title: Substrate control of internal electron transfer in bacterial nitric-oxide reductase.
abstract: Nitric -oxide reductase (NOR) from Paracoccus denitrificans catalyzes the reduction of nitric oxide (NO) to nitrous oxide (N(2)O) (2NO + 2H(+) + 2e(-) -->N(2)O + H(2)O) by a poorly understood mechanism. NOR contains two low spin hemes c and b, one high spin heme b(3), and a non-heme iron Fe(B). Here, we have studied the reaction between fully reduced NOR and NO using the "flow-flash" technique. Fully (four-electron) reduced NOR is capable of two turnovers with NO. Initial binding of NO to reduced heme b(3) occurs with a time constant of approximately 1 micros at 1.5 mM NO, in agreement with earlier studies. This reaction is [NO]-dependent, ruling out an obligatory binding of NO to Fe(B) before ligation to heme b(3). Oxidation of hemes b and c occurs in a biphasic reaction with rate constants of 50 s(-1) and 3 s(-1) at 1.5 mM NO and pH 7.5. Interestingly, this oxidation is accelerated as [NO] is lowered; the rate constants are 120 s(-1) and 12 s(-1) at 75 microM NO. Protons are taken up from solution concomitantly with oxidation of the low spin hemes, leading to an acceleration at low pH. This effect is, however, counteracted by a larger degree of substrate inhibition at low pH. Our data thus show that substrate inhibition in NOR, previously observed during multiple turnovers, already occurs during a single oxidative cycle. Thus, NO must bind to its inhibitory site before electrons redistribute to the active site. The further implications of our data for the mechanism of NO reduction by NOR are discussed.
authors.affiliation: Department of Biochemistry and Biophysics, The Arrhenius Laboratories for Natural Sciences, Stockholm University, SE-106 91 Stockholm, Sweden.
authors.forename: Peter
authors.initials: P
authors.lastname: Lachmann
authors.forename: Yafei
authors.initials: Y
authors.lastname: Huang
authors.forename: Joachim
authors.initials: J
authors.lastname: Reimann
authors.forename: Ulrika
authors.initials: U
authors.lastname: Flock
authors.forename: Pia
authors.initials: P
authors.lastname: Adelroth
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D004579:Electron Transport / Q000235:genetics / Q000502:physiology; D006863:Hydrogen-Ion Concentration; D009569:Nitric Oxide / Q000378:metabolism; D009609:Nitrous Oxide / Q000378:metabolism; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000235:genetics / Q000378:metabolism*; D010231:Paracoccus denitrificans / Q000201:enzymology
chemical_list: D001426:Bacterial Proteins; D009569:Nitric Oxide; D010088:Oxidoreductases; C034747:nitric-oxide reductase; D009609:Nitrous Oxide