Food Lipids: Chemistry, Nutrition, and Biotechnology

isolated from spinach (Spinacia oleracea) and safflower (Carthamus tinctorius), has
broad specificity toward 4- to 16-carbon acyl-ACP substrates, with maximal activity
(evaluated in the reverse reaction) toward the 8-carbon acyl-ACP derivative
(45,66,90). In comparison, there is some controversy regarding whether one or three
isoforms of HAD (gene not yet identified) exist in the E. coli FAS system (11). In
any event, HAD is not to be confused with 3-hydroxydecanoyl–ACP dehydrase
(FabA) in E. coli; this latter enzyme medicates the specific step of isomerizing some
of the transient 2-trans decanoyl–ACP product into the cis-3 derivative, providing
for unsaturated fatty acid synthesis (vaccenic acid, 18:1�11) inE. coli and related
organisms under anaerobic conditions (11) [plant and animal systems require oxygen
for fatty acid desaturation (10)].
To complete the reduction cycle, the 2-trans-enoyl–ACP reductose (EAR)
uses NAD(P)H to yield the saturated acyl-ACP. Two dominant EAR isoforms exist,
although four isoforms have been reported [65]. In contrast, E. coli has only one
form: FabI (91). The major EAR isoform, type I, is a soluble homotetramer, appears
to be present in leaf tissue and avocado fruit mesocarp, utilizes NADH, and has
broad specificity toward acyl-ACP derivatives of 4 to 16 carbons, with greatest
activity toward 2-hexenoyl– and 2-octenoyl–ACP derivatives (45,46,92,93). Type
II EAR may be membrane-associated (65), prefers NADPH as a source of reducing
power (but also uses NADPH), and exhibits a preference for enoyl-ACP of 10 or
more acyl carbons (93). Both types have been isolated from castor (Ricinus communis)
bean and safflower and rape (Brassica napus) seeds, hence appear to be
generally present in seeds (45,66,90).
5. Elongation/Condensation
Following the reduction cycle, the saturated acyl-ACP is now ready to undergo
condensation cycles initiated by KAS I and KAS II (Fig. 1). KAS I is primarily
responsible for chain lengthening of 4- to 14-carbon acyl-ACP substrates, whereas
KAS II is active only on 14- to 16-carbon acyl-ACP derivatives (66,73). KAS I
and II isoforms have been identified in barley (Hordeum vulgare) chloroplasts, and
each can exist as a homo- or heterodimer (94a). In E. coli, KAS I (FabB) is a
homodimer, and KAS II (FabF) is the condensing enzyme for (un)saturated fatty
acid lengthening from 16 to 18 acyl carbons (11). Each step initiated by KAS I or
KAS II is followed by the reduction cycle (KAR-, HAD-, and EAR-mediated steps)
to yield the corresponding saturated acyl-ACP derivative.
Recent suggestions of the presence of KAS IV isoforms in plant species that
may mediate medium chain fatty acid elongation are founded on the preparation of
cDNA clones from Cuphea wrightii (Kas A1), Cuphea hookeriana (Ch Kas 4), and
Cuphea pulcherrima (Cpu Kas4) species (94b–d). These species also contain medium
chain TE and accumulate medium chain fatty acids in seed oils. The deduced
sequences of these cDNA clones are 98% identical to each other, about 60% identical
to KAS I, and 75–85% identical to KAS II. Expression of these KAS isoforms in
Brassica species leads to more than fourfold enhanced rates of extension of 6:0- and
8:0-ACP over nontransformed plants in cerulenin-inhibited seed extracts. Furthermore,
coexpression of Kas4 and the gene encoding for the medium chain TE in
Brassica plants gives rise to medium fatty acids in seed oils whereas none were
produced from wild-type plants. Although the KAS IV has not yet been isolated,
current thinking is that it is a condensing/elongation enzyme specific for medium
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