Nitrate Utilization

The ability to reduce nitrate (NO 3-) to ammonia (NH 3) is common to virtually all plants, fungi, and bacteria. It occurs in the following four steps:

> NO 3- -> NO 2- -> NO- -> NH 2 OH -> NH 3.

The first step, conversion of NO 3- to NO 2-, is catalyzed by a large and complex enzyme called nitrate reductase. Nitrate reductase is a multi-subunit enzyme with Mr of about 800 kD. It contains bound FAD, molybdenum, and a cytochrome called cytochrome 557 (which contains an Fe 4S 4 complex). Nitrate reductase carries out the following reaction:

> NO 3- + NAD(P)H + H+ -> NO 2- + NADP+ + H 2O

Plants use NADH. Fungi and bacteria use NADPH. The molybdenum is bound to a cofactor containing a pteridine ring to form molybdopterin (see here). All molybdenum-requiring enzymes except nitrogenase contain a structure similar to molydopterin.

Another interesting molybdenum-containing enzyme is dimethylsulfoxide reductase (Figure 20.6). It contains two molecules of bound molybdopterin covalently linked to a serine residue in the enzyme.

The last three steps in the reduction of nitrate to ammonia are carried out by an enzyme called nitrite reductase. It contains one Fe 2 S 2 center and one molecule of siroheme, a partially reduced iron porphyrin. The electron donor for each step is ferredoxin.

See also: The Nitrogen Cycle, Nitrogen Fixation, Utilization of Ammonia

INTERNET LINK: Nitrate Uptake and Reduction