Molecular mechanism of enzymatic
Eckhard Hofmann a,*,a Biophysics, Department of Biology and Biotechnology, Ruhr-Univers
b Plant Physiology, Department of Biology and Biotechnology, Ruhr-Univ
and are widespread throughout a variety of different plant is the involvement of JA in wound response and pathogenesis
A1 DAD1 . The subsequent oxygenation of a-LA at theC-13 position is catalyzed by 13-lipoxygenase . The resulting13-hydroperoxide, 13(S )-hydroperoxy-9(Z ),11(E ),15(Z )-octadecatrienoic acid (13-HPOT), is further dehydrated with
noic acid reductase; 13(S )-HPOT, 13(S )-hydroperoxy-9(Z ),11(E ),15(Z )-octa-decatrienoic acid; 13-LOX, 13-lipoxygenase.
* Corresponding author. Tel.: 49 234 32 24463; fax: 49 234 32 14238.E-mail address: email@example.com (E.
Available online at www.sciencedirect.com
Plant Physiology and Biochemisphyla . Although the jasmonic acid methyl ester (MeJA)was demonstrated to be a constituent of the essential oil of
[8,18,37]. Additionally, jasmonates play a crucial role in re-production , metabolic regulation , and as a signaltransducer in mechanotransduction [31,38,39]. JA is also re-quired for protection from ozone damage [26,27], and hasa pivotal role in the production of protective secondary metab-olites in cell cultures of Eschscholtzia californica [2,3].
The pathway of jasmonic acid biosynthesis is shown inFig. 1. Jasmonic acid and its octadecanoid precursors are syn-thesized from a-linolenic acid (a-LA) which is found in greatextent in plastidial membranes. From there, a-LA is suggestedto be released by the action of lipases, e.g. the phospholipase
Abbreviations:ACS, acyl-CoA synthase; AOC, allene oxide cyclase; 12,13-EOT, 12,13(S )-epoxy-9(Z ),11,15(Z )-octadecatrienoic acid; 12,13-EOD,
12,13(S )-epoxy-9(Z ),11-octadecatrienoic acid; AOS, allene oxide synthase;
CESG, Center for Eucaryotic Structural Genomics; CTS/PXA1, ABC trans-
porter for OPDA or OPDA-CoA import; HPOD, 13(S )-hydroperoxy-9(Z ),11(E )-octadecadienoic acid; JA, jasmonic acid; MeJA, jasmonic acid
methylester; OPC-8:0, 3-oxo-2(20(Z )-pentenyl)-cyclopentane-1-octanoic acid;LA,a-linolenic acid;OPDA, 12-oxo-phytodienoic acid;OPR, 12-oxo-phytodie-identified signal molecules with phytohormone properties,
like functions, the jasmonates are among the most recently [4,33]. To date, jasmonic acid and its derivatives are associated
with diverse physiological functions. The most prominent onefertility, and a multitude of developmental processes. In the course of JA biosynthesis, the first oxylipin with signal character, cis-()-12-oxo-phytodienoic acid (OPDA), is produced in a cyclization reaction catalyzed by allene oxide cyclase (AOC). This enzyme-catalyzed ring closure isof particular importance, as it warrants the enantiomeric structure at the cyclopentenone ring which in the end results in the only bioactive JAenantiomer, cis-()-JA. In this review, we focus on the structural and molecular mechanisms underlying the above mentioned cyclization re-action. In this context, we will discuss the crystal structure of AOC2 of Arabidopsis thaliana with respect to putative binding sites of the instablesubstrate, 12,13-epoxy-9(Z ),11,15(Z )-octadecatrienoic acid (12,13-EOT), as well as possible intermolecular rearrangements during the cycliza-tion reaction. 2007 Elsevier Masson SAS. All rights reserved.
