Nitrate reductase is a key enzyme involved in the first step of nitrate assimilation in plants, fungi and bacteria. H2O + NAD(+) + nitrite = H(+) + NADH + nitrate Binds 1 FAD (per monomer/chain). Binds 1 Mo-molybdopterin (Mo-MPT) cofactor per subunit. Binds 1 heme group. The heme group is called cytochrome b-557. kcat is 4900 sec(-1) for NADH in the ferricyanide reduction by the FAD-binding domain (at ph 7.0 and 25 degrees Celsius). Homodimer. By nitrate. Repressed by ammonium, the repression is not removed by the addition of extra nitrate. Urea weakens the effect of nitrate, but does not repress expression itself, implying it possibly reduces nitrate uptake rather than nitrate reduction. Up-regulated by nitrite. Belongs to the nitrate reductase family.