title: An alternative mechanism of bicarbonate-mediated peroxidation by copper-zinc superoxide dismutase: rates enhanced via proposed enzyme-associated peroxycarbonate intermediate.
abstract: Hydrogen peroxide can interact with the active site of copper-zinc superoxide dismutase (SOD1) to generate a powerful oxidant. This oxidant can either damage amino acid residues at the active site, inactivating the enzyme (the self-oxidative pathway), or oxidize substrates exogenous to the active site, preventing inactivation (the external oxidative pathway). It is well established that the presence of bicarbonate anion dramatically enhances the rate of oxidation of exogenous substrates. Here, we show that bicarbonate also substantially enhances the rate of self-inactivation of human wild type SOD1. Together, these observations suggest that the strong oxidant formed by hydrogen peroxide and SOD1 in the presence of bicarbonate arises from a pathway mechanistically distinct from that producing the oxidant in its absence. Self-inactivation rates are further enhanced in a mutant SOD1 protein (L38V) linked to the fatal neurodegenerative disorder, familial amyotrophic lateral sclerosis. The 1.4 A resolution crystal structure of pathogenic SOD1 mutant D125H reveals the mode of oxyanion binding in the active site channel and implies that phosphate anion attenuates the bicarbonate effect by competing for binding to this site. The orientation of the enzyme-associated oxyanion suggests that both the self-oxidative and external oxidative pathways can proceed through an enzyme-associated peroxycarbonate intermediate.
authors.affiliation: Department of Biochemistry and the X-ray Crystallography Core Laboratory, University of Texas Health Science Center, San Antonio, Texas 78229-3900, USA.
authors.forename: Jennifer Stine
authors.initials: JS
authors.lastname: Elam
authors.forename: Kevin
authors.initials: K
authors.lastname: Malek
authors.forename: Jorge A
authors.initials: JA
authors.lastname: Rodriguez
authors.forename: Peter A
authors.initials: PA
authors.lastname: Doucette
authors.forename: Alexander B
authors.initials: AB
authors.lastname: Taylor
authors.forename: Lawrence J
authors.initials: LJ
authors.lastname: Hayward
authors.forename: Diane E
authors.initials: DE
authors.lastname: Cabelli
authors.forename: Joan Selverstone
authors.initials: JS
authors.lastname: Valentine
authors.forename: P John
authors.initials: PJ
authors.lastname: Hart
mesh_terms: D000690:Amyotrophic Lateral Sclerosis / Q000378:metabolism; D001639:Bicarbonates / Q000378:metabolism*; D018360:Crystallography, X-Ray; D004789:Enzyme Activation; D006801:Humans; D006861:Hydrogen Peroxide / Q000378:metabolism*; D009046:Motor Neurons / Q000201:enzymology; D009154:Mutation; D018384:Oxidative Stress; D017434:Protein Structure, Tertiary; D013482:Superoxide Dismutase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D001639:Bicarbonates; D006861:Hydrogen Peroxide; D013482:Superoxide Dismutase