Aldehyde ferredoxin oxidoreductase

In enzymology, an aldehyde ferredoxin oxidoreductase (EC 1.2.7.5) is an enzyme that catalyzes the chemical reaction

Aldehyde ferredoxin oxidoreductase
Identifiers
EC no.1.2.7.5
CAS no.138066-90-7
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AFOR_N
structure of a hyperthermophilic tungstopterin enzyme, aldehyde ferredoxin oxidoreductase
Identifiers
SymbolAFOR_N
PfamPF02730
InterProIPR013983
SCOP21aor / SCOPe / SUPFAM
Available protein structures:
Pfam  structures / ECOD  
PDBRCSB PDB; PDBe; PDBj
PDBsumstructure summary
AFOR_C
Identifiers
SymbolAFOR_C
PfamPF01314
InterProIPR001203
SCOP21aor / SCOPe / SUPFAM
Available protein structures:
Pfam  structures / ECOD  
PDBRCSB PDB; PDBe; PDBj
PDBsumstructure summary
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

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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

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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

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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]

 
Molybdopterin cofactor, shown in the dithiol protonation state.

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

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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]

 
AOR mechanism at the active site.

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

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  1. ^ a b Majumdar A, Sarkar S (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. ^ a b Kisker C, Schindelin H, Rees DC (1997). "Molybdenum-cofactor-containing enzymes: structure and mechanism" (PDF). Annu. Rev. Biochem. 66: 233–67. doi:10.1146/annurev.biochem.66.1.233. PMID 9242907.
  3. ^ a b c d e f g h i j k l m n o p q Kletzin A, Adams MW (March 1996). "Tungsten in biological systems". FEMS Microbiol. Rev. 18 (1): 5–63. doi:10.1111/j.1574-6976.1996.tb00226.x. PMID 8672295.
  4. ^ White H, Strobl G, Feicht R, Simon H (September 1989). "Carboxylic acid reductase: a new tungsten enzyme catalyses the reduction of non-activated carboxylic acids to aldehydes". Eur. J. Biochem. 184 (1): 89–96. doi:10.1111/j.1432-1033.1989.tb14993.x. PMID 2550230.
  5. ^ Trautwein T, Krauss F, Lottspeich F, Simon H (June 1994). "The (2R)-hydroxycarboxylate-viologen-oxidoreductase from Proteus vulgaris is a molybdenum-containing iron-sulphur protein". Eur. J. Biochem. 222 (3): 1025–32. doi:10.1111/j.1432-1033.1994.tb18954.x. PMID 8026480.
  6. ^ Mukund S, Adams MW (April 1995). "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. doi:10.1074/jbc.270.15.8389. PMID 7721730.
  7. ^ Ma K, Hutchins A, Sung SJ, Adams MW (September 1997). "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. Bibcode:1997PNAS...94.9608M. doi:10.1073/pnas.94.18.9608. PMC 23233. PMID 9275170.
  8. ^ a b c d e f g h i j k l m Roy R, Dhawan IK, Johnson MK, Rees DC, Adams MW (2006-04-15). Handbook of Metalloproteins: Aldehyde Ferredoxin Oxidoreductase (5 ed.). John Wiley & Sons, Ltd.
  9. ^ a b c d e f g h Kisker C, Schindelin H, Rees DC (1997). "Molybdenum-cofactor-containing enzymes: structure and mechanism" (PDF). Annual Review of Biochemistry. 66: 233–67. doi:10.1146/annurev.biochem.66.1.233. PMID 9242907.
  10. ^ a b c d e f Bevers LE, Hagedoorn PL, Hagen WR (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

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This article incorporates text from the public domain Pfam and InterPro: IPR013983