Mn(III) ions. The Mn(III) chelate interacts with the active site to facilitate product release from the enzyme. The chelator can have an effect on the kinetic rate and even the catalyzed reaction. If the substrate Mn(II) is chelated with lactate, MnP instead catalyzes the evolution of O2. However, this side reaction has little impact on enzymatic activity because it follows slower third order kinetics. Structural studies As of late 2007, 6 structures have been solved for this class of enzymes, with PDB accession codes , , , , , and . Although MnP, like other lignin peroxidases, is a Class II peroxidase, it has a similar tertiary structure to prokaryotic Class I peroxidases, but contains disulfide bridges like the Class III peroxidases in plants. MnP has a globular structure containing 11-12 α-helices, depending on the species it is produced in. It is stabilized by 10 cystine amino acid residues which form 5 disulfide bridges, one of which is near the C-terminal area. The active site contains a heme cofactor which is bound by two Ca2+ ions, one above and one below the heme. Near the internal heme propionate are three acidic residues which are used to stabilize Mn(II) or Mn(III) when it is bound to the enzyme. The specific residues vary between species, but their number and relative location in the folded protein is conserved. There are a total of 357 amino acid residues in the MnP of P. chrysosoporium, and a similar number in enzymes produced by other basidiomycetes. Biochemical significance The major function of the Mn(III) ions produced by MnP is oxidation and degradation of lignin. For this purpose, basidiomycetes secrete MnP, rather than Mn(III), and the enzyme functions outside of the fungal cell. Mn(III) ions from MnP can oxidize the phenolic compounds in lignin directly, but they can also oxidize some organic sulfur compounds and unsaturated fatty acids. This oxidation forms thiyl and peroxyl radicals, which in the presence of O2, can oxidize lignin or react with water to form H2O2. The Mn3+ ion itself can degrade lignin by catalyzing alkyl-aryl cleavages and α-carbon oxidation in phenols. Regulation MnP activity is controlled via transcriptional regulation. MnP is up-regulated by increases in extracellular Mn(II) and H2O2 concentrations. It has been found that increased O2 concentration and heat stress also activate MnP. References Further reading EC 1.11.1 Heme enzymes Enzymes of known structure