrangements in a wide variety of enzymes including ribonucleotide reductase (4-7), DNA photolyase (8), cytochrome c peroxidase (9), pyruvateformate lyase (10), lysine-2,3-aminomutase (11), prostaglandin H synthase (12), glyoxal oxidase (13), and galactose oxidase (14).*Authors to whom correspondence should be addressed. Email: dmdooley@montana.edu, Tel: 406-994-4373, FAX: 406 -994-7989; Email: m.j.mcpherson@leeds.ac.uk, Tel: +44 113 233-2595, FAX: +44 113 233-3167. 1 Data deposition: The atomic coordinates and structure factors for W290G, W290F and W290H have been deposited in the Protein Data Bank, www.rcsb.org. † This work was supported by a grant from the National Institutes of Health (GM27659 DMD) and from the Biotechnology and Biological Sciences Research Council (MJM).
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Author ManuscriptBiochemistry. Author manuscript; available in PMC 2008 September 9.
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