Food Lipids: Chemistry, Nutrition, and Biotechnology

The three KAS isoforms are assigned on basis of sensitivity to cerulenin and
thiolactomycin (Fig. 2) (44,56,82–84), acyl-ACP chain length specificities (43,
44,66,73), and status as specific gene products of E. coli (11). KAS I is very sensitive
(100% inhibition to 10 �M) to cerulenin and is the product of fabB; KAS II is
moderately sensitive (50% inhibition at 50 �M) to cerulenin and is the product of
fabF; KAS III is insensitive to cerulenin, but sensitive to thiolactomycin and is the
product of fabH (85). Complementary DNA (cDNA) of plant KAS III is 45% identical
to E. coli KAS III (FabH) and has little homology with other plant KAS
isoforms (86). The three KAS isoforms have a functional cysteine at the active site,
consistent with their common mechanism (85) in forming an S-acyl intermediate
(and then thio exchange of the condensation product with ACP-SH to allow subsequent
reactions: Sec. III.B.4) and the expected mode of action of these inhibitors
(83,85,87). The differential inhibitor sensitivities have been attributed to different
active site homologies and topographies, which in turn modulate the effectiveness
of these �SH-directed inhibitors (87). KAS I acts on 4- to 14-carbon acyl-ACP
substrates, KAS II acts only on 14- to 16-carbon acyl-ACP substrates, and both use
malonyl-ACP as the condensing unit (13,36,87). KAS III principally forms acetoacetyl-ACP
from malonyl-ACP and acetyl-CoA, but in isolated FAS systems where
KAS I and II are both inhibited by cerulenin, acyl-ACP products of only up to four
to six carbons are formed (44,87).
A KAS IV isoform in E. coli (assigned to fabJ) was inferred from genetic and
cerulenin sensitivity studies (57). This isoform was also projected to be in plants to
account for the observed accumulation of 8- to 10-carbon acyl-ACP in cerulenininhibited
FAS systems. However, a subsequent report suggests that KAS IV (presumptively
FabJ) was incorrectly assigned, and is the fabF gene product, or KAS II
(88). More recent reports suggesting the presence of a KAS IV isoform in selected
Cuphea species of plants that accumulate medium chain length fatty acids are addressed
in Sec. B.5, ‘‘Elongation/Condensation.’’
4. Reduction Cycle
The next several reactions serve to reduce the 3-ketoacyl-ACP residue to a saturated
acyl-ACP derivative and prepare the acyl chain for a repetitive cycle(s) of elongation/condensation
reactions (Fig. 1). The 3-ketoacyl–ACP reductase (KAR) uses
NAD(P)H to mediate the first reduction step. Two KAR isoforms exist in plants
(45,46,89); the dominant form uses NADPH for reducing equivalents, whereas the
other isoform uses NADH. In contrast, the FAS type II system in E. coli has only
the NADPH-specific KAR (FabG) (11).
The KAR reduction step is followed by 3-hydroxyacyl–ACP dehydrase
(HAD) action involving NAD(P)H to yield the 2-trans-enoyl–ACP derivative. HAD,
Figure 2 Inhibitors used to differentiate between 3-ketoacyl–ACP synthetase isoforms, I,
II, and III.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.