Oxidation and reduction of leghemoglobin in root nodules of leguminous plants

Reactions involving changes that affect the function of leghemoglobin (Lb) are reviewed. The chemical nature of Lb and conditions inside nodules, such as slightly acid pH and the presence of metal ions, chelators, and toxic metabolites (nitrite, superoxide radical, peroxides), are conducive for oxidation of ferrous Lb (Lb²⁺) or its oxygenated form (LbO₂) to nonfunctional ferric Lb (Lb³⁺) and ferryl Lb. Because Lb³⁺ is nearly nonexistent in nodules and undergoes observable reduction in vivo, mechanisms must operate in nodules to maintain Lb in the Lb²⁺ state. Redox reactions of Lb are mediated, for the most part, by activated oxygen species: (a) oxidation of LbO₂ to Lb³⁺ involves superoxide; (b) excess peroxide oxidizes LbO₂ and Lb³⁺ to ferryl Lb and may cause breakdown of heme, release of iron, and generation of hydroxyl radicals (protein radicals may be formed in this process); (c) enzymatic reduction of Lb³⁺ requires active flavin and thiol groups and involves formation of peroxide; and (d) direct reduction of Lb³⁺ by NADH is mediated by superoxide and peroxide. Transition metal ions and certain small molecules of nodules such as flavins may act as intermediate electron carriers between NADH and Lb³⁺, increasing the rate of reaction, which then proceeds via superoxide or flavin radicals, respectively.