Catalyzes the four-electron reduction of molecular oxygen to water. In fact, functions as the terminal component of an NADH oxidase (NADH:O(2) oxidoreductase) when using NADH:rubredoxin oxidoreductase (NROR) and rubredoxin (Rd) as electron transport intermediaries between NADH and FDP. Is thus able to reductively scavenge intracellular dioxygen and is part of an oxidative stress defense system in C.acetobutylicum, an obligate anaerobic bacterium. Can also serve as the terminal component of an NADH:nitric oxide oxidoreductase (NOR) with a catalytic efficiency comparable to that of its NADH oxidase activity, and therefore might have an in vivo role in scavenging nitric oxide. 2 H(+) + 2 NADH + O2 = 2 H2O + 2 NAD(+) Binds 1 FMN per subunit. Binds 2 iron ions per subunit. Homotetramer. Up-regulated upon exposure to O(2). Repressed by PerR. Consists of an N-terminal non-heme diiron domain and a C-terminal flavodoxin-like domain. In the N-terminal section; belongs to the zinc metallo-hydrolase group 3 family.