Proton transfer in the quinol-dependent nitric oxide reductase from Geobacillus stearothermophilus during reduction of oxygen

► Quinol-dependent nitric oxide reductase from G. stearothermophilus can reduce O2. ► The fully reduced qNOR is oxidised by O2 in a flow-flash experiment. ► A proton-coupled electron transfer with time constant = 1.8 ms is observed. ► The pH dependence of this phase is different from cNOR. ► Possible proton transfer pathways in qNOR are discussed.