Oxidation of glycine by Phaseolus leghaemoglobin with associated catabolic reactions at the haem

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 (Lb(2+)) or its oxygenated form (LbO(2)) to nonfunctional ferric Lb (Lb(3+)) and ferryl Lb. Because Lb(3+) is nearly nonexistent in nodules and undergoes observable reduction in vivo, mechanisms must operate in nodules to maintain Lb in the Lb(2+) state. Redox reactions of Lb are mediated, for the most part, by activated oxygen species: (a) oxidation of LbO(2) to Lb(3+) involves superoxide; (b) excess peroxide oxidizes LbO(2) and Lb(3+) 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 Lb(3+) requires active flavin and thiol groups and involves formation of peroxide; and (d) direct reduction of Lb(3+) 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 Lb(3+), increasing the rate of reaction, which then proceeds via superoxide or flavin radicals, respectively.

The prosthetic group of milk lactoperoxidase is protoheme IX

The prosthetic group of milk lactoperoxidase has been isolated from a Pronase hydrolysate of the enzyme and identified spectroscopically and chromatographically as protoheme IX. Partial degradation of the heme occurred during the proteolysis, possibly as a result of coupled oxidation in the presence of glycine and oxygen. The heme is assumed to be buried in the protein molecule in a crevice, which allows ligands to bind to the iron on one side only. The pyridine hemochrome of lactoperoxidase with alpha-band at 563 nm is interpreted as a mixed ligand complex with pyridine on one side of the heme and a ligand originating from the protein on the other. No experimental evidence supports the view that the heme is covalently bound to the protein through an ester linkage.