View source for Trimethylamine N-oxide reductase

{{infobox enzyme
| Name       = trimethylamine-N-oxide reductase
| EC_number  = 1.7.2.3
| CAS_number = 37256-34-1
| GO_code    = 0009033
| image      =
| width      = 
| caption    =  
}}
{{DISPLAYTITLE:Trimethylamine ''N''-oxide reductase}}
'''Trimethylamine ''N''-oxide reductase''' (TOR or TMAO reductase, EC 1.7.2.3) is a microbial enzyme that can reduce [[trimethylamine N-oxide|trimethylamine ''N''-oxide]] (TMAO) into [[trimethylamine]] (TMA), as part of the [[electron transport chain]]. The enzyme has been purified from ''[[E. coli]]'' and the photosynthetic bacteria ''[[Roseobacter|Roseobacter denitrificans]]''.<ref>{{cite journal | vauthors = Arata H, Shimizu M, Takamiya K | title = Purification and properties of trimethylamine N-oxide reductase from aerobic photosynthetic bacterium Roseobacter denitrificans | journal = Journal of Biochemistry | volume = 112 | issue = 4 | pages = 470–475 | date = October 1992 | pmid = 1337081 | doi = 10.1093/oxfordjournals.jbchem.a123923 }}</ref>

Trimethylamine oxide is found at high concentrations in the tissues of fish, and the bacterial reduction of this compound to foul-smelling trimethylamine is a major process in the spoilage of fish.<ref>{{cite journal | vauthors = Barrett EL, Kwan HS | title = Bacterial reduction of trimethylamine oxide | journal = Annual Review of Microbiology | volume = 39 | pages = 131–149 | year = 1985 | pmid = 3904597 | doi = 10.1146/annurev.mi.39.100185.001023 }}</ref>

== Classification ==
TMAO reductase has an [[Enzyme Commission number|enzyme commission (EC) number]] of 1.7.2.3. EC numbers are a system of enzyme nomenclature, and each part of this nomenclature refers to a progressive classification of the enzyme with regards to its reaction. The first number defines the reaction type, the second number provides information on involved compounds, the third number specifies the type of reaction, and the fourth number completes the unique serial number for each enzyme.<ref>{{cite journal | vauthors = Ann Benore M | title = What is in a name? (or a number?): The updated enzyme classifications | journal = Biochemistry and Molecular Biology Education | volume = 47 | issue = 4 | pages = 481–483 | date = July 2019 | pmid = 31063221 | doi = 10.1002/bmb.21251 | hdl = 2027.42/150538 | hdl-access = free }}</ref>

Trimethylamine N-oxide reductase has the EC number 1.7.2.3, and these components refer to the following enzyme classifications:

* EC 1 enzymes are [[oxidoreductase]] enzymes, where an oxidation reduction reaction occurs, and the substrate being oxidized is either an oxygen or hydrogen donor
* EC 1.7 enzymes act on other nitrogenous compounds as donors
* EC 1.7.2 enzymes have a cytochrome as an acceptor
* EC 1.7.2.3 is the enzyme TMAO reductase, which reduces the cytochrome TorC<ref>{{cite journal | vauthors = Gon S, Giudici-Orticoni MT, Méjean V, Iobbi-Nivol C | title = Electron transfer and binding of the c-type cytochrome TorC to the trimethylamine N-oxide reductase in Escherichia coli | journal = The Journal of Biological Chemistry | volume = 276 | issue = 15 | pages = 11545–11551 | date = April 2001 | pmid = 11056172 | doi = 10.1074/jbc.m008875200 | doi-access = free }}</ref>

== Species distribution ==
[[Trimethylamine N-oxide|TMAO]] is an organic osmolyte that has the useful biological function of protecting proteins against denaturing stresses such as high concentration of urea.<ref name=":0">{{cite journal | vauthors = Dos Santos JP, Iobbi-Nivol C, Couillault C, Giordano G, Méjean V | title = Molecular analysis of the trimethylamine N-oxide (TMAO) reductase respiratory system from a Shewanella species | journal = Journal of Molecular Biology | volume = 284 | issue = 2 | pages = 421–433 | date = November 1998 | pmid = 9813127 | doi = 10.1006/jmbi.1998.2155 }}</ref> Various bacteria grow anaerobically using TMAO as an alternative [[electron transport chain]], allowing for growth on non-fermentable carbon sources such as [[glycerol]].<ref>{{cite journal | vauthors = Méjean V, Iobbi-Nivol C, Lepelletier M, Giordano G, Chippaux M, Pascal MC | title = TMAO anaerobic respiration in Escherichia coli: involvement of the tor operon | journal = Molecular Microbiology | volume = 11 | issue = 6 | pages = 1169–1179 | date = March 1994 | pmid = 8022286 | doi = 10.1111/j.1365-2958.1994.tb00393.x | s2cid = 27347013 }}</ref> Bacteria capable of reducing TMAO to TMA are found throughout three different ecological niches. TMAO-reducing, to date, has been observed in marine bacteria, photosynthetic bacteria living in shallow ponds, and in [[Enterobacteriaceae|enterobacteria]].<ref name=":0" />

