Biology:Aldehyde ferredoxin oxidoreductase
| Aldehyde ferredoxin oxidoreductase | |||||||||
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| Identifiers | |||||||||
| EC number | 1.2.7.5 | ||||||||
| CAS number | 138066-90-7 | ||||||||
| Databases | |||||||||
| IntEnz | IntEnz view | ||||||||
| BRENDA | BRENDA entry | ||||||||
| ExPASy | NiceZyme view | ||||||||
| KEGG | KEGG entry | ||||||||
| MetaCyc | metabolic pathway | ||||||||
| PRIAM | profile | ||||||||
| PDB structures | RCSB PDB PDBe PDBsum | ||||||||
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| AFOR_N | |||||||||
|---|---|---|---|---|---|---|---|---|---|
 structure of a hyperthermophilic tungstopterin enzyme, aldehyde ferredoxin oxidoreductase | |||||||||
| Identifiers | |||||||||
| Symbol | AFOR_N | ||||||||
| Pfam | PF02730 | ||||||||
| InterPro | IPR013983 | ||||||||
| SCOP2 | 1aor / SCOPe / SUPFAM | ||||||||
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| AFOR_C | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Identifiers | |||||||||
| Symbol | AFOR_C | ||||||||
| Pfam | PF01314 | ||||||||
| InterPro | IPR001203 | ||||||||
| SCOP2 | 1aor / SCOPe / SUPFAM | ||||||||
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In enzymology, an aldehyde ferredoxin oxidoreductase (EC 1.2.7.5) is an enzyme that catalyzes the chemical reaction
- an aldehyde + H2O + 2 oxidized ferredoxin ⇌ an acid + 3 H+ + 2 reduced ferredoxin
This enzyme belongs to the family of oxidoreductases, specifically those acting on the aldehyde or oxo group of donor with an iron-sulfur protein as acceptor. The systematic name of this enzyme class is aldehyde:ferredoxin oxidoreductase. This enzyme is also called AOR. It is a relatively rare example of a tungsten-containing protein.[1]
Occurrence
The active site of the AOR family feature an oxo-tungsten center bound to a pair of molybdopterin cofactors (which does not contain molybdenum) and an 4Fe-4S cluster.[2][3] This family includes AOR, formaldehyde ferredoxin oxidoreductase (FOR), glyceraldehyde-3-phosphate ferredoxin oxidoreductase (GAPOR), all isolated from hyperthermophilic archea;[2] carboxylic acid reductase found in clostridia;[4] and hydroxycarboxylate viologen oxidoreductase from Proteus vulgaris, the sole member of the AOR family containing molybdenum.[5] GAPOR may be involved in glycolysis,[6] but the functions of the other proteins are not yet clear. AOR has been proposed to be the primary enzyme responsible for oxidising the aldehydes that are produced by the 2-keto acid oxidoreductases.[7]
AOR is found in hyperthermophillic archaea, Pyrococcus furiosus.[1] The archaeons Pyrococcus ES-4 strain and Thermococcus ES-1 strain differ by their substrate specificity: AFOs show a broader size range of its aldehyde substrates. Its primary role is to oxidize aldehyde coming derived from the metabolism of amino acids and glucoses.[8] Aldehyde Ferredoxin Oxidoreductase is a member of an AOR family, which includes glyceraldehyde-3-phosphate ferredoxin oxidoreductase (GAPOR) and Formaldehyde Ferredoxin Oxidoreductase.[3]
Function
AOR functions at high temperature conditions (~80 degrees Celsius) at an optimal pH of 8-9. It is oxygen-sensitive as it loses bulk of its activity from oxygen exposure and works in the cytoplasm where it is a reducing environment. Thus, either exposure to oxygen or lowering of the temperature causes an irreversible loss of its catalytic properties. Also, as a result of oxygen sensitivity of AOR, purification of the enzyme is done under anoxic environments.[8]
It is proposed that AOR has a role in the Entner-Doudoroff pathway (glucose degradation) due to its increased activity with maltose incorporation.[3] However, other proposals include its role in oxidation of amino acid metabolism aldehyde side products coming from de-aminated 2-ketoacids. The main substrates for aldehyde ferredoxin oxidoreductase are acetaldehyde, phenylacetaldehyde, and isovalerdehyde, which is a metabolic product from common amino acids and glucose.[8] For example, acetaldehyde reaches its kcat/KM value up to 22.0 μM-1s-1.[8] In fact, some microorganisms only make use of amino acids as a carbon source, such as Thermococcus strain ES1; thus, they utilize aldehyde ferredoxin oxidoreductase to metabolize the amino acid carbon source.[8]