Keywords: Allene oxide cyclase; Allene oxide synthase; Jasmonate; 12-Oxo-phytodienoic acid; Oxylipins; X-ray structure
Besides brassinosteroids and oligopeptides with hormone-
Jasminum grandiflorum in the early 1960s , it took nearlyanother twenty years until the first physiological effects ofMeJA and the occurrence of the free acid were describedReceived 14
Jasmonates, a collective term combining both jasmonic acid (JA) aThey are characterized as lipid-derived signal molecules which mediate0981-9428/$ - see front matter 2007 Elsevier Masson SAS. All rights reserved.doi:10.1016/j.plaphy.2007.12.007w
allene oxide cyclization in plants
Stephan Pollmann b
ity Bochum, Universitaetsstrasse 150, D-44801 Bochum, Germanyersity Bochum, Universitaetsstrasse 150, D-44801 Bochum, Germany
related derivatives, are ubiquitously distributed in the plant kingdom.lethora of physiological functions, in particular stress responses, male
try 46 (2008) 302e308www.elsevier.com/locate/plaphy
Fig. 1. Pathway of jasmonic acid biosynthesis in plants. Intermediates are ab-
breviated as: 13-HPOT, 13(S )-hydroperoxy-9(Z ),11(E ),15(Z )-octadecatrie-
noic acid; 12,13-EOT, 12,13(S )-epoxy-9(Z ),11,15(Z )-octadecatrienoic acid;OPDA, cis-()-12-oxo-phytodienoic acid; OPDA-CoA, cis-()-12-oxo-phyto-dienoic acid-coenzyme A; OPC8:0, 3-oxo-2(20(Z )-pentenyl)-cyclopentane-1-octanoic acid. The enzymes are indicated as: LIP, lipase; LOX, lipoxygenase;
AOS, allene oxide synthase; AOC, allene oxide cyclase; OPR, oxo-phytodie-
noic acid reductase; CTS/PXA1, comatose, ABC transporter for OPDA or
OPDA-CoA import; ACS, acyl-CoA synthase.
E. Hofmann, S. Pollmann / Plant Physiologythe help of allene oxide synthase [29,30], providing the unstableintermediate, 12,13-epoxy-9(Z ),11,15(Z )-octadecatrienoicacid (12,13-EOT). Allene oxide cyclase [14,32] catalyzes thereaction within the octadecanoid pathway which guaranteesenantiomeric specificity, by converting 12,13-EOT to 12-oxo-10,15(Z )-phytodienoic acid (OPDA). OPDA is then trans-ferred from the chloroplast to the peroxisome where it is furthermetabolized by reduction of the D10-double bond catalyzedby oxo-phytodienoic acid reductase , yielding 3-oxo-2(20(Z )-pentenyl)-cyclopentane-1-octanoic acid (OPC-8:0). Due to radiotracer experiments , it is generally agreedthat OPC-8:0 undergoes three consecutive cycles of b-oxidationwhich results in the production of bioactive JA with (3R,7S )-configuration, i.e. ()-7-iso-JA.
Allene oxide cyclase (AOC) has been described for the firsttime from Zea mays [14,41], followed by the cloning of thecorresponding genes from tomato , Arabidopsis , andbarley . While in tomato AOC is encoded as a singlegene, in A. thaliana four isogenes can be found, which mostlikely evolved from one ancestral isoform by gene duplicationevents. With respect to functional differences of the four isoen-zymes, it has been shown that especially AOC2mRNA accumu-lates in the case of local as well as systemic wound response,whereas AOC1 mRNA seems to be preferentially transcribedin systemic wound response. By reason that allene oxide syn-thase (AOS) transcription is also systemically induced afterwounding [20,21], a specific interaction of AOS and AOC1might be supposable in systemically responding leaves. Furtherevidence for functional differences of the AOCs is emphasizedby the occurrence of dinor-oxo-phytodienoic acid (dnOPDA)which is synthesized from hexadecatrienoic acid. Possibly,in this context, the isoenzymes possess diverse substratespecificities . Unfortunately, investigations of either en-zyme kinetic or substrate specificity of the AOCs have beenhampered by the instability of their substrate. So far, all activityassays utilized a coupled test system, determining the combinedactivity of both AOS and AOC.
The AOCs from Arabidopsis contain a predicted plastidialtarget sequence which facilitates the import of the enzymesinto the chloroplast. Functional import of AOC into the chloro-plast, investigated by immunocytochemical means, has alreadybeen described . However, differentiation between the indi-vidual isoforms was not possible, suggesting that a more de-tailed examination of the import of the single isoforms ofAOC is needed. Intriguingly, the expression of ArabidopsisAOCs has been shown for all plant organs, including roots. This finding is in contrast to that obtained from tomatowhere AOC expression is described to be restricted to floralorgans and vascular bundles . However, there are stillmany open questions which mark challenges for future work.One of the most urgent topics, after unraveling the molecularmechanism of AOC catalyzed 12,13-EOT cyclization, is theelucidation of the functional interconnection of AOS andAOC. Although a covalent interaction of AOS and AOC hasbeen described as unnecessary , the close vicinity of the
303and Biochemistry 46 (2008) 302e308two proteins seems to enhance their combined activity(P. Zerbe, personal communication).