NADH dehydrogenase and nitrate reductase can form the anaerobic respiratory chain that can be used for transferring electrons from NADH to nitrate with proton-motive force across cytoplasmic membrane. In E. coli K-12, NDH-I and NDH-II is the two energy conserving NADH dehydrogenases that do not contribute to proton gradient; but both of the enzymes are involved in anaerobic nitrate respiration. NDH-I might be acted as proton pump for translocating 4H+ per NADH oxidised (2e-). In E. coli K-12, there are also two energy conserving (H+/e- = 1) nitrate reductases (nitrate reductase A (NRA) and nitrate reductase Z (NRZ)). Nitrate reductase A can express under the condition of high levels of nitrate in environment; while the expression of nitrate reductase Z doesn't depend on nitrate levels or anaerobiosis. Nitrate and hydrogen atom will be catalyzed to form nitrite and water during nitrate reduction.