Nitrate reductase is a key enzyme involved in the first step of nitrate assimilation in plants, fungi and bacteria. a quinol + nitrate = a quinone + H2O + nitrite Binds 1 [4Fe-4S] cluster. Binds 1 molybdenum-bis(molybdopterin guanine dinucleotide) (Mo-bis-MGD) cofactor per subunit. Nitrogen metabolism; nitrate reduction (denitrification); dinitrogen from nitrate: step 1/4. Belongs to the prokaryotic molybdopterin-containing oxidoreductase family. NasA/NapA/NarB subfamily.