Table A4.

Heme–copper superfamily active site (modified from Hemp & Gennis, 2008)

Enzyme	Active‐site residue	Predicted no. of proton channels	Present in
Oxygen reductase	Y	0–2	All domains; varies by enzyme subfamily
cNOR	E	0	Proteobacteria (Shiro, 2012; Hendriks, 2000)
qNOR	E	0	Proteobacteria, Cyanobacteria, Firmicutes, Archaea (Shiro, 2012; Hemp & Gennis, 2008; Heylen et al., 2007; Hendriks et al, 2000)
sNOR	N	0	Betaproteobacteria, Silicibacter, Deinococcus, Geobacillus (Stein et al., 2007)
eNOR	Q	1	Archaea (Hemp & Gennis, 2008, this study), Chloroflexi (Ward, McGlynn et al., 2018 this study), Proteobacteria (this study)
gNOR	D	0	Sulfurimonas, Sulfurovum, Persephonella (Sievert et al., 2008)
CuANor	N	1	Bacillus (Al‐Attar & De Vries, 2015; Suharti et al., 2001)

Abbreviations: cNOR, cytochrome‐type nitric oxide reductase; qNOR, quinol‐dependent nitric oxide reductases.