Oxidative cyclase

Figure 8.6 A diagram of the oxylipin pathway. LOX lipoxygenase AOS allene oxide synthase HPL hydroperoxide lyase AOC allene oxide cyclase OPDA Reductase 12-oxo-phytodienoic acid-10,11-reductase.

ALLANTOICASE ALLANTOINASE ALLENE OXIDE CYCLASE ALLENE OXIDE SYNTHASE ALU IN LYASE... 

Nitric oxide Cyclase activation, smooth muscle relaxation Nitric oxide synthase... 

JAs are derived from linolenic acid via an octadecanoid pathway consisting of several enzymatic steps (Figure 36). Multiple compartments in plant cells participate in JA synthesis. The early steps of this pathway occur in chloroplasts, where linolenic acid is converted to OPDA by means of the three enzymes lipoxygenase (LOX), allene oxide synthase (AOS), and allene oxide cyclase (AOC).867-869 Linolenic acid is oxygenated by 13-LOX producing a peroxidized fatty acid 13-hydroperoxylinolenic acid. The product is subsequently metabolized by AOS to an unstable compound allene oxide. Allene oxide is sequentially converted by AOC to produce OPDA. An alternative pathway from another trienoic fatty acid, hexadecatrienoic acid (16 3), is present in chloroplasts.870 In this pathway, dinor OPDA is produced instead of OPDA. OPDA and dinor OPDA are transported into the peroxisome. An ABC transporter involved in this transport was identified in... 

Prostacyclin, PGEi, PGD2, adenosine, forskolin, coleonol endothelium derived relaxing 6ctor DRF, nitric oxide), cyclases nitroglycerine, nitropmsside, Sin-1, nitrosoglutathione. 

The most well known and well investigated degradation reaction of 13S-HPOTE is the generation of jasmonic acid. 13S-HPOTE is converted by allene oxide hydroperoxydehydrase by an intramolecular elimination of water to an allene oxide [160]. The latter is cyclized by allene oxide cyclase to 9S,13S,12-oxo-10,15-phytodienoic acid [160] followed by a trifold P-oxidation and a hydrogenation step (Scheme 7) [108]. 

Fig. 8. Metabolism of 18 3 fatty acid to oxylipins. I. Lipoxygenase 2, alleneoxide synthase 3, allene oxide cyclase 4, 12-oxo-phytodienoic acid reductase, -oxidation 5, hydroperoxide lyase.

When, however, 13(S)-HPOD was incubated with an ammonium sulphate precipitate of defatted com germ, the resulting 12-oxo-PDA was not optically pure but a mixture of enantiomers in a ratio of 82.18 [35]. The 12-oxo-PDA obtained by incubation of 13(S)-HPOT with flaxseed extracts was found to be a racemic mixture [36]. These results may be due to low or absent allene oxide cyclase activity in the preparations used [34]. The allene oxide cyclase, a novel enzyme in the metabolism of oxygenated fatty acids was partially characterised and found to be a soluble protein with an apparent molecular weight of about 45 kDa which catalysed specifically the conversion of allene oxide into 12-oxo-PDA [34]. The allene oxide was very unstable and was rapidly hydrolysed in aqueous media into stable 12-oxo-13-hydroxy-9(z),15(z)-octadecadienoic acid (a-ketols) [34]. 

The final step in the biosynthetic pathway to chlorophylls is the esterification of the chlorophyllides with phytyl diphosphate or geranylgeranyl diphosphate (followed by reduction of the three extra double bonds) via the enzyme chlorophyll synthetase. Once again, when zinc or cadmium replaces magnesium, esterification is unaffected. However, when nickel or copper are used, esterification is hindered. Presumably the labile coordinating power of the group Ila and Ilb metals, as opposed to the more inert coordination of transition metals, is involved in the function of the two enzymes, oxidative cyclase and chlorophyll synthetase. 

ALLENE OXIDE CYCLASE FROM CORN PARTIAL PURIFICATION AND CHARACTERIZATION... 

The last route is important for the formation of jasmonic acid. The action of allene oxide synthase on 13-hydroperoxy linolenic acid initially results in the generation of 12,13-epoxy-octadecatrienoic acid. This unstable epoxide is either chemically hydrolyzed to a- and y-ketols and racemic 12-oxo-phytodienoic acid or, in the presence of allene oxide cyclase (AOC), is further converted to enantiomeric pure 12-oxo-PDA [7]. The ring double bond of PDA is then reduced in a NADPH dependent reaction by 12-oxo-PDA reductase and after shortening of the side chain containing the carboxy group by 3 rounds of fi>-oxidation, the biosynthesis of jasmonic acid is completed. 

To understand the flow of metabolites through this pathway it is necessary to investigate the regulation of the participating enzymes. In this report we focus our interest on the partial purification and characterization of allene oxide cyclase. 

Hamberg, M. and Fahlstadius, P. Allene oxide cyclase a new enzyme in plant lipid metabolism. Arch. Biochem. Biophys. 276(2) (1990), 518-526... 

Allene Oxide Cyclase from Com Partial Purification and Characterization. 

Fig, 1 Schematic representation of the octadecanoid pathway leading to jasmonic acid biosynthesis. 12-OH-JA, 12-hydroxy-jasmonic acid AOC, allene oxide cyclase AOS, allene oxide synthase JA, jasmonic acid JARl, enzyme responsible for the conjugation of JA with isoleucine (JA-Ile) JMT, 5 -adenosyl-L-methionine jasmonic acid carboxyl methyl transferase LA, a-linolenic acid LOX, lipoxygenase MeJA, methyl jasmonate OPDA, 12-oxo-phytodienoic acid OPR3, OPDA reductaseS PL, phospholipase... 

Stenzel I, Hause B, Miersch O, Kurz T, Maucher H, Weichert H, Ziegler J, Feussner 1, Wastemack C (2003) Jasmonate biosynthesis and the allene oxide cyclase family of Arabidopsis thaliana. PlantMolBiol51 895-911... 

For the cyclization to occur, the a-region is essential for the enzyme cyclase. A second enzymatic control is involved in the /S-region for the double bond to orient properly on the incoming carbonium ion. The reaction is therefore believed to proceed nonstop until a tetracyclic system is obtained. The role of the squalene oxide cyclase is to maintain the carbon skeleton in place to maximize orbital overlap which allows generation of the ff-bonds in the sterol molecule. This is represented below ... 

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