Arsenite oxidase

 

Arsenite oxidase oxidises arsenite (As(III)O33-)to the less toxic arsenate (As(V)O43-). The electrons produced are transferred towards the soluble periplasmic electron carriers cytochrome c and/or amicyanin. Arsenite oxidase is a heterodimeric enzyme containing a large and a small subunit. The large catalytic subunit harbours the molybdopterin cofactor (comprising two molybdopterin guanosine dinucleotide cofactors bound to molybdenum), and a [3Fe-4S] cluster; the small subunit belongs to the structural class of the Rieske proteins and contains a Rieske-type [2Fe-2S] clusters.

 

Reference Protein and Structure

Sequences
Q7SIF4 UniProt (1.20.9.1)
Q7SIF3 UniProt (1.20.9.1) IPR014066 (Sequence Homologues) (PDB Homologues)
Biological species
Alcaligenes faecalis (Bacteria) Uniprot
PDB
1g8k - CRYSTAL STRUCTURE ANALYSIS OF ARSENITE OXIDASE FROM ALCALIGENES FAECALIS (1.64 Å) PDBe PDBsum 1g8k
Catalytic CATH Domains
3.40.228.10 CATHdb 3.30.200.200 CATHdb 2.102.10.10 CATHdb (see all for 1g8k)
Cofactors
Molybdopterin (1), Molybdenum(4+) (1), Tri-mu-sulfido-mu3-sulfido-triiron(0) (1), Di-mu-sulfido-diiron(2+) (1), Molybdopterin guanine dinucleotide (1) Metal MACiE
Click To Show Structure

Enzyme Reaction (EC:1.20.9.1)

water
CHEBI:15377ChEBI
+
arsenite(1-)
CHEBI:29242ChEBI
+
copper(2+)
CHEBI:29036ChEBI
→
arsenate(2-)
CHEBI:48597ChEBI
+
hydron
CHEBI:15378ChEBI
+
copper(1+)
CHEBI:49552ChEBI
Alternative enzyme names: Arsenite oxidase,

Enzyme Mechanism

Introduction

The arsenic of the arsenite substrate attacks one of the molybdenum coordinating oxo groups, resulting in a Mo(VI) to Mo(IV) reduction. The oxo group forms a second bond to the arsenic, forming the product arsenate and resulting in the loss of the oxo group from the molybdenum complex and the remaining oxo group binding in a much stronger interaction. An unidentified base activates water to attack the Mo(IV), forming a hydroxide group in place of the lost oxo group in a nucleophilic addition to the Mo(IV). A second unidentified base deprotonates the Mo-bound hydroxide, reforming the oxo group. This results in a single electron transfer through the pterin portion of the cofactor, main chain carbonyl of Ser238, thiolate of Cys24, an iron-sulfur cluster, the peptide bond of Ser99-Ser98, His62B, a second iron-sulfur complex and His81B and finally yields the electron to a bound azurin and resulting in Mo(V). A second single electron is transferred from Mo(V) via the same route regenerating the enzyme's Mo(VI) oxidation state.

Catalytic Residues Roles

UniProt PDB* (1g8k)
Ser100 (main-N), His104, Ser239 (main-C), Cys25, Ser99 (main-C), His123 Ser99E (main-N), His62F, Ser238E (main-C), Cys24E, Ser98E (main-C), His81F Forms the electron relay chain that transports single electrons from the active site to the bound azurin electron acceptor. single electron relay, single electron acceptor, single electron donor, hydrogen bond donor
*PDB label guide - RESx(y)B(C) - RES: Residue Name; x: Residue ID in PDB file; y: Residue ID in PDB sequence if different from PDB file; B: PDB Chain; C: Biological Assembly Chain if different from PDB. If label is "Not Found" it means this residue is not found in the reference PDB.

Chemical Components

bimolecular nucleophilic addition, overall reactant used, cofactor used, intermediate formation, unimolecular elimination by the conjugate base, decoordination from a metal ion, intermediate collapse, overall product formed, proton transfer, coordination to a metal ion, electron transfer, native state of cofactor regenerated, electron relay

References

  1. Conrads T et al. (2002), J Am Chem Soc, 124, 11276-11277. The Active Site of Arsenite Oxidase fromAlcaligenesfaecalis. DOI:10.1021/ja027684q. PMID:12236735.
  2. Warelow TP et al. (2017), Sci Rep, 7, 1757-. The active site structure and catalytic mechanism of arsenite oxidase. DOI:10.1038/s41598-017-01840-y. PMID:28496149.
  3. Hoke KR et al. (2004), Biochemistry, 43, 1667-1674. Electrochemical Studies of Arsenite Oxidase:  An Unusual Example of a Highly Cooperative Two-Electron Molybdenum Center†. DOI:10.1021/bi0357154. PMID:14769044.
  4. Ellis PJ et al. (2001), Structure, 9, 125-132. Crystal Structure of the 100 kDa Arsenite Oxidase from Alcaligenes faecalis in Two Crystal Forms at 1.64 Å and 2.03 Å. DOI:10.1016/s0969-2126(01)00566-4. PMID:11250197.

Step 1. Arsenic of the arsenite substrate attacks one of the molybdenum coordinating oxo groups, resulting in a Mo(VI) to Mo(IV) reduction.

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Catalytic Residues Roles

Residue Roles
Ser238E (main-C) hydrogen bond acceptor
Cys24E metal ligand
Ser99E (main-N) hydrogen bond donor
Ser98E (main-C) hydrogen bond acceptor
His62F hydrogen bond donor, metal ligand
His81F metal ligand

Chemical Components

ingold: bimolecular nucleophilic addition, overall reactant used, cofactor used, intermediate formation

Step 2. The attacking oxo group forms a second bond to the arsenic, forming the product arsenate and resulting in the loss of the oxo group from the molybdenum complex and the remaining oxo group binding in a much stronger interaction.

