Sandbox GGC1

<StructureSection load='6BKA' size='340' side='right' caption='Crystal structure of yeast nitronate monooxygenase from Cyberlindera saturnas' scene=''>

Nitronate Monooxygenase (NMO) is an FMN-dependent (flavin mononucleotide) enzyme that oxidizes the neurotoxin propionate 3-nitronate (P3N). FMN is produced from riboflavin or Vitamin B2 by riboflavin kinase and can function as a prosthetic group for NADH dehydrogenase <ref>Huerta C, Borek D, Machius M, Grishin NV, Zhang H. Structure and Mechanism of a Eukaryotic FMN Adenylyltransferase. Journal of molecular biology. 2009;389(2):388-400. doi:10.1016/j.jmb.2009.04.022.</ref>. NMO is widely known as the best system for P3N detoxification in many different organisms.

NMOs are FMN-dependent enzymes that can quickly and efficiently catalyze the oxidation of P3N. They can also oxidize alkyl nitronates but with lower catalytic efficiency in comparison to P3N.<ref> Francis K, Nishino SF, Spain JC, Gadda G. A novel activity for fungal nitronate monooxygenase: detoxification of the metabolic inhibitor propionate-3-nitronate. Arch Biochem Biophys. 2012;521(1–2):84–89.</ref><ref>Gadda G, Francis K. Nitronate monooxygenase, a model for anionic flavin semiquinone intermediates in oxidative catalysis. Arch Biochem Biophys. 2010;493(1):53–61.</ref> Recent structural studies suggest there are two classes of NMOs, Class I and Class II.<ref>Salvi F, Agniswamy J, Yuan H, et al. The combined structural and kinetic characterization of a bacterial nitronate monooxygenase from Pseudomonas aeruginosa PAO1 establishes NMO class I and II.J Biol Chem. 2014;289(34):23764–23775.</ref> Class I NMOs contain about 450 NMO gene products from bacteria, fungi, and animals. The enzymes in this class only oxidize P3N and nitronate analogues. Class II NMOs consists of small groups of ten fungal gene products and can oxidize nitronate and nitroalkaline analogues.<ref> Salvi F, Agniswamy J, Yuan H, et al. The combined structural and kinetic characterization of a bacterial nitronate monooxygenase from Pseudomonas aeruginosa PAO1 establishes NMO class I and II.J Biol Chem. 2014;289(34):23764–23775.</ref>

In Class I NMOs, the enzyme mechanism is first initiated by a single electron transfer from P3N to the flavin that is enzyme-bound. The product is then oxidized by dioxygen and forms superoxide. The superoxide and P3N radial will then interact in the active site and form 3-peroxy-3-nitro-propanoate, which will decay to products with time.<ref>Smitherman C, Gadda G. Evidence for a transient peroxynitro acid in the reaction catalyzed by nitronate monooxygenase with propionate 3-nitronate. Biochemistry. 2013;52(15):2694–2704.</ref>

P3N can be considered a toxic compound that is commonly found in legumes, fungi, and leaf beetles. During hydrolysis, P3N is released from esters and acts as an irreversible inhibitor of mitochondrial succinate dehydrogenase. <ref>Hipkin CR, Simpson DJ, Wainwright SJ, Salem MA. Nitrification by plants that also fix nitrogen. Nature. 2004;430(6995):98–101</ref> Succinate dehydrogenase is a key enzyme in the Kreb's cycle and the electron transport chain for oxidative phosphorylation. Because this is inhibited, it can lead to a variety of neurological disorders and can even cause death. <ref>Francis K, Smitherman C, Nishino SF, Spain JC, Gadda G. The biochemistry of the metabolic poison propionate 3-nitronate and its conjugate acid, 3-nitropropionate. IUBMB Life. 2013;65(9):759–768.</ref>

This is the structure of the yeast CsNMO. <scene name='75/752263/Alpha_beta_sheets/1'>This view</scene> shows the alpha helixes and beta pleated sheets of nitronate monooxygenase. There are eight parallel beta pleaded strands that are depicted in pink surrounded by eight alpha helixes in blue.<ref>Agniswamy J, Reis R, Wang Y, et. al. Crystal structure of yeast nitronate monooxygenase from <i>Cyberlindnera saturnus</i>. Proteins: Structure, Function, and Bioinformatics, 2018.</ref>

The active site is located in an area formed by the FMN-binding domain and the substrate-binding domain. Residues that surround the active site and form a ring around FMN consist of M23, A24, F79, Y119, H147, H197, Y321, Y325, and L348.<ref>Agniswamy J, Reis R, Wang Y, et. al. Crystal structure of yeast nitronate monooxygenase from <i>Cyberlindnera saturnus</i>. Proteins: Structure, Function, and Bioinformatics, 2018.</ref> <scene name='75/752263/Active_site_fmn/1'>The active site is depicted with the surrounding residues that form a ring around FMN.</scene>

FMN Binding site is formed with hydrogen bonds with main chain amino acids of G240, G261, and T262. <ref>Agniswamy J, Reis R, Wang Y, et. al. Crystal structure of yeast nitronate monooxygenase from <i>Cyberlindnera saturnus</i>. Proteins: Structure, Function, and Bioinformatics, 2018.</ref> <scene name='75/752263/Fmn_binding_sitefinal/1'>This is a view of the FMN Binding site with labeled amino acid residues.</scene> Also, the phosphate group of FMN forms a hydrogen bond with the side-chain hydroxyl group of T262.