P-ketoacyl-ACP synthase III

The last two carbons of the fatty acid chain (i.e., those most distal from the carboxylate group) are the first introduced into the nascent chain, and acetyl-CoA can be thought of as the primer molecule of fatty acid synthesis in E. coli. The initial condensation reaction, catalyzed by P-ketoacyl-ACP synthase III (FabH), utilizes acetyl-CoA and malonyl-ACP to form the four-carbon acetoacetyl-ACP with concomitant loss of COj (Fig. 2). FabH also possesses acetyl-CoA ACP transacylase activity, and for many years it was thought that acetyl-ACP was the actual primer. However, acetyl-ACP appears to be a product of a side reaction, and the role, if any, played by this intermediate in the pathway is unknown. 

Despite the presence of acetyl-CoA ACP acyltransferase activity in plant fatty acid synthase preparations, acetyl-ACP does not appear to play a major role in plant fatty acid synthesis (J. Jaworski, 1993). Instead, the first condensation takes place between acetyl-CoA and malonyl-ACP. This reaction is catalyzed by P-ketoacyl-ACP synthase III, one of three ketoacyl synthases in plant systems (Fig. 2). The acetoacetyl-ACP product then undergoes the standard reduction-dehydration-reduction sequence to produce 4 0-ACP, the initial substrate of ketoacyl-ACP synthase I. KAS I is responsible for the condensations in each elongation cycle up through that producing 16 0-ACP. The third ketoacyl synthase, KAS II, is dedicated to the final plastidial elongation, that of 16 0-ACP to 18 0-ACP. 

Fatty acid synthesis in plants is carried out by a type I dissociable fatty acid synthase (FAS). There are three p-ketoacyl-ACP synthases (KAS) associated with FAS in plants. The short-chain condensing enzyme (KAS III) catalyses the initial condensation of acetyl-CoA with malonyl-ACP [1,2] to form acetoacetyl-ACP (4C) and may catalyse further rounds of condensation in vivo [3]. Further rounds of condensation are carried out by KAS I which catalyses the condensation of malonyl-ACP with intermediate length acyl-ACPs from acetoacetyl-ACP to myristoyl-ACP (14C) to give palmitoyl-ACP (16C). KAS II elongates palmItoyl-ACP (16C) to stearoyl-ACP (18C) [4J. 

It has also been demonstrated that 3-ketoacyl-ACP reductase (FabG) can accept not only acyl-ACP but also acyl-CoA as a substrate and is capable of supplying mcl-(R)-3HA-CoA from fatty acid p-oxidation in Escherichia co/i. FabG, which is known as a homologue of PhaB, has been reported to serve as a monomer supplier for PHA biosynthesis in recombinant E. colt. 3-Ketoacyl-AGP synthase III (FabH) is also a constituent of fatty... 

The FA elongation stage is catalyzed by P-ketoacyl synthase III (KAS III), a single polypeptide enzyme with several catalytic sites, which allows the enzyme to promote different reactions during this stage (Voelker Kinney, 2001). The condensation of malonyl-ACP and acetyl-CoA to yield 3-ketobutyryl-ACP and CO is the first reaction. The second reaction is the reduction of 3-ketobutyl-ACP to 3-hydroxylacyl-... 

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