title: Dithionite reduction kinetics of the dissimilatory copper-containing nitrite reductase of Alcalegenes xylosoxidans. The SO(2)(.-) radical binds to the substrate binding type 2 copper site before the type 2 copper is reduced.
abstract: We report here the first detailed study of the dithionite reduction kinetics of a copper-containing dissimilatory nitrite reductase (NiR). The reduction of the blue type 1 copper (T1Cu) center of NiR preparations that contained both type 1 and type 2 copper atoms, followed biphasic kinetics. In contrast, NiR that was deficient in type 2 copper (T2DNiR), followed monophasic kinetics with a second-order rate constant (T2D)k = 3.06 x 10(6) m(-1) s(-1). In all cases the SO(2)(.-) radical rather than S(2)O(4)(2-) was the effective reductant. The observed kinetics were compatible with a reaction mechanism in which the T1Cu of the fully loaded protein is reduced both directly by dithionite and indirectly by the type 2 Cu (T2Cu) site via intramolecular electron transfer. Reduction kinetics of the T2Cu were consistent with SO(2)(.-) binding first to the T2Cu center and then transferring electrons (112 s(-1)) to reduce it. As SO(2)(.-) is a homologue of NO(2)(-), the NiR substrate, it is not unlikely that it binds to the catalytic T2Cu site. Effects on the catalytic activity of the enzyme using dithionite as a reducing agent are discussed. Reduction of the semireduced T1Cu(I)T2Cu(II) state followed either second-order kinetics with k(2) = 3.33 x 10(7) m(-1) s(-1) or first-order kinetics with 52.6 s(-1) < (T1red)k(1) < 112 s(-1). Values of formation constants of the T1Cu(II)T2Cu(II)-SO(2)(.-) and T1Cu(I)T2Cu(II)-SO(2)(.-) adducts showed that the redox state of T1Cu affected binding of SO(2)(.-) at the catalytic T2Cu center. Analysis of the kinetics required the development of a mathematical protocol that could be applied to a system with two intercommunicating sites but only one of which can be monitored. This novel protocol, reported for the first time, is of general application.
authors.affiliation: Department of Biological Chemistry, John Innes Center, Colney, Norwich NR4 7UH, United Kingdom. faridoon.yousafzai@bbsrc.ac.uk
authors.forename: Faridoon K
authors.initials: FK
authors.lastname: Yousafzai
authors.forename: Robert R
authors.initials: RR
authors.lastname: Eady
mesh_terms: D000421:Alcaligenes / Q000201:enzymology*; D001665:Binding Sites; D003300:Copper / Q000378:metabolism*; D004227:Dithionite / Q000494:pharmacology*; D005609:Free Radicals; D007700:Kinetics; D008433:Mathematics; D009572:Nitrite Reductases / Q000737:chemistry / Q000378:metabolism*; D010084:Oxidation-Reduction; D013458:Sulfur Dioxide / Q000378:metabolism*
chemical_list: D005609:Free Radicals; D013458:Sulfur Dioxide; D004227:Dithionite; D003300:Copper; D009572:Nitrite Reductases