Flavin reductase

File:Flavin mononucleotide.png|This is the structure of flavin mononucleotide.

File:Flavin mononucleotide reduced.svg|This is the structure of reduced flavin mononucleotide.

File:NADP+ structure.svg|This is the structure of NADP+

Flavin reductase is a dimer made up of two subunits. Each subunit is similar. Flavin reductase P, FRP, was studied by Tanner, Lei, Tu and Krause and was discovered to have a structure made up of two subunits each containing a sandwich domain and an excursion domain. The excursion domains of each subunit reach out to connect the sandwich domain of the other subunit. This creates a large hydrophobic core in flavin reductase{{cite journal |author1 = Tanner, JJ | author2 = B. Lei | author3 = SC Tu |author4 =KL Krause | title = Flavin Reductase P: Structure of a Dimeric Enzyme That Reduces Flavin. | journal = Biochemistry |date= 22 Oct 1996 | pmid = 8885832 | doi=10.1021/bi961400v | volume=35 | issue=42 | pages=13531–9}} The enzyme has two binding sites, one for NADPH and one for the flavin mononucleotide substrate. The isoalloxazine ring of flavin mononucleotide is where reduction occurs. Therefore, this is where flavin creates a variety of hydrogen bonds to connect to the amino acid side chains of flavin reductase. Side chains 167–169 in FRP block the isoalloxazine ring in FAD from binding the enzyme, making FRP an FMN specific flavin reductase. The placement of methyl groups in the isoalloxazine ring can also have an effect on the binding and specificity of the enzyme for substrate.{{cite journal |vauthors = Fieschi F, Niviere V, Frier C, Decout JL, Fontecave M| title = The Mechanism and Substrate specificity of the NADPH:Flavin Oxidoreductase from Escherichia Coli. | date= 22 Dec 1995 | pmid = 8530465 | volume=270 | issue=51 | journal=J. Biol. Chem. | pages=30392–400 | doi=10.1074/jbc.270.51.30392| doi-access=free }} There is a depletion of a C-terminal extension that allows for the binding of NADPH, and studies show that if it is removed, it is depleted, catalytic activity increases.{{cite journal | vauthors = Seo D, Asano T, Komori H, Sakurai T| title = Role of the C-terminal extension stacked on the re-face of the isoalloxazine ring moiety of flavin. | date= 30 Jan 2014 | pmid = 24529496 | doi=10.1016/j.plaphy.2014.01.011 | volume=81 | journal=Plant Physiol. Biochem. | pages=143–8| url = https://kanazawa-u.repo.nii.ac.jp/?action=repository_action_common_download&item_id=10641&item_no=1&attribute_id=26&file_no=1 | hdl= 2297/36899 | hdl-access= free }}

File:Enzyme with FMN.png|This shows the hydrogen bonding of flavin reductase with flavin mononucleotide.

File:Mechanismpingpong.png|The ping pong mechanism is shown with NADPH binding first and leaving as NADP+ before FMN is bound by Flavin Reductase.

The mechanism of the flavin reductase process is described above and most likely follows the ping pong kinetic pattern. This means that it is a bisubstrate-biproduct mechanism. First the flavin reductase enzyme binds NADPH and stabilizes the release of the hydride. Because of sterics, it is not possible for the enzyme to bind both NADPH and the flavin. For this reason, NADP+ is released and then the flavin substrate is bound to the enzyme. In this step, the hydride attacks Nitrogen on the flavin, which allows for another protonation. Then, reduced flavin is released from flavin reductase as the second product. In this way, the reduction of flavin is dependent on flavin reductase binding first to NADPH, or in some cases NADH.

Flavin reductases exist in a variety of organisms, including animals and bacteria. In luminous organisms, flavin reductase is important in the luciferase process.{{cite journal | author1 = Takahito Imagawa | author2 = Toshiharu Tsurumura | author3 = Yasasushi Sugimoto | author4 = Kenji Aki | author5 = Kazumi Ishido | author6 = Seiki Kuramitsu | author7 = Hideaki Tsuge| title = Structural Basis of Free Reduced Flavin Generation by Flavin Reductase from Thermus Thermophilus. | date= 3 Nov 2011 | pmid = 22052907 | doi=10.1074/jbc.M111.257824 | pmc=3243531 | volume=286 | issue=51 | journal=J. Biol. Chem. | pages=44078–85| doi-access = free }}

In an experiment with P. fischeri and B. harveyi cells, bioluminescence was increased as the in vivo concentration of flavin reductase was increased. This suggests a connection between either a flavin reductase-luciferase complex or reduced flavin and the luminescence process in bacteria.{{cite journal |author1 = Warren Duane |author2 =JW Hastings| title = Flavin Nomonucleotide Reductase of Luminous Bacteria. | date= 10 June 1974 | pmid = 47604 | volume=6 | issue=1 | pages=53–64 | doi=10.1007/BF01731866 | journal=Mol. Cell. Biochem.|s2cid =21243671}} The bacteria oxidize the reduced flavin mononucleotide to oxidized FMN and transfer it through free fusion to generate light.{{cite journal | vauthors = Tinikul R, Pisawong W, Sucharitakul J, Nijvipakul S, Ballou DP, Chaiyen P| title = The transfer of reduced flavin mononucleotide from LuxG oxidoreductase to luciferase via free diffusion. | date= 1 Oct 2013 | pmid = 24004065 | doi=10.1021/bi4006545 | volume=52 | issue=39 | journal=Biochemistry | pages=6834–43}}

