Covello1 1 Plant Biotechnology Institute, Saskatoon, SK, Canada;2Department of Chemistry, Carleton University, Ottawa, Ontario, Canada The mechanism by which the fatty acid 1,4-desaturas
Trang 1Mechanism of 1,4-dehydrogenation catalyzed by a fatty acid
Darwin W Reed1, Christopher K Savile2, Xiao Qiu1, Peter H Buist2and Patrick S Covello1
1
Plant Biotechnology Institute, Saskatoon, SK, Canada;2Department of Chemistry, Carleton University, Ottawa, Ontario, Canada
The mechanism by which the fatty acid (1,4)-desaturase of
Calendula officinalisproduces calendic acid from linoleic acid
has been probed through the use of kinetic isotope effect
(KIE) measurements This was accomplished by incubating
appropriate mixtures of linoleate and regiospecifically
dideuterated isotopomers with a strain of Saccharomyces
cerevisiaeexpressing a functional (1,4)-desaturase GC-MS
analysis of methyl calendate obtained in these experiments
showed that the oxidation of linoleate occurs in two discrete
steps since the cleavage of the C11-H bond is very sensitive to
isotopic substitution (kH/kD¼ 5.7 ± 1.0) while no isotope effect (kH/kD¼ 1.0 ± 0.1) was observed for the C8-H bond breaking step These data indicate that calendic acid is pro-duced via initial H-atom abstraction at C11 of a linoleoyl substrate and supports the hypothesis that this transforma-tion represents a regiochemical variatransforma-tion of the more com-mon C12-initiated D12desaturation process
Keywords: desaturase; kinetic isotope effect; conjugated fatty acid; deuterium labelling; Calendula
The D12-oleate desaturase (FAD2) family of enzymes are
membrane-bound nonheme iron-containing proteins that
carry out a fascinating array of related oxidative
transfor-mations [1,2] The prototypical reaction features the
intro-duction of a cis-double bond at the 12,13-position of an
oleoyl substrate – a ubiquitous biosynthetic reaction of
higher plants [3,4] (Fig 1A) Species-specific mechanistic
variations of this process include 12-hydroxylation of oleate
(Ricinus communis) [5] and 12,13-epoxidation (Crepis
palaestina) [2] or 12,13-acetylenation (Crepis alpina) [2] of
linoleate Recently, it has been shown that the production of
conjugated trienoic acids, such as calendic acid from a
linoleate precursor are also carried out by FAD2 variants
[6–8](Fig 1B) The latter reaction is particularly noteworthy
given the current interest in conjugated fatty acids with
respect to their role in human nutrition [9] as well as
commercial applications [10]
As part of ongoing research into the structure–function
relationships of FAD2 type enzymes, a closer examination
of calendate formation is clearly warranted Early labelling
experiments using marigold seed homogenates and labelled
linoleate precursors demonstrated that calendic acid is
produced by an apparent (1,4)-dehydrogenation process whereby a linoleoyl substrate loses one hydrogen from C8 and C11, respectively [11] No oxygenated intermediates were detected These results as well as related substrate specificity data [6] point to a mechanism which is analogous
to that proposed for the more common (1,2)-dehydrogen-ation reactions of fatty acid desaturases (Fig 2) The mechanistic model [12] for the latter process features an initial, energetically difficult hydrogen abstraction step, which generates a very short-lived, carbon-centered radical intermediate, or its iron-bound equivalent (not shown) This species collapses rapidly to give an unsaturated product by what is formally a second hydrogen abstraction, although a one electron oxidation/proton removal sequence cannot be rigorously excluded at this time The stepwise nature of this transformation is supported by kinetic isotope effect (KIE) studies of several membrane-bound fatty acid desaturases
In all cases examined, one C-H cleavage was found to be subject to a large primary deuterium kinetic isotope effect while the second C-H bond rupture was insensitive to isotopic substitution [12–22] This pattern of KIEs is precisely what one would expect for a disproportionation mechanism [23] of the type showed in Fig 2 and the data were used to pinpoint the site of the initial oxidative attack (cryptoregiochemistry) for these systems In several cases, additional independent evidence is available to support the cryptoregiochemical assignments That is, the location of the putative diiron oxidant relative to substrate can be ascertained by inducing the desaturase to behave as a regioselective oxygenase through modifications of substrate
or enzyme [19,20,24–26]
The availability of a convenient yeast expression system for Fac2 – a Calendula officinalis gene encoding the (1,4)-desaturase involved in calendic acid production [6,8] offered
a unique opportunity to study the mechanism of this reaction using KIE methodology Specifically, we wished to correlate the site of initial oxidation for this process with that determined for D12-desaturation [13] We report here, the results of our collaborative investigation
Correspondence to P H Buist, Department of Chemistry,
Carleton University, 1125 Colonel By Drive, Ottawa, Ontario,
Canada, K1S 5B6.
