Transfer of electrons from NADH to the respiratory chain. The immediate electron acceptor for the enzyme is believed to be ubiquinone. a ubiquinone + 5 H(+)(in) + NADH = a ubiquinol + 4 H(+)(out) + NAD(+) Binds 1 FAD per subunit. Homodimer. Belongs to the class-II pyridine nucleotide-disulfide oxidoreductase family.