Food Lipids: Chemistry, Nutrition, and Biotechnology

plants (79). This observation is similar to those that form the basis of speculation
that a KAS IV isoform exists in plants.
2. Flux and Feedback Control
Since KAS III causes the initial condensation step in fatty acid biosynthesis, some
features of the control of this enzyme have been evaluated. ACP and acyl-ACP
levels have impact on KAS III activity in vitro. Feedback inhibition of acyl-ACP,
especially 10:0-ACP, ofKAS III occurs in Cuphea lanceolata, a species that accumulates
decanoylglycerides (59); KAS III from rape and spinach behaves similarly.
Feedback inhibition of KAS III also occurs in E. coli, with increased inhibition
with increasing acyl chain length, leading to an accumulation of malonyl-ACP (123);
however, free ACP and fatty acids are not inhibitory.
Low levels of ACP inhibit KAS III in C. lanceolata (124), indicating a need
for FAS to ‘‘budget’’ available ACP between initiation reactions and ongoing chain
lengthening. Relatedly, overexpression of E. coli KAS III in E. coli and rape results
in an increase in 14:0 and corresponding decrease in 18:1�9 (120). This may result
from a shift toward overcommitment of ACP to initiating chain synthesis and greater
opportunity for premature chain termination by TE action. In accord with this interpretation,
overexpressed TE leads to uncontrolled fatty acid biosynthesis in E. coli
(125), where elevated steady-state ACP levels would be expected. Overexpression
of plant medium chain TE in E. coli accelerates fatty acid biosynthesis with attendant
increases in steady-state malonyl-ACP and fatty acid levels, mediated by rapid removal
of long chain acyl-ACP (119). A cDNA clone encoding for KAS III from
Cuphea wrightii should allow for further evaluation of FAS control in rapidly developing
embryos of this organism (126).
Despite a recent assertion (57) and subsequent rebuttal (88) regarding the presence
of a KAS IV in E. coli, several studies have led to the speculation that a KAS
IV does exist in plants (and thus, likely in E. coli) (56,59,124). This postulation is
based on the formation of acyl-ACP products having 6–10 acyl carbons in cerulenininhibited
preparations (where only KAS III, and not KAS I/II, can act). Since KAS
III specificity prohibits formation of acyl chain lengths beyond 4:0, another KAS
isoform is suggested to account for the observed condensation steps beyond butyrate
in cerulenin-inhibited FAS systems. In fact, recent reports (discussed in Sec. B.5)
have suggested the presence of KAS IV in Cuphea spp. (94b–d). KAS IV appears
to have a regulatory role in fatty acid biosynthesis in that transformed plants expressing
KAS IV and medium chain TE accumulate medium chain length fatty acids
in oils to a greater extent than seed oil plants expressing this TE alone. This provides
evidence for a ‘‘flux-forward’’ regulatory aspect of KAS IV in that it could enhance
steady-state levels of medium chain ACP species and provide greater opportunity
for medium chain TE action to terminate elongation and allow accumulation of
medium chain fatty acids in the oils.
Although ACCase in plants is not subject to the same mechanisms of control
as in animal FAS systems, it may be subject to feedback inhibition (13).
3. Developmental Control on FAS
Developmental control of fatty acid biosynthesis is responsible for redirecting plant
metabolism toward a rapid deposition of storage lipid in maturing seed and fruit
tissues. Central to developmental control is the genomic expression of organ-specific
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.