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Abstract
The first example of an O(2) adduct of an active Co-substituted oxygenase has been
observed in the extradiol ring cleavage of the electron-poor substrate 4-nitrocatechol
(4NC) by Co(II)-homoprotocatechuate 2,3-dioxygenase (Co-HPCD). Upon O(2) binding to
the high-spin Co(II) (S = (3)/(2)) enzyme-substrate complex, an S = (1)/(2) EPR signal
exhibiting (59)Co hyperfine splitting (A = 24 G) typical of a low-spin Co(III)-superoxide
complex was observed. Both the formation and decay of the new intermediate are very
slow in comparison to the analogous steps for turnover of 4NC by native high-spin
Fe(II)-HPCD, which is likely to remain high-spin upon O(2) binding. A similar but
effectively stable S = (1)/(2) intermediate was formed by the inactive [H200N-Co-HPCD(4NC)]
variant. The observations presented shed light on the key roles played by the substrate,
the second-sphere His200 residue, and the spin state of the metal center in facilitating
O(2) binding and activation.
[1
]Department
of Chemistry and ‡Department of Biochemistry, Molecular Biology, and Biophysics
and
Center for Metals in Biocatalysis, University of Minnesota, Minneapolis, Minnesota
55455, United States