In humans, flavin reductase often catalyzes an NADPH dependent reduction of flavin mononucleotide which occurs in methemoglobin in erythrocytes and the liver.{{cite journal |vauthors = Shalloe F, Elliott G, Ennis O, Mantle TJ | title = Evidence that biliverdin-IX beta reductase and flavin reductase are identical. | date= 1 Jun 1996 | pmid = 8687377 | pmc=1217361 | volume=316 | journal=Biochem. J. | pages=385–7 | issue=2 | doi=10.1042/bj3160385}}

It has also been suggested that flavin reductases play a role in the production of hydrogen peroxide. This would be biologically helpful as H₂O₂ assists the body in maintaining homeostatic microbiota. A study showed that women with lactobacillus that produced hydrogen peroxide were less likely to develop bacterial vaginosis prebirth.{{cite journal | author1 = Hertzberger R | author2 =Arents Jos | author3 =Dekker H | author4 =Pridmore R | author5 =Gysler C | author6 =Kleerebezem M | author7 =Teixeira de Mattos M| title = H₂O₂ Production in species of the lactobacillus acidophilus group, a central role for a novel NADPH dependent flavin reductase. | date= 31 January 2014 | pmid = 24487531 | doi=10.1128/AEM.04272-13 | volume=80 | issue=7 | journal=Appl. Environ. Microbiol. | pages=2229–39| pmc=3993133 | bibcode =2014ApEnM..80.2229H }} It was also seen in Trichomonas vaginalis that decreased levels of flavin reductase increased the cycling of metronidazole because flavin reductase has an antioxidative effect, which decreases oxygen levels, maintaining the metronidazole population.{{cite journal|last1=Leitsch|first1=David|last2=Janssen|first2=Brian D.|last3=Kolarich|first3=Daniel|last4=Johnson|first4=Patricia J.|last5=Duchêne|first5=Michael|title=Trichomonas vaginalisflavin reductase 1 and its role in metronidazole resistance|journal=Molecular Microbiology|volume=91|issue=1|year=2014|pages=198–208|issn=0950-382X|doi=10.1111/mmi.12455|pmid=24256032|pmc=4437529}}

Currently, it is seen that bacterial flavin reductase can be used to sensitize carcinomas, or tumors to pro drugs. At first, flavin reductases were used to target the hypoxia of tumors. However, current research is showing an interest in these reductase molecules, specifically, MSuE from Pseudomonas aeruginosa which has been shown to increase the effectiveness of the prodrugs for cancerous tumors.{{cite journal | vauthors = Green LK, Storey MA, Williams EM, Patterson AV, Smaill JB, Copp JN, Ackerley DF| title = The Flavin Reductase MsuE is a Novel Nitroreductase that can efficiently Activate two promising next Generation Prodrugs for Gene-Directed Enzyme Prodrug Therapy. | date= 8 Aug 2013 | pmid = 24202330 | doi=10.3390/cancers5030985 | pmc=3795375 | volume=5 | issue=3 | journal=Cancers (Basel) | pages=985–97| doi-access = free }} A dual flavin reductase has been shown to participate in the activation of anticancer drugs.{{cite journal |vauthors = Paine MJ, Garner AP, Powell D, Sibbald J, Sales M, Pratt N, Smith T, Tew DG, Wolf CR| title = Cloning and Characterization of a novel human dual flavin reductase. | date= 14 Jan 2000 | pmid = 10625700 | doi=10.1074/jbc.275.2.1471 | volume=275 | issue=2 | pages=1471–8 | journal=J. Biol. Chem.| doi-access=free }} There are also molecules that when oxidized can be carcinogenic. In this case, it is helpful to have flavin reductase to reduce these molecules, such as carcinogenic chromate.{{cite journal|author4-link=David M. Kramer (biophysicist) | vauthors = Puzon GJ, Petersen JN, Roberts AG, Kramer DM, Xun L| title = A bacterial flavin reductase system reduces chromate to a soluble chromium(III)-NAD(+) complex. | date= 31 May 2002 | pmid = 12054743 | doi=10.1016/S0006-291X(02)00438-2 | volume=294 | issue=1 | journal=Biochem. Biophys. Res. Commun. | pages=76–81}}

{{Reflist}}

{{CH-NH oxidoreductases}}

{{Enzymes}}

{{Portal bar|Biology|border=no}}

Category:EC 1.5.1

Category:NADPH-dependent enzymes

Category:Enzymes of unknown structure