Food Lipids: Chemistry, Nutrition, and Biotechnology

with either CoA or ACP. CoA and ACP are similar in that they both possess a 4phosphopantetheine
unit that furnishes the �SH functional group. Imported acetate
is first converted to acetyl-CoA by plastidic acetyl-CoA synthetase (72), the activity
of which is more than sufficient to supply carbon toward fatty acid synthesis (73).
Commitment of acetyl-CoA to fatty acid biosynthesis is achieved by acetyl-CoA
carboxylase (ACCase), which yields malonyl CoA (Fig. 1). The only plastidic fate
of malonyl-CoA is for FAS (74), although malonyl-CoA can be used for acyl chain
elongation (74) and flavonoid synthesis (75,76) in extraplastidic compartments.
Plastidic ACCase in plants exists in two forms: a multifunctional protein (MF-
ACCase: single protein domain or ‘‘eukaryotic’’ form) and a multisubunit complex
(MS-ACCase: ‘‘prokaryotic’’ form), both of which have the same three functional
regions and activities relevant to fatty acid biosynthesis (13,65). These functional
regions are biotin carboxyl carrier protein (BCCP), biotin carboxylase (BC), and
carboxyltransferase (CT). BC requires Mg 2� /Mn 2� �
and utilizes and ATP to
HCO 3
carboxylate the biotin prosthetic group of BCCP. CT then transfers the carboxyl
group to acetyl-CoA to yield malonyl-CoA. The MF-ACCase also recognizes propionyl-CoA
as a substrate for carboxylation (which would give rise to fatty acids of
odd-numbered chain length) and has a reduced K m for acetyl-CoA relative to the
MS-ACCase (13). It appears that plastids of both di- and monocotyledons generally
have MS-ACCase, whereas some members of the Gramineae family of plants
(grasses and cereals) have MF-ACCase as the plastidic form. Although little else is
known regarding the role of ACCase types in fatty acid biosynthesis, the differential
organization of ACCase seems to explain the sensitivity of some Gramineae plants
to grass-specific herbicides (77,78).
The next requirement for fatty acid biosynthesis is to import acetyl-CoA and
malonyl-CoA groups into the FAS system via thioester exchange with ACP (Fig. 1).
This exchange is achieved by the activities of acetyl–CoA:ACP transacylase
(ACAT) and malonyl–CoA:ACP transacylase (MCAT), respectively (10,11,13,15).
With the realization that 3-ketoacyl–ACP synthetase III (KAS III) (see Secs.
III.B.3 and III.C) is primarily responsible for initiation of fatty acid biosynthesis and
utilizes acetyl-CoA instead of acetyl-ACP as substrate (43,44), the exact role of
ACAT has become rather nebulous. Two isoforms for both ACAT and MCAT have
been reported (79,80), and for MCAT, there appears to be considerable variability
of the enzyme within the plant kingdom (74). Both MCAT isoforms appear to be
in leaf tissue, but only one form is dominant in seeds (80).
3. Initiation
The ‘‘initiation’’ step involves the conjugation of a malonyl and an acetyl group and
results in the formation of the 3-ketobutanoyl–ACP (acetoacetyl-ACP) derivative
with the liberation of CO 2 (Fig. 1). In both plants and E. coli, this step is now
believed to be catalyzed principally by KAS III (11,43,44,81). KAS exists in two
other isoforms, KAS I and KAS II, both of which are involved in chain elongation
steps (Sec. III.B.5). KAS III is the most recently identified isoform, first in E. coli
(43) and then in plants (44,56). KAS III is unique in that it strictly (59) utilizes
acetyl-CoA (instead of acetyl-ACP) and malonyl-ACP in forming acetoacetyl-ACP.
Thus, the former view (and dogma) that ACAT and acetyl-ACP levels were central
to control of flux in fatty acid biosynthesis is now very doubtful (81).
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.