Structure
AOR is homodimeric. Each 67kDa subunit contains 1 tungsten and 4-5 Iron atoms.[3] The two subunits are bridged by a low spin Iron center. It is believed that the two subunits function independently.[3]
- Tungsten-pterin
Tungsten in the active site of AOR adopts a distorted square pyramidal geometry bound an oxo/hydroxo ligand and the dithiolene substituents of two molybdopterin cofactors.[3]
Two molybdopterin cofactors bind tungsten,[9] as observed in many related enzymes.[9] Tungsten is not bonded directly to the protein.[9] Phosphate centers pendant on the cofactor are bound to a Mg2+, which is also bound by Asn93 and Ala183 to complete its octahedral coordination sphere.[3][9] Thus, pterin and Tungsten atoms are connected to the AOR enzyme primarily through pterin's Hydrogen bonding networks with the amino acid residues.[3] In addition, two water ligands that occupy the octahedral geometry take part in hydrogen bonding networks with pterin, phosphate, and Mg2+.[9] While [Fe4S4] cluster is bound by four Cys ligands, Pterin - rich in amino and ether linkages - interacts with the Asp-X-X-Gly-Leu-(Cys/Asp) sequences in the AOR enzyme.[3] In such sequence, Cys494 residue is also hydrogen bonded to the [Fe4S4] cluster.[3] This indicates that Cys494 residue connects the Tungsten site and the [Fe4S4] cluster site in the enzyme.[3] Iron atom in the cluster is additionally bound by three other Cystein ligands: .[9] Also, another linker amino acid residue between ferredoxin cluster and pterin is the Arg76, which hydrogen bonds to both pterin and ferredoxin.[3] It is proposed that such hydrogen bonding interactions imply pterin cyclic ring system as an electron carrier.[3] Additionally the C=O center of the pterin binds Na+.[8] The W=O center is proposed, not verified crystallographically.[9]
AOR consists of three domains, domain 1, 2, and 3.[8] While domain 1 contains pterin bound to tungsten, the other two domains provide a channel from tungsten to protein's surface (15 Angstroms in length) in order to allow specific substrates to enter the enzyme through its channel.[8] In the active site, this pterin molecules is in a saddle-like conformation (500 to the normal plane) to “sit” on the domain 1 which also takes on a form with beta sheets to accommodate the Tungsten-Pterin site.[8]
- Iron
The iron center in between the two subunits serve a structural role in AOR.[8] Iron metal atoms takes on a tetrahedral conformation while the ligand coordination comes from two histidines and glutamic acids.[8] This is not known to have any functional role in the redox activity of the protein.[8]
- Fe4S4 centre
[Fe4S4] cluster in AOR is different in some aspects to other ferredoxin molecules.[3] EPR measurements confirm that it serves as a one-electron shuttle.[3]
Aldehyde ferredoxin oxidoreductase mechanism
In the catalytic cycle, W(VI) (tungsten "six") converts to W(IV) concomitant with oxidation of the aldehyde to a carboxylic acid (equivalently, a carboxylate).[3] A W(V) intermediate can be detected by EPR spectroscopy.[3][8]
General Reaction Mechanism of AOR:[10]
- RCHO + H2O → RCO2H + 2H+ + 2 e−
The redox equivalents are provided by the 4Fe-4S cluster.
A tyrosine residue is proposed to activate the electrophilic centre of aldehydes by H-bonding to the carbonyl oxygen atom, coordinated to the W centre.[10] A glutamic acid residue near the active site activates a water molecule for a nucleophilic attack on aldehyde carbonyl center.[10] After nucleophilic attack by water, hydride is transferred to oxo-tungsten sie thus, .[10] Subsequently, W(VI) is regenerated by electron transfer to the 4Fe-4S center.[10] With formaldehyde ferredoxin oxidoreductase, Glu308 and Tyr 416 would be involved while Glu313 and His448 is shown to be present in AOR active site.[9][10]
References
- ↑ 1.0 1.1 Majumdar, Amit; Sarkar, Sabyasachi (May 2011). "Bioinorganic chemistry of molybdenum and tungsten enzymes: A structural–functional modeling approach". Coordination Chemistry Reviews 255 (9–10): 1039–1054. doi:10.1016/j.ccr.2010.11.027.
