title: A hyperactive cobalt-substituted extradiol-cleaving catechol dioxygenase.
abstract: Homoprotocatechuate 2,3-dioxygenase from Brevibacterium fuscum (HPCD) has an Fe(II) center in its active site that can be replaced with Mn(II) or Co(II). Whereas Mn-HPCD exhibits steady-state kinetic parameters comparable to those of Fe-HPCD, Co-HPCD behaves somewhat differently, exhibiting significantly higher [Formula: see text] and k (cat). The high activity of Co-HPCD is surprising, given that cobalt has the highest standard M(III/II) redox potential of the three metals. Comparison of the X-ray crystal structures of the resting and substrate-bound forms of Fe-HPCD, Mn-HPCD, and Co-HPCD shows that metal substitution has no effect on the local ligand environment, the conformational integrity of the active site, or the overall protein structure, suggesting that the protein structure does not differentially tune the potential of the metal center. Analysis of the steady-state kinetics of Co-HPCD suggests that the Co(II) center alters the relative rate constants for the interconversion of intermediates in the catalytic cycle but still allows the dioxygenase reaction to proceed efficiently. When compared with the kinetic data for Fe-HPCD and Mn-HPCD, these results show that dioxygenase catalysis can proceed at high rates over a wide range of metal redox potentials. This is consistent with the proposed mechanism in which the metal mediates electron transfer between the catechol substrate and O(2) to form the postulated [M(II)(semiquinone)superoxo] reactive species. These kinetic differences and the spectroscopic properties of Co-HPCD provide new tools with which to explore the unique O(2) activation mechanism associated with the extradiol dioxygenase family.
authors.affiliation: Department of Chemistry, Center for Metals in Biocatalysis, University of Minnesota, 207 Pleasant Street, Minneapolis, MN 55455, USA.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Fielding
authors.forename: Elena G
authors.initials: EG
authors.lastname: Kovaleva
authors.forename: Erik R
authors.initials: ER
authors.lastname: Farquhar
authors.forename: John D
authors.initials: JD
authors.lastname: Lipscomb
authors.forename: Lawrence
authors.initials: L
authors.lastname: Que
mesh_terms: D001951:Brevibacterium / Q000201:enzymology; D020134:Catalytic Domain; D003035:Cobalt / Q000378:metabolism*; D018360:Crystallography, X-Ray; D049308:Dioxygenases / Q000737:chemistry / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D007501:Iron / Q000378:metabolism; D008345:Manganese / Q000378:metabolism; D011485:Protein Binding
chemical_list: D003035:Cobalt; D008345:Manganese; D007501:Iron; D049308:Dioxygenases; C020055:3,4-dihydroxyphenylacetate 2,3-dioxygenase