Fax: + 1 613 5203749 or 3830, Tel.: + 1 613 5202600 Ext 3643,
E-mail: pbuist@ccs.carleton.ca
or P S Covello, National Research Council, Plant Biotechnology
Institute, 110 Gymnasium Place, Saskatoon, SK, Canada S7N 0W9.
Fax: + 1 306 9754839, Tel.: + 1 306 9755269;
E-mail: Patrick.Covello@nrc.ca
Abbreviations: KIE, kinetic isotope effect.
Definition: The term (1,4)-desaturase denotes an enzyme that converts
an isolated carbon-carbon double bond in a fatty acid into two
conjugated double bonds by what is formally a 1,4-dehydrogenation
reaction Such enzymes have also been termed conjugases [7].
(Received 23 July 2002, accepted 28 August 2002)
Trang 2E X P E R I M E N T A L P R O C E D U R E S
Materials
Methyl linoleate (> 99%) was purchased from
Nu-Chek-Prep, Inc The two regiospecifically dideuterated methyl
linoleates ([8,82H2]-1,11,11-2H2]-1) required for the KIE
study were prepared by routes which were very similar to
those reported previously for the synthesis of the
corres-ponding chiral monodeutero analogues [27] Thus, the
tosylate of 8-hydroxy-[8,82H2]octanoic acid was reacted
with lithium acetylide-ethylenediamine complex to give
[8,82H2]dec-9-ynoic acid which was in turn coupled with the
tosylate of 2-octyn-1-ol to give [8,82H2]-1 after
semihydro-genation and methyl esterification of the intermediate diyne
In a similar manner, dec-9-ynoic acid was C-alkylated with
the tosylate of [1,1-2H2]-2-octyn-1-ol to give [11,11-2H2]-1
after semihydrogenation and methyl esterification The
overall yields of [8,82H2]-1 and [11,11-2H2]-1 obtained via
these procedures was 5% and 14%, respectively
Purifica-tion of substrates was carried out by flash chromatography
(silica gel, 0.5% v/v ethyl acetate/hexane) and HPLC
fractionation as previously described [16] GC-MS analysis
[16] of the final deuterated products revealed that each
isotopomer consisted essentially entirely of dideuterated
species (m/z 296; 294 for nondeuterated analogue).1H and
13C NMR analysis confirmed the position of the two
deuterium atoms for each isotopomer as indicated by the
presence/absence of the diagnostic bisallylic signals at d
2.77 p.p.m (1H) and 25.67 p.p.m (13C) for [11,11-2H2]-1,
respectively, and an approximately two-fold attenuation of
the overlapping allylic signals (C-8,C-13) at d 2.02 p.p.m
(1H) and 27.25 p.p.m (13C) for [8,82H2]-1
Incubation experiments
For characterization of the Calendula fatty acid conjugase,
the yeast strain DTY10-a2 (MATa, fas2D::LEU2,
can1-100, ura3-1, ade2-1, his3-11, his3-15) [28] was transformed
with a plasmid (pYJ) comprised of the pYES2 vector
Tergitol solution and 2· 10 mL H2O prior to lipid extraction
Analytical procedures For fatty acid analysis, yeast pellets were saponified by adding 2 mL 10% KOH/methanol and heating at 80C for
2 h The mixture was then cooled and pre-extracted with
2· 2 mL hexane to remove nonsaponifiable lipids The reaction mixture was then neutralized with 50% acetic acid
to pH 5 and the fatty acids were extracted with 2 · 2 mL hexane The hexane was removed under a nitrogen stream and the mixture, including the conjugated fatty acids, was esterified with 2 mL 1% H2SO4in methanol at 50C for 1
h (This methylation method has been found to be most suitable for conjugated fatty acid ester analysis; W Christie, Mylnefield Research Services Ltd., Dundee, Scotland, personal communication.) The cooled mixture was extrac-ted with 2· 2 mL hexane The pooled hexane was washed with 2 mL H2O and concentrated under N2 for HPLC purification, GC or GC-MS analysis
GC-MS analysis of yeast lipids was performed using a Fisons VG TRIO 2000 mass spectrometer (VG Analytical, UK) controlled by Masslynx version 2.0 software, coupled
to a GC 8000 Series gas chromatograph as previously described [16] except that a narrow EI+scan range of 285–
305 m/z was used A representative mass spectrum of biosynthetic methyl calendate is shown in Fig 3
R E S U L T S A N D D I S C U S S I O N
Our methodology for determining the intermolecular primary deuterium KIE on each C-H cleavage step in fatty acid desaturation reactions involves GC-MS analysis of olefinic products derived from direct competition between nondeuterated substrate and the appropriate regiospecifi-cally dideuterated (-C2H2-) fatty acid As has been pointed out on a previous occasion [13], the magnitude of the primary deuterium KIE determined in this manner must be regarded as an estimate since the observed value may incorporate a small (< 10%) a-secondary isotope effect [40] In addition, partial masking of the intrinsic KIE by
headgroup.
