Bacterial nitric oxide reductases (NOR) are integral membrane proteins that
catalyse the reduction of nitric oxide to nitrous oxide, often as a step in
the process of denitrification. Most functional data has been obtained with
NORs that receive their electrons from a soluble cytochrome c in the periplasm
and are hence termed cNOR. Very recently, the structure of a different type of
NOR, the quinol-dependent (q)-NOR from the thermophilic bacterium Geobacillus
stearothermophilus was solved to atomic resolution [Y. Matsumoto, T. Tosha,
A.V. Pisliakov, T. Hino, H. Sugimoto, S. Nagano, Y. Sugita and Y. Shiro, Nat.
Struct. Mol. Biol. 19 (2012) 238–246]. In this study, we have investigated the
reaction between this qNOR and oxygen. Our results show that, like some cNORs,
the G. stearothermophilus qNOR is capable of O2 reduction with a turnover of ~
3 electrons s− 1 at 40 °C. Furthermore, using the so-called flow-flash
technique, we show that the fully reduced (with three available electrons)
qNOR reacts with oxygen in a reaction with a time constant of 1.8 ms that
oxidises the low-spin heme b. This reaction is coupled to proton uptake from
solution and presumably forms a ferryl intermediate at the active site. The pH
dependence of the reaction is markedly different from a corresponding reaction
in cNOR from Paracoccus denitrificans, indicating that possibly the proton
uptake mechanism and/or pathway differs between qNOR and cNOR. This study
furthermore forms the basis for investigation of the proton transfer pathway
in qNOR using both variants with putative proton transfer elements modified
and measurements of the vectorial nature of the proton transfer. This article
is part of a Special Issue entitled: 17th European Bioenergetics Conference
(EBEC 2012)