Nitrate reductase

Nitrate reductases are molybdoenzymes that reduce nitrate (NO) to nitrite (NO). This reaction is critical for the production of protein in most crop plants, as nitrate is the predominant source of nitrogen in fertilized soils. Eukaryotic nitrate reductases are part of the sulfite oxidase family of molybdoenzymes. They transfer electrons from NADH or NADPH to nitrate. Prokaryotic nitrate reductases belong to the DMSO reductase family of molybdoenzymes and have been classified into three groups, assimilatory nitrate reductases (Nas), respiratory nitrate reductase (Nar), and periplasmic nitrate reductases (Nap). The active site of these enzymes is a Mo ion that is bound to the four thiolate functions of two pterin molecules. The coordination sphere of the Mo is completed by one amino-acid side chain and oxygen and/or sulfur ligands. The exact environment of the Mo ion in certain of these enzymes (oxygen versus sulfur as a sixth molybdenum ligand) is still debated. The Mo is covalently attached to the protein by a cysteine ligand in Nap, and an aspartate in Nar. Prokaryotic nitrate reductases have two major types, transmembrane nitrate reductases and periplasmic nitrate reductases. The transmembrane nitrate reductase (NAR) does proton translocation and can contribute to the generation of ATP by the proton motive force. The periplasmic nitrate reductase (NAP) does not do proton translocation and does not contribute to the proton motive force. The transmembrane respiratory nitrate reductase is composed of three subunits; an 1 alpha, 1 beta and 2 gamma. It is the second nitrate reductase enzyme which it can substitute for the NRA enzyme in Escherichia coli allowing it to use nitrate as an electron acceptor during anaerobic respiration. A transmembrane nitrate reductase that can function as a proton pump (similar to the case of anaerobic respiration) has been discovered in a diatom Thalassiosira weissflogii.