Fig 2 Generic mechanistic scheme showing
the stepwise removal of hydrogens in fatty acid
(1,2)-desaturation Structure of the putative
diiron oxidizing species is speculative.
Trang 3other enzymic steps in the catalytic cycle such as substrate
binding may also be occurring [35] None of these
considerations affect the conclusions reached in this paper
The use of a competitive rather than a noncompetitive
experimental design has allowed KIE determinations to be
carried out for both in vitro and in vivo desaturase systems
The results have correlated well with KIE data obtained by
other methods [30–34] Our methodology dictates that
interference by endogenous d0-substrate, if present, must be
eliminated: this has been accomplished previously through
the use of unnatural chain-shortened substrates or
ana-logues bearing a remote thia- or deuterium mass label
Such measures proved unnecessary in the case of the
linoleate-calendate reaction since the host yeast system used
for this purpose does not biosynthesize the relevant
substrate
Optimal incubation conditions for our KIE studies were
set up in a preliminary experiment: methyl linoleate 1
(100 mgÆL)1) was administered to cultures (50 mL) of the
pYJ/DTY10a2 strain of S cerevisiae incubated at 20C for
3 days to permit relatively rapid growth and then at 15C
for a further 3 days to reach saturation at a temperature
which has been found to give better substrate conversion
rates The cells were harvested by centrifugation and the
lipids were isolated via a hydrolysis/methylation sequence
known to be suitable for conjugated fatty acid esters (10%
w/v KOH/CH3OH, 80C, 2 h; neutralization to pH 5
with acetic acid, hexane extraction, 1% w/v H2SO4/
CH3OH, 50C, 1 h) Analysis of the fatty acids as methyl
esters by GC-MS revealed that exogenous linoleate had
been converted to calendate to the extent of 1% of total
cellular fatty acids, a result similar to that observed
previously [6,8] Control experiments previously indicated
that the production of 2 was dependent on expression of the
FAC2 enzyme
The two regiospecifically dideuterated linoleates
([8,8-2H2]-1, [11,11-2H2]-1) required for the KIE study
(Fig 4) were prepared via established synthetic routes (See
Experimental section) A mixture of each deuterated
material with its nondeuterated parent (1 mg) was
admini-stered to growing cultures (10 mL) of the S cerevisiae
transformant (pYJ/DTY10-a2) using conditions identical to
that of the trial experiment The deuterium content of the
olefinic fatty acid methyl esters in the cellular lipid extract
was assessed by GC-MS as described in the Experimental section The d2/d0ratio of the linoleate isotopomers found
in the cells was essentially identical to that of the starting material in both incubations, as is required for these types of competitive KIE measurements [35] No loss of label due to reversible exchange of deuterated linoleate at C-8 or C-11 could be detected Mass spectral analysis of the calendate fraction revealed that in both incubations, this material consisted entirely of a d0/d1mixture indicating a loss of one deuterium from the d2-substrate as expected Product kinetic isotope effects (kH/kD) were calculated using the ratio: [% d0(product)/% d1(product)]/[% d0(substrate)/%
d2(substrate)] and this analysis indicated the presence of a large primary deuterium isotope effect (5.7 ± 1.0, average
of four experiments) for the carbon-hydrogen bond clea-vage at C11 while the C8-H bond breaking step was shown
to be essentially insensitive to deuterium substitution (KIE¼ 1.0 ± 0.1, average of four experiments) (Table 1) According to our mechanism (Fig 2), these results demon-strate that calendate production is initiated by an energeti-cally difficult and hence isotopienergeti-cally sensitive hydrogen abstraction at C11 and completed by a second facile and kinetically unimportant hydrogen abstraction at C8 The fast formation of an allylic radical at C8 followed by rate-determining hydrogen abstracton at C11 cannot be rigor-ously excluded but seems far less likely given the intrinsically high energy content of radical intermediates relative to product
