User:Cmarchak/Enzyme: AOS-LOX

AOS-LOX Fusion Protein
AOS-LOX fusion protein is a catalase-like allene oxide synthase (AOS) and 8R-lipoxygenase (LOX) gene that fuse together to encode AOS-LOX. The AOS-LOX is the primary pathway to the arachidonate metabolism in corals and plants. Lipoxygenase (LOX) and allene oxide synthase (AOS) are key enzymes in the jasmonic acid (JA) biosynthesis pathway.

Enzyme Reaction Pathway & EC Numbers
AOS EC Number: 4.2.1.92 (Allene Oxide Synthase/ Hydroperoxide isomerase)

Reaction Catalyzed- (13S)-hydroperoxy-(9Z,11E,15Z)-octadecatrienoate <=> (9Z,13S,15Z)-12,13-epoxyoctadeca-9,11,15-trienoate + H2O

Comments:


 * Acts on a number of unsaturated fatty-acid hydroperoxides, forming the corresponding allene oxides.
 * The product of the above reaction is unstable and is acted upon by EC 5.3.99.6 to form the cyclopentenone derivative (15Z)-12-oxophyto- 10,15-dienoate (OPDA), which is the first cyclic and biologically active metabolite in the jasmonate biosynthesis pathway.

LOX EC Number- 1.13.11.60 (linoleate 8R-lipoxygenase)

Reaction Catalyzed- (9Z,12Z)-octadecadienoate + O2 ⇔ (8R,9Z,12Z)-8-hydroperoxyoctadeca-9,12-dienoate)

Comments:


 * The bifunctional enzyme from Aspergillus nidulans uses different heme-binding domains to catalyze two separate reactions.
 * Linoleic acid is oxidized within the N-terminal heme peroxidase domain to (8R,9Z,12Z)-8-hydroperoxyoctadeca-9,12-dienoate, which is subsequently isomerized by the C-terminal P450 heme thiolate domain to (5S,8R,9Z,12Z)-5,8-dihydroxyoctadeca-9,12-dienoate (cf. EC 5.4.4.5).
 * The bifunctional enzyme from Gaeumannomyces graminis also catalyzes the oxidation of linoleic acid to (8R,9Z,12Z)-8-hydroperoxyoctadeca- 9,12-dienoate, but its second domain isomerizes it to (7S,8S,9Z,12Z)- 5,8-dihydroxyoctadeca-9,12-dienoate (cf. EC 5.4.4.6).
 * Formerly EC 1.13.11.44

-Description-
-AOS catalyses the production of an unstable epoxide (an allene oxide) from the fatty acid hydroperoxide generated by the lipoxygenase activity.

-The coral AOS occurs as the N-terminal domain of a fusion protein with a C-terminal domain of 8R-LOX. Covalent fusion of the AOS and LOX should greatly enhance any mutual affinity of the two domains by increasing their effective concentrations, facilitating rapid transfer of the labile 8R-hydroperoxide from the LOX to the AOS active site.

-The AOS-LOX route is a rapid-onset stress response pathway in corals.

Organisms which AOS-LOX is Found
AOS-LOX enzyme is found in many species of plants and Corals and are key enzymes in the jasmonic acid (JA) biosynthesis pathway. Corals are the main model organisms for temperature-imposed declines in coral population and are being used in many studies especially relating to metabolic processes. The AOS-LOX is the primary pathway to the arachidonate metabolism in corals and is identified as an early stress response gene induced by mechanical injury in coral. This article will mainly examine soft coral known as C. imbricata as well as the jasmonic acid biosynthesis pathway in plants.

Lipoxygenase (LOX) and allene oxide synthase (AOS) are key enzymes in the jasmonic acid (JA) biosynthesis pathway found in plants. Jasmonic acid (JA) or otherwise referred to as jasmonates (JAs) are vital molecules in the regulation of many physiological processes in plant growth and development.

In plants, the conversion of α-linolenic acid by the LOX and allene oxide synthase (AOS) pathway results in 12-oxo-phytodienoic acid and jasmonic acid (JA), which regulate the expression of defense genes. The expression of LOX and AOS is regulated separately, whereas in coral the AOS-LOX fusion protein enforces strict stoichiometric coupling of both catalytic steps.

Covalent Catalysis in Plants
''Jasmonate biosynthetic pathway in plants. LOX, 13-lipoxygenase; AOS, allene oxide synthase; AOC, Allene oxide cyclase; OPR3, OPDA reductase 3; OPCL1, OPC-8:0 CoA ligase 1; 13-HPOT, 13-hydroperoxylinoleic acid; OPDA, (9S, 13S)-12-oxo-phytodienoic acid; OPC-8:0, 3-oxo-2(29-[Z]- pentenyl)cyclopentane-1-octanoic acid;''

Reference & Visual Representation Found Here- Shen, Q. (2014, March). ''Jasmonate biosynthetic pathway in plants. LOX, 13-lipoxygenase; AOS ...'' Retrieved October 25, 2022, from https://www.researchgate.net/figure/Jasmonate-biosynthetic-pathway-in-plants-LOX-13-lipoxygenase-AOS-allene-oxide_fig7_260915186

''The different branches in the LOX pathway. AOS, Allene oxide synthase; DES, divinyl ether synthase; a-DOX, a-dioxygenase; EAS, epoxyalcohol synthase; HPL, hydroperoxide lyase; POX, peroxygenase; LOX, lipoxygenase; PUFAs, polyunsaturated fatty acids.''

Reference & Visual Representation Found Here- Hause, B. (2002, February). ''The different branches in the LOX pathway. AOS, allene oxide synthase ...'' Retrieved October 25, 2022, from https://www.researchgate.net/figure/The-different-branches-in-the-LOX-pathway-AOS-Allene-oxide-synthase-DES-divinyl-ether_fig2_11183230

Known Active Sites
Sequencing of C.Imbricata AOX-LOX cDNA results in two complete AOS-LOX sequence designated as AOS-LOXa (NCBI ID: KF000373) and AOX-LOXb (NCBI ID: KF000374). Sequence analysis of coral AOS-LOX fusion proteins shows that all catalytically important amino acids of LOX domains are conserved. In the primary structure of the LOX domain of both AOS-LOXa and AOS-LOXb, the active sites were reported to be determinants of R and S stereospecificity of LOXs (glycine in R- and alanine in S-specific LOXs) Thus, the LOX domains of AOS-LOX fusion proteins convert the substrate fatty acid (AA) into 8R-HpETE.

-Function in Cell (Metabolism)

 * Lipid metabolism; arachidonate metabolism
 * Lipid metabolism; fatty acid metabolism.

The AOS-LOX pathway is central to the arachidonate metabolism in corals, its biological function in coral homeostasis is unclear

Lipoxygenase (LOX) pathway metabolis- Arachindonic acid is coverted to an 8R-hydroperoxide that transforms to an allene oxide (epoxide).

This pathway is very similar to the pathway of jasmonic acid biosynthesis in plants. Although in corals the pathway may lead to cyclid products such as clavulones- this connection remains unproven.

''Comparison of allene oxide biosynthesis in coral and plants. The conversion of allene oxide to clavulone remains unproven.''

References and Visual Representation Found Here- Oldham, M. L., Brash, A. R., & Newcomer, M. E. (2005). The structure of coral allene oxide synthase reveals a catalase adapted for metabolism of a fatty acid hydroperoxide. Proceedings of the National Academy of Sciences, 102(2), 297-302. doi:10.1073/pnas.0406352102

Crystal Structure
-The Crystal Structure is Unknown for AOS-LOX