Malonyl-CoA.ACP transacylases

The now-vacant ACP accepts a three-carbon malonate unit from malonyl CoA. Domain Malonyl CoA-ACP-transacylase. 

Next, a malonyl-group is transferred from a Coenzyme A to the pantetheinyl-S of ACP2 by Malonyl-CoA ACP transacylase. 

Fig. 2. Initiation of fatty acid synthesis. (1) Malonyl-CoA ACP transacylase (FabD) transfers the malonyl group from CoA to ACP and then (2) (5-ketoacyl-ACP synthase lit (FabH) catalyzes the initial irreversible condensation of malonyl-ACP with acetyl-CoA to form acetoacetyl-ACP.

Malonyl-CoA + ACP <=> Malonyl-ACP + CoASH (catalyzed by Malonyl-CoA-ACP Transacylase)... 

ACP is covalently linked to acetyl-CoA and malonyl-CoA by the enzymes acetyl-CoA-ACP transacylase and malonyl-CoA-ACP transacylase, respectively. 

Pathway III involves 3-hydroxyacyI-ACP-CoA transferase (PhaG) and malonyl-CoA-ACP transacylase (FabD), which help supply 3-hydroxyacyl-ACP to form PHA monomer 3-hydroxyacyI-CoA, leading to PHA formation under the action of PHA synthase (Sudesh et al. 2000 Zheng et al. 2005 Taguchi et al. 1999). 

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... 

Although no attempt has yet been made to isolate and purify all the enzymes responsible for the synthesis of palmitoyl-ACP, undoubtedly ace-tyl-CoA ACP transacetylase, malonyl-CoA ACP transacylase, j8-ketoacyl-ACP reductase, /3-acyl-ACP dehydrase, and enoyl-ACP reductase are present in sufficient concentrations to allow effective synthesis of palmitoyl-ACP. These enzymes are probably present as discrete, separable, nonasso-ciated proteins, because prolonged centrifugation at high gravitational force does not sediment the activities. However, in the near future, careful purification and characterization studies should be carried out in order to define more precisely the molecular properties of these enzymes and thereby allow them to be compared with their counterparts in prokaryotic systems. It is conceivable that even these activities may be associated with two or three polypeptides rather than with six or seven discrete proteins. 

The transfer of the malonyl group from the SH group of malonyl CoA to ACP is catalyzed by malonyl CoA-ACP transacylase. 

The malonyl-CoA-ACP transacylase has been purified to homogeneity from E. coli and has a molecular weight of 36,500. A malonyl enzyme intermediate is formed, and the malonyl moiety is known to be attached to a serine residue of the enzyme. 

The malonyl-coenzyme A then reacts with acyl-carrier protein (ACP), in the presence of malonyl-CoA-ACP transacylase, to give the malonyl-ACP complex. Acetyl-coenzyme A is then coupled with ACP in the presence of acetyl-CoA-ACP transacylase, and this reacts with the malonyl-ACP, the chain length being increased by two carbon atoms to give the butyryl-ACP complex. The reactions involved are shown in Fig. 9.18. 

Functional assay for PKS ACPs was developed [ 168,177] long before the development of a cell-free system to assay the entire PKS complex [181,182]. ACPs can be assayed as a substrate to accept a malonate from malonyl CoA, catalyzed by a crude malonyl CoArACP acyltransferase (MAT) preparation [168,174,175, 177,178]. Only holo-ACPs were biochemically active in the malonyl CoA ACP transacylase assay, confirming that 4 -phosphopantetheinylation is a prerequisite for loading the malonyl CoA extender unit to ACP [168,177,186]. 

Figure 7.7 Common metabolic pathways that are involved in the biosynthesis of PHA in microorganisms. FabC malonyl-CoA acyl carrier protein (ACP) transcylase, FabD malonyl-CoA-ACP transacylase, FabG 3-ketoacyl-CoA reductase, PhaA P-ketothiolase, PhaB NAOH-dependent acetoacetyl-CoA reductase, PhaC polyhydroxyalkanoates synthase, PhaG 3-hydroxyacyl-ACP GoA transferase, PhaJ (R)-enoyl-GoA hydratase and TCA tricarboxylic acid...

European Atomic Energy Community Ethylene-vinyl alcohol Malonyl-CoA acyl carrier protein Malonyl-CoA-ACP transacylase 3-Ketoacyl-CoA reductase... 

A.T. Keatinge-Qay et al.. Catalysis, specificity, and ACP docking site of Streptomyces coeli-co/or malonyl-CoA ACP transacylase. Structure 11, 147-154 (2003)... 

Two Isoenzymes of malonyl-CoA ACP transacylase (MTA) (7), as well as two forms of ACP (8,9) have been reported in the literature, adding credibility to the possibility of two Isoenzymes of ATA. Investigation of the chemical and physical properties of the two forms of ATA are underway. This paper describes the purification scheme of the two forms of ATA. 

Figure 4 (A) Coordinated interaction of members of the condensing (KAS) enzyme family results in biosynthesis of fatty acids in E. coli. The genes encoding each KAS as well as major destinations of the fatty acyl products are shown. Lipoic acid is a precursor of the coenzyme lipoamide. Lipid A consists of p-hydroxymyristate linked to saccharides in the cell membrane. PL are the membrane phospholipids. (B) How do KASes interact with one another and the other members of the FAS complex ACP, acyl carrier protein KR, p-ketoacyl-ACP reductase DH, P-hydroxyacyl-ACP dehydrase ER, enoylacyl-ACP reductase MAL TR, malonyl-CoA ACP transacylase TE, thioesterase AC TR acetyl-CoA ACP transacylase whose contribution to fatty acid synthesis is uncertain since the discovery and characterization of KAS III [33,38].

Type II PKS Systems in Bacteria Many bio-medicaUy important aromatic natural products are derived from bacterial type II PKSs [23]. Within these biosynthetic machineries, individual enzymes form a functional complex, the so-called minimal PKS. This is composed of a KSot/KSp heterodimer and a dissociated ACP that serves as product tether (cf. Scheme 2.3b). ACP loading with building blocks is catalyzed by a malonyl-CoA ACP transacylase (MAT), an activity that can be provided by FabD from fatty acid... 

Figure 22.4 A simplified illustration of saturated fatty acid biosynthesis in microalgal chloroplast. ACCase, Acetyl-CoA carboxylase ACP, acyl carrier protein CoA, coenzyme A ENR, enoyl-ACP reductase HD, 3-hydroxyacyl-ACP dehydratase KAR, 3-ketoacyl-ACP reductase KAS, 3-ketoacyl-ACP synthase MAT, malonyl-CoA ACP transacylase.

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