title: Electronic and geometric structures of the organophosphate-degrading enzyme from Agrobacterium radiobacter (OpdA).
abstract: The organophosphate-degrading enzyme from Agrobacterium radiobacter (OpdA) is a highly efficient catalyst for the degradation of pesticides and some nerve agents such as sarin. OpdA requires two metal ions for catalytic activity, and hydrolysis is initiated by a nucleophilic hydroxide that is bound to one of these metal ions. The precise location of this nucleophile has been contentious, with both a terminal and a metal-ion-bridging hydroxide as likely candidates. Here, we employed magnetic circular dichroism to probe the electronic and geometric structures of the Co(II)-reconstituted dinuclear metal center in OpdA. In the resting state the metal ion in the more secluded α site is five-coordinate, whereas the Co(II) in the solvent-exposed β site is predominantly six-coordinate with two terminal water ligands. Addition of the slow substrate diethyl 4-methoxyphenyl phosphate does not affect the α site greatly but lowers the coordination number of the β site to five. A reduction in the exchange coupling constant indicates that substrate binding also triggers a shift of the μ-hydroxide into a pseudoterminal position in the coordination sphere of either the α or the β metal ion. Mechanistic implications of these observations are discussed.
authors.affiliation: School of Chemistry and Molecular Biosciences, The University of Queensland, St. Lucia, Australia.
authors.forename: Fernanda
authors.initials: F
authors.lastname: Ely
authors.forename: Kieran S
authors.initials: KS
authors.lastname: Hadler
authors.forename: Nataša
authors.initials: N
authors.lastname: Mitić
authors.forename: Lawrence R
authors.initials: LR
authors.lastname: Gahan
authors.forename: David L
authors.initials: DL
authors.lastname: Ollis
authors.forename: Nicholas M
authors.initials: NM
authors.lastname: Plugis
authors.forename: Marie T
authors.initials: MT
authors.lastname: Russo
authors.forename: James A
authors.initials: JA
authors.lastname: Larrabee
authors.forename: Gerhard
authors.initials: G
authors.lastname: Schenk
mesh_terms: D016960:Agrobacterium tumefaciens / Q000737:chemistry / Q000201:enzymology*; D001426:Bacterial Proteins / Q000737:chemistry*; D002942:Circular Dichroism; D003035:Cobalt / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004583:Electrons; D008958:Models, Molecular; D010755:Organophosphates / Q000378:metabolism*; D011487:Protein Conformation
chemical_list: D001426:Bacterial Proteins; D010755:Organophosphates; D003035:Cobalt