- ↑ 2.0 2.1 "Molybdenum-cofactor-containing enzymes: structure and mechanism". Annu. Rev. Biochem. 66: 233–67. 1997. doi:10.1146/annurev.biochem.66.1.233. PMID 9242907. https://authors.library.caltech.edu/630/1/KISarb97.pdf.
- ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 3.13 3.14 3.15 3.16 "Tungsten in biological systems". FEMS Microbiol. Rev. 18 (1): 5–63. March 1996. doi:10.1111/j.1574-6976.1996.tb00226.x. PMID 8672295.
- ↑ "Carboxylic acid reductase: a new tungsten enzyme catalyses the reduction of non-activated carboxylic acids to aldehydes". Eur. J. Biochem. 184 (1): 89–96. September 1989. doi:10.1111/j.1432-1033.1989.tb14993.x. PMID 2550230.
- ↑ "The (2R)-hydroxycarboxylate-viologen-oxidoreductase from Proteus vulgaris is a molybdenum-containing iron-sulphur protein". Eur. J. Biochem. 222 (3): 1025–32. June 1994. doi:10.1111/j.1432-1033.1994.tb18954.x. PMID 8026480.
- ↑ "Glyceraldehyde-3-phosphate ferredoxin oxidoreductase, a novel tungsten-containing enzyme with a potential glycolytic role in the hyperthermophilic archaeon Pyrococcus furiosus". J. Biol. Chem. 270 (15): 8389–92. April 1995. doi:10.1074/jbc.270.15.8389. PMID 7721730.
- ↑ "Pyruvate ferredoxin oxidoreductase from the hyperthermophilic archaeon, Pyrococcus furiosus, functions as a CoA-dependent pyruvate decarboxylase". Proc. Natl. Acad. Sci. U.S.A. 94 (18): 9608–13. September 1997. doi:10.1073/pnas.94.18.9608. PMID 9275170. Bibcode: 1997PNAS...94.9608M.
- ↑ 8.00 8.01 8.02 8.03 8.04 8.05 8.06 8.07 8.08 8.09 8.10 8.11 8.12 Roy, Roopali; Dhawan, Ish K.; Johnson, Michael K.; Rees, Douglas C.; Adams, Michael W. (2006-04-15). Handbook of Metalloproteins: Aldehyde Ferredoxin Oxidoreductase (5 ed.). John Wiley & Sons, Ltd.
- ↑ 9.0 9.1 9.2 9.3 9.4 9.5 9.6 9.7 "Molybdenum-cofactor-containing enzymes: structure and mechanism". Annual Review of Biochemistry 66: 233–67. 1997. doi:10.1146/annurev.biochem.66.1.233. PMID 9242907. https://authors.library.caltech.edu/630/1/KISarb97.pdf.
- ↑ 10.0 10.1 10.2 10.3 10.4 10.5 Bevers, Loes E.; Hagedoorn, Peter-Leon; Hagen, Wilfred R. (February 2009). "The bioinorganic chemistry of tungsten". Coordination Chemistry Reviews 253 (3–4): 269–290. doi:10.1016/j.ccr.2008.01.017.
Further reading
- "The novel tungsten-iron-sulfur protein of the hyperthermophilic archaebacterium, Pyrococcus furiosus, is an aldehyde ferredoxin oxidoreductase. Evidence for its participation in a unique glycolytic pathway". J. Biol. Chem. 266 (22): 14208–16. 1991. doi:10.1016/S0021-9258(18)98669-2. PMID 1907273.
- "Identification of molybdopterin as the organic component of the tungsten cofactor in four enzymes from hyperthermophilic Archaea". J. Biol. Chem. 268 (7): 4848–52. 1993. doi:10.1016/S0021-9258(18)53474-8. PMID 8444863.
- "Aldehyde oxidoreductases from Pyrococcus furiosus". Methods Enzymol. 331: 132–44. 2001. doi:10.1016/S0076-6879(01)31052-2. PMID 11265456.
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