In enzymology, a leghemoglobin reductase () is an enzyme that catalyzes the chemical reaction NAD(P)H + H+ + 2 ferrileghemoglobin NAD(P)+ + 2 ferroleghemoglobin In other words, a leghemoglobin (or phytoglobin in general) with a Fe3+ is reduced to one with the ferrous ion, Fe2+. This enzyme belongs to the family of oxidoreductases, specifically those acting on NADH or NADPH with a heme protein as acceptor. The systematic name of this enzyme class is NAD(P)H:ferrileghemoglobin oxidoreductase. This enzyme is also called ferric leghemoglobin reductase. Role in legume nodules Leghemoglobin (Lb) is a heme-containing protein that reversibly binds and transports O2 into the N2-fixing nodules of leguminous plants. In order to function as an O2-carrier Lb must be in the ferrous oxidation state (Lb2+). Oxygenated Lb2+ (Lb2+O2) readily autoxidizes to ferric Lb (Lb3+) generating O2− in the presence of trace amounts of transition metals, chelators and toxic metabolites (such as nitrite, superoxide radical and peroxides), however Lb2+ is the predominant form in nodules. Therefore, mechanisms exist in vivo for maintaining Lb in the functional ferrous status. History Burris and Hass were the first to propose that reduced pyridine nucleotides might function as reductants of Lb3+ in leguminous root nodules and in 1969 Appleby reported that Lb3+ was reduced to Lb2+ by a suspension of bacteroids. In 1982 Kretovich and collaborators purified an enzyme from lupine nodules which catalyzed the reduction of Lb3+ to Lb2+ using NADH as reductant. This enzyme (named by these authors as Legoglobin Reductase -LR) is similar to NADH:cytochrome b5 reductase (EC 1.6.2.2) from erythrocytes and bovine muscle. Lupin LR is a flavoprotein with a molecular mass of 60 kDa and its activity is specific for NADH. In 1984 Klucas and collaborators purified a protein with ferric Lb reductase (FLbR) activity from soybean nodules. The activity of soybean FLbR was 90% in the nodule cytosol and 10% in the bacteroids. NADH was the best reductant for soybean FLbR, although NADPH also functioned at rates that were three-fold less than NADH. These investigations by Klucas and collaborators also showed that the oxidation of NADH and reduction of Lb3+ was undetectable when O2 was removed from the reaction system, but all were restored upon re-addition of O2, which indicated that the FLbR activity is O2-dependent. In legumes Soybean FLbR is a flavoprotein with flavin adenine dinucleotide (FAD) as the prosthetic group and consists of two identical subunits, each having a