Some decades ago, Morris and Marshall [36] speculated that conjugated trienoic fatty acids are produced in plants from linoleic acid via an allylic radical intermediate Crombie and coworkers [11] provided evidence that calen-dic acid was indeed biosynthesized from linoleic acid via removal of hydrogens at C8 and C11 More recently, it was suggested that C8 might be the site of initial oxidation for this process based on a comparison with the putative site of initial attack catalyzed by a soluble plant D9desaturase [37] However our results clearly demonstrate that calendate production is in fact initiated at C11 as might be expected for a process which is catalyzed by a homolog of FAD2 – an enzyme which initiates the conversion of oleate to linoleate
at C12 [12] Thus, the switch between 1,2 and 1,4-dehydrogenation could conceivably be controlled by a fairly small change in oxidant position relative to substrates which both adopt a conformation allowing syn removal of
Fig 4 Isotopomers of 1 used to probe the kinetic isotope effects on the fatty acid (1,4)-desaturase reaction involved in calendate biosynthesis.
Fig 3 Mass spectrum of biosynthetic methyl calendate Arrow
indi-cates the molecular ion cluster used to calculate the isotopic content of
deuterated samples.
Trang 4two proximal hydrogens (H-H distance in both cases
2.5 A˚) This model (Fig 5) can be tested by determining
the stereochemistry of H-removal for calendate formation
using chiral monodeutero probes [27] and comparing this
result with the known pro-R enantioselectivity at C12,13
observed for D12-desaturation [38]
Further evidence for the close relationship between 1,2
and 1,4-dehydrogenation has been obtained recently for a
Spodoptera littoralis desaturating system which converts
11(Z)- tetradecenoate to 10(E),12(E)-tetradecadienoate by
initial H-abstraction at C10 and 11(E)-tetradecenoate
to 9(Z),11(E)-tetradecadienoate by initial oxidative attack
at C9 [39] Whether these two transformations are
catalyzed by separate enzymes in this case remains to be
determined
In summary, the cryptic site of initial oxidation for an
important plant fatty acid conjugase-mediated reaction has
been determined to be at the carbon furthest from C-1 In
contrast, all other desaturase-catalyzed oxidations studied
to date are initiated at the carbon closest to the acyl
headgroup Thus it would be interesting to apply our KIE
methodology to the study of related FAD2-like enzymes [7]
involved in the formation of a-eleostearic acid
[9(Z),11(E),13(E)-octadecatrienoic acid] and a-parinaric
acid [9(Z),11(E),13(E),15(Z)-octadecatetraenoic acid] from linoleic acid In so doing, we would hope to correlate the various regioselectivities observed for this important set of catalysts with the geometric relationship between oxidant and substrate
Note added in proof: Recent site-directed mutagenesis experiments using a D12desaturase/hydroxylase system have validated the mechanistic paradigm underlying our crypto-regiochemical determinations [41]
A C K N O W L E D G E M E N T S
We wish to thank the National Science and Engineering Research Council (NSERC) for financial support of the synthetic work performed at Carleton University (PHB), Steve Ambrose for perform-ing the GC-MS analysis, Charles Martin for providperform-ing the yeast strain DTY-10a2 and Michele Loewen and Robert Sasata for reviewing the manuscript.
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Fig 5 Mechanistic model showing the
rela-tionship between oxidant position in D12
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(1.03 ± 0.11) c
a The isotopic ratio of each species is given as an average value based on four GC-MS runs d 0 , d 1 , d 2 refer to undeuterated, monodeuterated and dideuterated species, respectively.bThe isotopic ratios of the substrates were determined by mass spectral examination of the linoleate isotopomers isolated from the cellular pool These values are similar (within experimental error) to those obtained for the two starting mixtures prior to incubation: 1.02 (d 0 : 11d 2 ); 1.92 (d 0 : 8d 2 ) c The average KIE (four incubations) ± standard deviation.
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