TMAO reductases have been studied in several organisms, and a common conserved feature is the presence of a [[molybdenum]] cofactor in all the known terminal enzymes.<ref name=":0" />

Based on their substrate specificity, these enzymes can be divided into two groups:

* TMAO reductases which have high substrate specificity
* [[DMSO reductase|DMSO]]/TMAO reductases which can reduce a broad range of N and S-oxide substrates.

The first group consists of species such as ''Escherichia coli'', ''[[Shewanella putrefaciens]]'', and ''Roseobacter denitrificans'' while the second group consists of species such as ''[[Proteus vulgaris]]'', ''[[Rhodobacter capsulatus]]'', and ''[[Rhodobacter sphaeroides]]''.<ref name=":0" />

The TMAO respiratory system has been mostly widely studied at the molecular level in ''E. coli'' and ''Rhodobacter'' species.

== Reaction mechanism ==
In ''E. coli'', TMAO reductase is encoded by the ''tor''CAD operon. The torC gene encodes a pentahemic [[Cytochrome c|c-type cytochrome]] (TorC). TorC is likely to transfer electrons directly to the periplasmic TorA terminal enzyme encoded by the torA gene.<ref name=":0" /> The anaerobic expression of the torCAD operon is strictly controlled by the presence of TMAO or related compounds.<ref>{{cite journal | vauthors = Jourlin C, Ansaldi M, Méjean V | title = Transphosphorylation of the TorR response regulator requires the three phosphorylation sites of the TorS unorthodox sensor in Escherichia coli | journal = Journal of Molecular Biology | volume = 267 | issue = 4 | pages = 770–777 | date = April 1997 | pmid = 9135110 | doi = 10.1006/jmbi.1997.0919 }}</ref>

There are several different metabolic pathways that involve TMAO and TMA'''.''' The reduction of TMAO to TMA, catalyzed by TMAO reductase, as part of the electron transport chain follows the following reaction:

NADH + H<sup>+</sup> + trimethylamine N-oxide <math>\rightleftharpoons</math> NAD<sup>+</sup> + trimethylamine + H<sub>2</sub>O
However, both the ''R. denitrificans'' and ''E. coli'' enzymes can accept electrons from [[cytochrome]]s:<ref>{{cite journal | vauthors = Gon S, Giudici-Orticoni MT, Méjean V, Iobbi-Nivol C | title = Electron transfer and binding of the c-type cytochrome TorC to the trimethylamine N-oxide reductase in Escherichia coli | journal = The Journal of Biological Chemistry | volume = 276 | issue = 15 | pages = 11545–11551 | date = April 2001 | pmid = 11056172 | doi = 10.1074/jbc.M008875200 | doi-access = free }}</ref>
:trimethylamine + 2 (ferricytochrome c)-subunit + H<sub>2</sub>O → trimethylamine ''N''-oxide + 2 (ferrocytochrome c)-subunit + 2 H<sup>+</sup>
Other reactions involving TMAO and TMA include:<ref>{{cite journal | vauthors = Barrett EL, Kwan HS | title = Bacterial reduction of trimethylamine oxide | journal = Annual Review of Microbiology | volume = 39 | issue = 1 | pages = 131–149 | date = October 1985 | pmid = 3904597 | doi = 10.1146/annurev.mi.39.100185.001023 }}</ref>

* The oxidation of TMA to TMAO, which occurs in some [[methylotroph]]s as an initial step in utilizing TMA as a source of carbon
* The demethylation of TMAO to dimethylamine and formaldehyde by methylotrophs
* The oxidative demethylation of TMA to dimethylamine and formaldehyde by methylotrophs
* The production of methane from TMA and other methylamines by some [[methanogen]]s