Download: Image, Marvin File

Catalytic Residues Roles

Residue Roles
Cys24E metal ligand
His81F metal ligand
His62F metal ligand
Ser238E (main-C) hydrogen bond acceptor
Ser99E (main-N) hydrogen bond donor
Ser98E (main-C) hydrogen bond acceptor
His62F hydrogen bond donor

Chemical Components

ingold: unimolecular elimination by the conjugate base, decoordination from a metal ion, intermediate collapse, intermediate formation, overall product formed

Step 3. An unidentified base (shown as water for simplicity) activates water to attack the Mo(IV), forming a hydroxide group in place of the lost oxo group in a nucleophilic addition to the Mo(IV).

Download: Image, Marvin File

Catalytic Residues Roles

Residue Roles
Cys24E metal ligand
His81F metal ligand
His62F metal ligand
Ser238E (main-C) hydrogen bond acceptor
Ser99E (main-N) hydrogen bond donor
Ser98E (main-C) hydrogen bond acceptor
His62F hydrogen bond donor

Chemical Components

proton transfer, ingold: bimolecular nucleophilic addition, coordination to a metal ion, intermediate formation, overall product formed

Step 4. A second unidentified base (shown as water) deprotonates the Mo-bound hydroxide, reforming the oxo group. This results in a single electron transfer through the pterin portion of the cofactor, main chain carbonyl of Ser238, thiolate of Cys24, an iron-sulfur cluster, the peptide bond of Ser99-Ser98, His62B, a second iron-sulfur complex and His81B and finally yields the electron to a bound azurin and resulting in Mo(V).

Download: Image, Marvin File

Catalytic Residues Roles

Residue Roles
Ser98E (main-C) single electron acceptor
His81F single electron donor
Cys24E metal ligand
His81F metal ligand
His62F metal ligand
Ser238E (main-C) hydrogen bond acceptor
Ser99E (main-N) hydrogen bond donor
Ser98E (main-C) hydrogen bond acceptor
His62F hydrogen bond donor
Cys24E single electron donor
His62F single electron relay
His81F single electron relay
His62F single electron acceptor
Cys24E single electron acceptor
Ser238E (main-C) single electron donor
Ser99E (main-N) single electron donor
Ser238E (main-C) single electron acceptor
His81F single electron acceptor
Cys24E single electron relay
Ser98E (main-C) single electron relay
Ser99E (main-N) single electron relay
Ser238E (main-C) single electron relay
Ser98E (main-C) single electron donor
Ser99E (main-N) single electron acceptor
His62F single electron donor

Chemical Components

proton transfer, electron transfer, overall reactant used, native state of cofactor regenerated, cofactor used, intermediate formation, electron relay

Step 5. The second single electron is transferred from Mo(V) through the pterin portion of the cofactor, main chain carbonyl of Ser238, thiolate of Cys24, an iron-sulfur cluster, the peptide bond of Ser99-Ser98, His62B, a second iron-sulfur complex and His81B and finally yields the electron to a bound azurin. This regenerates the enzyme's Mo(VI) oxidation state.

Download: Image, Marvin File

Catalytic Residues Roles

Residue Roles
Cys24E metal ligand
His81F metal ligand
His62F metal ligand, single electron acceptor
Ser238E (main-C) single electron donor
His81F single electron donor
Ser238E (main-C) hydrogen bond acceptor
Ser99E (main-N) hydrogen bond donor
Ser98E (main-C) hydrogen bond acceptor
His62F hydrogen bond donor
His62F single electron relay
His81F single electron relay
Cys24E single electron relay
Ser98E (main-C) single electron relay
Ser99E (main-N) single electron relay
Ser238E (main-C) single electron relay
His62F single electron donor
Ser98E (main-C) single electron acceptor, single electron donor
Ser238E (main-C) single electron acceptor
Ser99E (main-N) single electron acceptor
Cys24E single electron acceptor
His81F single electron acceptor
Ser99E (main-N) single electron donor
Cys24E single electron donor

Chemical Components

electron transfer, overall reactant used, native state of cofactor regenerated, cofactor used, intermediate formation, electron relay
Download: Image, Marvin File

Step 1. Arsenic of the arsenite substrate attacks one of the molybdenum coordinating oxo groups, resulting in a Mo(VI) to Mo(IV) reduction.

Step 2. The attacking oxo group forms a second bond to the arsenic, forming the product arsenate and resulting in the loss of the oxo group from the molybdenum complex and the remaining oxo group binding in a much stronger interaction.

Step 3. An unidentified base (shown as water for simplicity) activates water to attack the Mo(IV), forming a hydroxide group in place of the lost oxo group in a nucleophilic addition to the Mo(IV).

Step 4. A second unidentified base (shown as water) deprotonates the Mo-bound hydroxide, reforming the oxo group. This results in a single electron transfer through the pterin portion of the cofactor, main chain carbonyl of Ser238, thiolate of Cys24, an iron-sulfur cluster, the peptide bond of Ser99-Ser98, His62B, a second iron-sulfur complex and His81B and finally yields the electron to a bound azurin and resulting in Mo(V).

Step 5. The second single electron is transferred from Mo(V) through the pterin portion of the cofactor, main chain carbonyl of Ser238, thiolate of Cys24, an iron-sulfur cluster, the peptide bond of Ser99-Ser98, His62B, a second iron-sulfur complex and His81B and finally yields the electron to a bound azurin. This regenerates the enzyme's Mo(VI) oxidation state.

Products.

Contributors

Gemma L. Holliday, Daniel E. Almonacid, Charity Hornby