Food Lipids: Chemistry, Nutrition, and Biotechnology

In developing castor seed, BC and ACCase expression are well coordinated,
and BC increases to 0.8% of total seed protein, compared to less than 0.05% in
young leaf or root tissue (110,115). This developmental signal to induce rapid deposition
of storage triacylglycerols in maturing seed suggests that the rate of lipid
synthesis is controlled by ACCase. In this case, the MS-ACCase is the key isoform,
and BC is found throughout embryo development, whereas the MF-ACCase was
detected only in early stages of embryo development.
Even though MS-ACCase is dissociable, there is evidence that complexes exist
in situ. In preparing pea (Pisum sativum) chloroplasts, BC copurifies with ACCase
(110), and antibodies to one of the CT subunits inhibit ACCase and interact with
BCCP (116). In pea leaves, two isoforms of CT have been identified and the �-CT
isoform is suggested to be membrane-associated (117). Similarly, in E. coli, homodimeric
BC copurifies with homodimeric BCCP, whereas CT exists as a heterotetramer
of isoforms (� 2� 2) (11). In terms of possible fine control mechanisms, castor
and pea ACCases are not inhibited by long chain acyl-ACP, fatty acids, or ACP
(110).
Greater understanding of in vivo regulation of ACCase will be possible when
studies are done with transformants that have antisense constructs to ACCase or
overexpress ACCase activity (118). Already there are indications of complex genomic
control of ACCase. InE. coli transformants expressing a medium chain
length–specific TE, increased levels of BCCP occurred, and this finding was related
to enhanced ACCase activity and carbon flux through FAS (119). Despite the preponderance
of evidence implicating ACCase as a key control point in FAS, it is
suggested that ACCase be considered as just one of multiple control points in fatty
acid biosynthesis in plants (120). For example, the examination of whether ACP
levels influence the rate of fatty acid biosynthesis and storage lipid deposition has
been suggested (16).
As as side note, the down-regulation of the plastidic (MS-)ACCase form is
viewed as a potential means to enhance production of the biodegradable polymer
polyhydroxybutyrate (121), by diverting carbon and energy resources away from
common storage lipid accumulation (111). Such polymers may have impact on food
packaging in the future.
ACAT was once thought to be rate-limiting to fatty acid synthesis, on the basis
of a comparison of in vitro activities of isolated components of type II FAS from
plant tissues (66,67). Compared to acetyl-CoA, 4:0-ACP, and 6:0-ACP, however,
acetyl-ACP is the least effective primer for fatty acid synthesis (122) and is now
judged to be a minor participant in FAS in plants (11,13,65); ACAT may simply
provide for a backup mechanism for KAS III in FAS (65). Although ACAT activity
can be resolved from KAS III activity (118), it has been suggested that ACAT
activity may reside with KAS III (11), or that ACAT and KAS III are structurally
related, including the possibility that ACAT is a proteolytic degradation product of
KAS III (118). Finally, it has been suggested that ACAT may be an artifact derived
from acyl–ACP synthetase activity that is important to phosphatidylethanolamine
metabolism, extrapolating from E. coli (118). Again, the importance of ACAT likely
awaits resolution from studies evaluating genetic constructs with overexpressed and
suppressed levels of ACAT. One interesting observation made with ACAT is that
when added to a crude FAS system, the proportion of 10:0 to 12:0 fatty acids increases,
suggesting that ACAT activity can modulate the product profile of FAS in
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.