Acyl-malonyl-ACP condensing enzyme

Condensation of acetyl-ACP and malonyl-ACP to form acetoacetyl-ACP, releasing free ACP and C02 (catalyzed by acyl-malonyl-ACP condensing enzyme). 

This first round of elongation produces the four-carbon butyryl-ACP. The cycle now repeats with malonyl-ACP adding two-carbon units in each cycle to the lengthening acyl-ACP chain. This continues until the 16-carbon palmitoyl-ACP is formed. This molecule is not accepted by the acyl-malonyl-ACP condensing enzyme, and so cannot be elongated further by this process. Instead it is hydrolyzed by a thioesterase to give palmitate and ACP. 

Malonyl-CoA is converted into malonyl-ACP (acyl-carrier protein) by a highly specific enzyme, ACP-malonyltransferase. Reactants participate in the synthetic pathway, only when linked to the ACP. Malonyl-ACP donates a two-carbon unit to elongate the acyl group attached to ACP. Initially, malonyl-ACP condenses with acetyl-ACP formed from acetyl-CoA by the action of ACP-acetyltrans-ferase. The enzyme is less specific than ACP-malo-nyltransferases and can also bond propionyl groups from propionyl-CoA to ACP to form pro-pionyl-ACP in the synthesis of fatty acids with an odd number of carbons. During the condensation reaction, CO2 is released to drive the formation of acetoacetyl-ACP, a reaction which otherwise would be thermodynamically unfavourable. The free energy provided by ATP in the carboxylation step is therefore employed to synthesize acetoacetyl-ACP from acetyl-ACP and malonyl-ACP. 

Following the formation of malonyl CoA, another nucleophilic acyl substitution reaction occurs in step 4 to form the more reactive malonyl ACP, thereby binding the malonyl group to an ACP arm of the multienzyme synthase. At this point, both acetyl and malonyl groups are bound to the enzyme, and the stage is set for their condensation. 

Both bacteria and plants have separate enzymes that catalyze the individual steps in the biosynthetic sequence (Fig. 17-12). The fatty acyl group grows while attached to the small acyl carrier protein (ACP).54 58 Control of the process is provided, in part, by the existence of isoenzyme forms. For example, in E. coli there are three different P-oxoacyl-ACP synthases. They carry out the transfer of any acyl primer from ACP to the enzyme, decarboxylate malonyl-ACP, and carry out the Claisen condensation (steps b, e, and/in Eq. 17-12)58a e One of the isoenzymes is specialized for the initial elongation of acetyl-ACP and also provides feedback regulation.58c The other two function specifically in synthesis of unsaturated fatty acids. 

Figure 22.23. Schematic Representation of Animal Fatty Acid Synthase. Each of the identical chains in the dimer contains three domains. Domain 1 (blue) contains acetyl transferase (AT), malonyl transferase (MT), and condensing enzyme (CE). Domain 2 (yellow) contains acyl carrier protein (ACP), P-ketoacyl reductase (KR), dehydratase (DH), and enoyl reductase (ER). Domain 3 (red) contains thioesterase (TE). The flexible phosphopantetheinyl group (green) carries the fatty acyl chain from one catalytic site on a chain to another, as well as between chains in the dimer. [After Y. Tsukamoto, H. Wong, J. S. Mattick, and S. J. Wakil. J. Biol. Chem. 258(1983) 15312.]...

After conversion to acetyl-AGP and malonyl-AGP, two carbons of the malonyl-AGP are introduced via the condensing enzyme, 6-ketoacyl-ACP synthetase. Loss of the malonyl carboxyl drives the reaction and in the first step of the sequence acetoacetyl-AGP is formed. The B-ketoacyl-ACP is then reduced to B-hydroxyacyl-AGP by NADPH and the enzyme B-ketoacyl-AGP reductase. The hydroxy acid is dehydrated to form a trans-2.3-enov1-ACP which can be reduced by NADH or NAOPH to the saturated AGP derivative (butyrate in the first series of steps). Gondensation with malony 1-ACP is then repeated and the cycle continues to produce acyl-ACP derivatives with two additional carbon atoms until palmitoy1-ACP results. A second B-ketoacyl-ACP synthetase accomplishes addition of two more malonyl carbon atoms to allow the formation of stearoyl-ACP. The B-ketoacyl-ACP synthetase has been shown to be a separate enzyme since it is more easily inhibited by arsenite and is less sensitive to the antibiotic, cerulenin, than the B-ketoacyl-ACP synthetase forming C to C g keto acids. 

Malonyl-CoA condenses with an acyl carrier protein (ACP). This reaction is catalyzed by malonyl-CoA ACP transacylase, an enzyme that has been purified from several plants (Ohlrogge et al., 1993). ACP is a relatively small heat-stable protein of 9000 molecular weight that occurs in several isoforms (Browse and Somerville, 1991 Lehninger, 1982). Both ACP and coenzyme A have a 4 -phosphopan-tetheine prosthetic group (Ohlrogge et al., 1993). Amino acid sequences for ACP molecules from more than 15 plants, including both monocots and dicots, are available. The... 

Three forms of 3-ketoacyl-ACP synthase have been discovered in plants. These forms may be distinguished by their substrate specificity they are homodimers with molecular weights of 43,000 to 45,000 per subunit. One, KAS III, appears to be responsible for the first condensation of acetyl-CoA and malonyl-ACP (Browse and Somerville, 1991 Ohlrogge et al., 1993). The activity of this enzyme in plants seems to bypass the need for acetyl-ACP, although that molecule is formed and accumulated in some plants (Ohlrogge et al., 1993). KAS I or 3-ketoacyl-ACP synthase elongates the acyl chain to palmitoyl-ACP, whereas KAS II converts palmitoyl-ACP to stearoyl-ACP (Ohlrogge et al., 1993). 

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. 

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