== Structure ==
In ''E. coli'', it has been shown that an inducible, periplasmic TMAO reductase is responsible for almost all TMAO reduction (with the rest being DMSO reduction). While no structural analysis of this ''E. coli'' enzyme has been reported, TMAO reductase from ''Shewanella massilia'' has been isolated and characterized at a resolution of 2.5 Å.<ref name=":1">{{cite journal | vauthors = Czjzek M, Dos Santos JP, Pommier J, Giordano G, Méjean V, Haser R | title = Crystal structure of oxidized trimethylamine N-oxide reductase from Shewanella massilia at 2.5 A resolution | journal = Journal of Molecular Biology | volume = 284 | issue = 2 | pages = 435–447 | date = November 1998 | pmid = 9813128 | doi = 10.1006/jmbi.1998.2156 }}</ref>

TMAO reductases have been studied in several organisms, and a common feature is the presence of a molybdenum cofactor in all the known terminal enzymes. The common form of the [[molybdopterin]] molecule is a tricyclic ring system comprising a [[pterin]] group fused to a [[pyran]] ring. The role of this pyran ring could be a way of controlling the oxidation state of the molybdenum cofactor and/or facilitating proton diffusion. Furthermore, the arrangement of aromatic residues in the funnel-like entrance leading to the active center is closely related to that of DMSO reductase structures. A hydrophobic pocket, formed by two [[tryptophan]] and two [[tyrosine]] residues, is also present in the TMAO reductase and contains highly conserved residues.<ref name=":1" />

When comparing TMAO reductase of ''S. massilia'' to DMSO reductase from ''R. Sphaeroides'' and ''R. capsulatus,'' the overall structure is strikingly similar. However, one major difference in TMAO reductase is a missing tyrosine (Tyr114), in DMSO reductase of ''R. capsulatus''. It is replaced by a threonine (Thr116) in the TMAO reductase, and the backbone stretch around this residue, from residue 100 to 116, is not identical to that in the DMSO reductases. A direct consequence of the missing residue is a wider accessible space, adjacent to the molybdenum active center, which potentially exists to accommodates the somewhat bulkier trimethylamine-oxide molecules more easily than the [[Dimethyl sulfoxide|dimethylsulfoxide molecules]].<ref name=":1" /> This different demonstrates how an enzyme's form is almost always directly tied to its function.

However, recent discrepancies have risen regarding the structure of the TMAO reductase active site. The proposed active site contains several anomalous bond lengths; one Mo-O bond length is too short for a Mo-O single-bond coordination, and the four Mo-S bond lengths are all considerably longer than expected. Moreover, the proposed molybdenum coordination of the active site is extremely crowded, with the distances between several supposedly nonbonding atoms being significantly shorter than the sum of their [[Van der Waals radius|van der Waals radii]] and some bond angles being unreasonably small. Now, it is being hypothesized that this overcrowding is due to the [[cocrystal]]lization of multiple forms of the enzyme.<ref>{{cite journal | vauthors = Zhang L, Nelson KJ, Rajagopalan KV, George GN | title = Structure of the molybdenum site of Escherichia coli trimethylamine N-oxide reductase | journal = Inorganic Chemistry | volume = 47 | issue = 3 | pages = 1074–1078 | date = February 2008 | pmid = 18163615 | doi = 10.1021/ic701956f }}</ref>

== See also ==
{{Portal|Biology}}
*[[Oxidative phosphorylation]]
*[[Microbial metabolism]]

== References ==
{{reflist}}

== Further reading ==
{{refbegin}}
* {{cite journal | vauthors = Unemoto T, Hayashi M, Miyaki K, Hayashi M | title = Intracellular localization and properties of trimethylamine-N-oxide reductase in Vibrio parahaemolyticus | journal = Biochimica et Biophysica Acta (BBA) - Enzymology and Biological Oxidation | volume = 110 | issue = 2 | pages = 319–328 | date = November 1965 | pmid = 4286289 | doi = 10.1016/s0926-6593(65)80039-x }}
{{refend}}

== External links ==
*[http://www.expasy.org/enzyme/1.7.2.3 ENZYME entry for EC 1.7.2.3]
*[http://www.brenda-enzymes.org/php/result_flat.php4?ecno=1.7.2.3 BRENDA entry for EC 1.7.2.3]

{{Electron transport chain}}
{{Cellular respiration}}
{{Nitrogenous donor oxidoreductases}}
{{NADH or NADPH oxidoreductases}}
{{Enzymes}}

[[Category:NADPH-dependent enzymes]]
[[Category:NADH-dependent enzymes]]
[[Category:Cellular respiration]]
[[Category:Metabolism]]
[[Category:EC 1.7.2]]