title: An unexpected formation of the spectroscopic Cu(I)-semiquinone radical by xenon-induced self-catalysis of a copper quinoprotein.
abstract: Plant copper/quinone amine oxidases are homodimeric enzymes containing Cu(II) and a quinone derivative of a tyrosyl residue (2,4,5-trihydroxyphenylalanine, TPQ) as cofactors. These enzymes catalyze the oxidative deamination of primary amines by a classical ping-pong mechanism, i.e. two distinct half-reactions, enzyme reduction by substrate followed by its re-oxidation by molecular oxygen. In the first half-reaction two forms of the reduced TPQ have been observed, the colorless Cu(II)-aminoquinol and the yellow Cu(I)-semiquinolamine radical so that this enzyme may be referred to as a "protein-radical enzyme". The interaction of xenon, in aqueous solutions, with the copper/TPQ amine oxidase from lentil (Lens esculenta) seedlings has been investigated by NMR and optical spectroscopy. NMR data indicate that xenon binds to the protein. Under 10 atm gaseous xenon and in the absence of substrates more than 60% native enzyme is converted into Cu(I)-semiquinolamine radical species, showing for the first time that both monomers in the dimer can generate the radical. Under the same experimental conditions the copper-free lentil enzyme is able to generate an intermediate absorbing at about 360 nm, which is assigned to the product Schiff base quinolaldimine which, to the best of our knowledge, has never been observed during the catalytic mechanism of plant amine oxidases. A possible role of the lysine residue responsible for the formation of Cu(I)-semiquinolamine and quinolaldimine, is proposed.
authors.affiliation: Department of Applied Sciences in Biosystems, University of Cagliari, 09042 Monserrato (CA), Cagliari, Italy.
authors.forename: Rosaria
authors.initials: R
authors.lastname: Medda
authors.forename: Anna
authors.initials: A
authors.lastname: Mura
authors.forename: Silvia
authors.initials: S
authors.lastname: Longu
authors.forename: Roberto
authors.initials: R
authors.lastname: Anedda
authors.forename: Alessandra
authors.initials: A
authors.lastname: Padiglia
authors.forename: Mariano
authors.initials: M
authors.lastname: Casu
authors.forename: Giovanni
authors.initials: G
authors.lastname: Floris
mesh_terms: D006631:Amine Oxidase (Copper-Containing) / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D016227:Benzoquinones / Q000737:chemistry / Q000378:metabolism*; D002384:Catalysis; D002851:Chromatography, High Pressure Liquid; D003300:Copper / Q000737:chemistry*; D027806:Lens Plant / Q000201:enzymology; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008956:Models, Chemical; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010940:Plant Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D036226:Seedlings / Q000201:enzymology; D013050:Spectrometry, Fluorescence / Q000379:methods; D013053:Spectrophotometry / Q000379:methods; D013056:Spectrophotometry, Ultraviolet / Q000379:methods; D014978:Xenon / Q000737:chemistry*; D014979:Xenon Isotopes
chemical_list: D016227:Benzoquinones; D010940:Plant Proteins; D014979:Xenon Isotopes; C025232:semiquinone radicals; D014978:Xenon; D003300:Copper; D006631:Amine Oxidase (Copper-Containing)