Malonyl transacylase

CoA to form malonyl CoA using C02 in the form of bicarbonate HC03 (Fig. 2). This reaction is catalyzed by the enzyme acetyl CoA carboxylase which has biotin as a prosthetic group, a common feature in C02-binding enzymes. One molecule of ATP is hydrolyzed in the reaction, which is irreversible. The elongation steps of fatty acid synthesis all involve intermediates linked to the terminal sulfhydryl group of the phosphopantetheine reactive unit in ACP phosphopantetheine is also the reactive unit in CoA. Therefore, the next steps are the formation of acetyl-ACP and malonyl-ACP by the enzymes acetyl transacylase and malonyl transacylase, respectively (Fig. 2). (For the synthesis of fatty acids with an odd number of carbon atoms the three-carbon propionyl-ACP is the starting point instead of malonyl-ACP.)... 

The elongation phase of fatty acid synthesis starts with the formation of acetyl ACP and malonyl ACP. Acetyl transacylase and malonyl transacylase catalyze these reactions. 

Malonyl transacylase is highly specific, whereas acetyl transacylase can transfer acyl groups other than the acetyl unit, though at a much slower rate. Fatty acids with an odd number of carbon atoms are synthesized starting with propionyl ACP, which is formed from propionyl CoA by acetyl transacylase. 

KS = /3-Ketoacyl synthase MT = malonyl transacylase AT = acetyl transacylase DH = dehydratase ER = enoyl reductase KR = /3-ketoacyl reductase ACP = acyl carrier site TE = thioesterase. [Reproduced with permission from S. J. Wakil, J. K. Stoops, and V. C. Joshi, Fatty acid synthesis and its regulation. Annu. Rev. Biochem. 52, 537 (1983). 1983 by Annual Reviews Inc.]... 

During the first reaction on fatty acid synthase, acetyl transacylase catalyzes the transfer of the acetyl group from an acetyl-CoA molecule to the SH group of a cysteinyl residue of /J-ketoacyl-ACP synthase. Malonyl-ACP is formed when malonyl transacylase transfers a malonyl group from malonyl-CoA to the SH group of the pantetheine prosthetic group of ACP (reaction 2). Then /J-keto-acyl-ACP synthase catalyzes a condensation reaction (reaction 3) in which ace-toacetyl-ACP is formed (Figure 12.14). 

Figure 11.2 A comparative picture of the fatty acid synthetase (FAS) systems in yeast, animal, bacterial and plant cells. fi-KS, -ketoacyl AGP synthetase P-KR, A-ketoacyl AGP reductase DH, -OH acyl-AGP dehydrase ER, enoyl AGP reductase AT, acetyl transacylase MT, malonyl transacylase TE, thioesterase AGP, acyl carrier protein. See Shimakata and Stumpf (19S2a,b) and Wakil etal, (1983) for details.

Proposed structure of the dimer of type I fatty acid synthase (based on studies of the chicken liver enzyme). ACP = acyl carrier protein, KR = p-ketoacyl reductase, MT = malonyl transacylase. 

Malonyl-CoA Acyl carrier protein transacylase (MCAT or FabD) catalyzes the transfer of a malonyl group between coenzyme A and acyl carrier proteins that are the privileged transporters of the FASH system. Although essential, this enzyme is present in excess and does not have a regulatory role in the FASH pathway, which might explain the relative paucity of reported inhibitors [1],... 

MALEATE ISOMERASE MALONYL-C0A.ACP TRANSACYLASE MALYL-CoA LYASE... 

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

After malonyl-CoA synthesis, the remaining steps in fatty acid synthesis occur on fatty acid synthase, which exists as a multienzyme complex. In the initial reactions acetyl-CoA and malonyl-CoA are transferred onto the protein complex by acetyl-CoA transacylase and malonyl-CoA transacylase (step 1 and step 2 in fig. 18.12a). The acceptor for the acetyl and malonyl groups is acyl carrier protein (ACP). ACP also carries all of the intermediates during fatty acid biosynthesis. The prosthetic group that binds these intermediates is... 

Fatty acid synthesis begins when the substrates, acetyl-CoA and malonyl-CoA, are transferred onto the protein by malonyl-CoA acetyl-CoA-ACP transacylase (MAT, steps 1 and 2 in fig. 18.12a). The numbers in parentheses below the abbreviation of the enzyme in this figure refer to the reactions shown in fig. 18.12. (Whereas E. coli has separate enzymes that catalyze the transfer of acetyl- and malonyl-CoA to ACP, both reactions are catalyzed by the same enzymatic activity (MAT) on the animal fatty acid synthase.) Subsequently, /3-ketobutyryl-ACP and CO2 are formed in a condensation reaction catalyzed by /3-ketoacyl-ACP synthase (KS, step 3 in fig. 18.12a). 

Figure 19.13 Biosynthesis of palmitate via fatty acid synthetase. The numbered enzyme activities (steps) are as follows (1) acetyl-CoA transacylase (2) malonyl-coA transacylase (3) /3-ketoacylsynthetase (4) j3-ketoacylreductase (5) /3-hydroxyacyldehydratase (6) enoyl reductase (7) fatty acyltransacylase. (Reproduced by permission from Wakil SJ, Stoops JK, Joshi VC. Fatty acid synthesis and its regulation. Annu Rev Biochem 52 537-579, 1983.)...

Fig. 4. X-ray determined protein crystal structures of multienzyme ensembly lines, (a) Mammalian fatty acid synthase at 4.5 A resolution (PDB 2cf2). Domain organization A starter substrate, acetyl-CoA or malonyl-CoA, gets loaded onto the acyl-carrler protein (ACP/absent in the structure) via the malonyl-CoA-/acetyl-CoA-ACP transacylase (MAT). Then, the ketoacyl synthase (KS) catalyzes a decarboxylative condensation reaction and forms the B-ketoacyl-ACP. This is followed from a reduction reaction catalyzed by the B-ketoacyl reductase (KR). Subsequently, the Intermediate gets dehydrated by a dehydratase (DH) and additionally reduced by a B-enoyl reductase (ER). The product gets released from the ACP by a thloesterase (absent in the structure), (b) Module 3 of 6-deoxyerthronolide B synthase at 2.6 A resolution (PDB 2qo3) bound to the inhibitor cerulin. The ketosynthase (KS) - acyltransferase (AT) di-domain is part of the large homodimeric polypeptide involved in biosynthesis of erythromycin from Saccharopolyspora erythraea...

The activities involved in yeast fatty acid biosynthesis are covalently linked as separate domains of two multifunctional polypeptides, a and p, encoded by the fas2 and fasl genes, respectively (Fig. 2) [57,58]. The functionalities associated with the 220 kDa a subunit include -ketoacyl synthase activity, -ketoacyl reductase activity, and an AGP domain which bears a phosphopantetheinylated serine. The 208 kDa -subunit has acetyl and malonyl CoA transacylase, palmi-toyl transferase, -hydroxyacyl-enzyme dehydratase, and enoyl acyl-enzyme reductase activities. The two subunits can be readily dissociated, and the individual activities maybe measured [57]. 

Elegant experiments, which capitalized on the ability of iodoacetamide to specifically alkylate the active site cysteine of the -ketoacyl synthase, were performed, which definitively proved the capability of the yeast FAS in decar-boxylating malonyl CoA [75]. Following alkylation, FAS activity is abolished however, the enzyme still transacylates malonyl CoA to the phosphopantetheine thiol, where it is decarboxylated before being transferred back to CoA by the transacylase prior to its release as acetyl CoA. 

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