Catalyzes the four-electron reduction of molecular oxygen to water. a ubiquinone + 5 H(+)(in) + NADH = a ubiquinol + 4 H(+)(out) + NAD(+) Binds 1 FAD per subunit. Probably essential, it was not disrupted in a global transposon mutagenesis study. Belongs to the class-III pyridine nucleotide-disulfide oxidoreductase family. Extended N-terminus.