Hydroperoxide dehydratase

 

Coral allene oxide synthase (cAOS) is a fusion protein with 8R-lipoxygenase (LOX) from Plexura homomalia. cAOS is a hemoprotein with sequence homology to catalase, and is involved in the catalysis of the production of an unstable epoxide (an allene oxide) from the fatty acid hydroperoxide generated by the lipoxygenase activity (8R-HPETE). Despite the sequence homology to catalase, cAOS does not exhibit catalase activity as is does not catalyse the dismutation of hydrogen peroxide to water.

 

Reference Protein and Structure

Sequence
O16025 UniProt (1.13.11.40, 4.2.1.92) IPR011614 (Sequence Homologues) (PDB Homologues)
Biological species
Plexaura homomalla (Black sea rod) Uniprot
PDB
1u5u - The structure of an Allene Oxide Synthase reveals a novel use for a catalase fold (2.0 Å) PDBe PDBsum 1u5u
Catalytic CATH Domains
2.40.180.10 CATHdb (see all for 1u5u)
Cofactors
Heme b (1)
Click To Show Structure

Enzyme Reaction (EC:4.2.1.92)

(9Z,11E,13S,15Z)-13-hydroperoxyoctadeca-9,11,15-trienoate
CHEBI:58757ChEBI
→
(9Z,13S,15Z)-12,13-epoxyoctadeca-9,11,15-trienoate
CHEBI:36438ChEBI
+
water
CHEBI:15377ChEBI
Alternative enzyme names: HPI, Hydroperoxide isomerase, Linoleate hydroperoxide isomerase, Linoleic acid hydroperoxide isomerase, Allene oxide synthase, (9Z,11E,14Z)-(13S)-hydroperoxyoctadeca-9,11,14-trienoate 12,13-hydro-lyase, (9Z,11E,14Z)-(13S)-hydroperoxyoctadeca-9,11,14-trienoate 12,13-hydro-lyase ((9Z)-(13S)-12,13-epoxyoctadeca-9,11-dienoate-forming), AOS,

Enzyme Mechanism

Introduction

Despite sequence homology to catalase, the catalytic activities of cAOS and catalase are very different. 8R-HPETE is the fatty acid hydroperoxide derivative of arachidonic acid, produced by LOX activity. cAOS catalyses the conversion of 8R-HPETE into an allene oxide.

His 67 hydrogen bonds to the terminal peroxide hydrogen, favouring homolytic cleavage. His 67 hydrogen bonds to Thr 66 and Asn 137, which keeps the distal His imidazole ring in the correct orientation (flipped, relative to in catalase) for enzymatic activity. A hydrogen bond from the main chain amide of Asn 137 also helps to promote this cleavage. Cleavage results in the formation of an alkoxyl radical. One of the peroxide oxygens becomes bound to the heme group as a hydroxyl, with the other oxygen forming the radical. The alkoxyl radical forms a carbon radical, which reduces Fe-IV. The carbon radical is converted into a short-lived epoxy allylic carbocation. This carbocation is deprotonated by His 67 to form the allene oxide.

Catalytic Residues Roles

UniProt PDB* (1u5u)
Asn137 Asn137A Asn 137 hydrogen bonds to His 67 to ensure favourable orientation of the histidine. Hydrogen bonding to the non-terminal peroxide oxygen of the substrate favours homolytic cleavage. electrostatic stabiliser
Thr66 Thr66A Thr 66 hydrogen bonds to His 67 to ensure favourable orientation of the His imidazole ring. electrostatic stabiliser
His67 His67A His 67 promotes homolyic cleavage by hydrogen bonding to the terminal peroxide oxygen. The position of the His imidazole ring with respect to the heme group is achieved by hydrogen bonds of the His residue to Thr 66 and Asn 137. His 67 also deprotonates the carbocation intermediate to form the allene oxide product. proton acceptor, electrostatic stabiliser, proton 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

intermediate formation, homolysis, radical formation, redox reaction, radical propagation, radical termination, proton transfer, overall product formed, native state of enzyme regenerated, inferred reaction step

References

  1. Oldham ML et al. (2005), Proc Natl Acad Sci U S A, 102, 297-302. The structure of coral allene oxide synthase reveals a catalase adapted for metabolism of a fatty acid hydroperoxide. DOI:10.1073/pnas.0406352102. PMID:15625113.
  2. De Luna P et al. (2013), J Phys Chem B, 117, 14635-14641. A molecular dynamics examination on mutation-induced catalase activity in coral allene oxide synthase. DOI:10.1021/jp408486n. PMID:24164352.
  3. Tosha T et al. (2006), J Biol Chem, 281, 12610-12617. On the Relationship of Coral Allene Oxide Synthase to Catalase: A SINGLE ACTIVE SITE MUTATION THAT INDUCES CATALASE ACTIVITY IN CORAL ALLENE OXIDE SYNTHASE. DOI:10.1074/jbc.m600061200. PMID:16513636.

Step 1. The Fe(III) ion causes homolysis of the hydroperoxide O-O bond and generates a free radical intermediate. His67 and Asn137 hydrogen bond to the substrate.

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Catalytic Residues Roles

Residue Roles
Asn137A electrostatic stabiliser
Thr66A electrostatic stabiliser
His67A electrostatic stabiliser
Tyr353A metal ligand

Chemical Components

intermediate formation, homolysis, radical formation, redox reaction

Step 2. The alkoxyl radical rearranges to form a carbon radical.

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Catalytic Residues Roles

Residue Roles
Thr66A electrostatic stabiliser
His67A electrostatic stabiliser
Asn137A electrostatic stabiliser
Tyr353A metal ligand

Chemical Components

radical propagation

Step 3. An electron is transferred to Fe(IV) centre, forming a carbocation and Fe(III).

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Catalytic Residues Roles

Residue Roles
Thr66A electrostatic stabiliser
His67A electrostatic stabiliser
Asn137A electrostatic stabiliser
Tyr353A metal ligand

Chemical Components

redox reaction, radical termination

Step 4. His67 deprotonates the carbocation intermediate to form the product.

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Catalytic Residues Roles

Residue Roles
Thr66A electrostatic stabiliser
Asn137A electrostatic stabiliser
Tyr353A metal ligand
His67A proton acceptor

Chemical Components

proton transfer, overall product formed

Step 5. The hydroxyl group accepts a proton from His67 and is released from the heme centre, restoring the active site.

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Catalytic Residues Roles

Residue Roles
Tyr353A metal ligand
Thr66A electrostatic stabiliser
Asn137A electrostatic stabiliser
His67A proton donor

Chemical Components

proton transfer, native state of enzyme regenerated, inferred reaction step
Download: Image, Marvin File

Step 1. The Fe(III) ion causes homolysis of the hydroperoxide O-O bond and generates a free radical intermediate. His67 and Asn137 hydrogen bond to the substrate.

Step 2. The alkoxyl radical rearranges to form a carbon radical.

Step 3. An electron is transferred to Fe(IV) centre, forming a carbocation and Fe(III).

Step 4. His67 deprotonates the carbocation intermediate to form the product.

Step 5. The hydroxyl group accepts a proton from His67 and is released from the heme centre, restoring the active site.

Products.

Contributors

Emma Penn, Gemma L. Holliday, Amelia Brasnett