Nitrate Reductase

Nitrate reductase catalyzes the first step in conversion of nitrate to ammonia, the reduction of nitrate to nitrite (NO2-). The reaction multisubunit nitrate reductase enzyme, with Mr of about 800 kilodaltons, contains bound FAD, molybdenum, and a cytochrome called cytochrome 557 (which contains an Fe4S4 complex). Nitrate reductase carries out the overall reaction:

NO3- + NAD(P)H + H+ ---> NO2- +   NADP+ + H2O

Plants use NADH as the electron donor, whereas fungi and bacteria use NADPH. The electrons are transferred to enzyme-bound FAD, then to cytochrome 557, then to molybdenum, and finally to the substrate.

See also: The Nitrogen Cycle, Nitrogen Fixation, Utilization of Ammonia, Dimethylsulfoxide Reductase, Nitrite Reductase, Siroheme, Ferredoxin, Figure 20.2

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1. Nitrate Uptake and Reduction