Acetyl carrier protein

We can descnbe the major elements of fatty acid biosynthesis by considering the for mation of butanoic acid from two molecules of acetyl coenzyme A The machinery responsible for accomplishing this conversion is a complex of enzymes known as fatty acid synthetase Certain portions of this complex referred to as acyl carrier protein (ACP), bear a side chain that is structurally similar to coenzyme A An important early step m fatty acid biosynthesis is the transfer of the acetyl group from a molecule of acetyl coenzyme A to the sulfhydryl group of acyl carrier protein... 

When both building block units are m place on the acyl carrier protein carbon-carbon bond formation occurs between the a carbon atom of the malonyl group and the carbonyl carbon of the acetyl group This is shown m step 1 of Figure 26 3 Carbon-carbon bond formation is accompanied by decarboxylation and produces a four carbon acetoacetyl (3 oxobutanoyl) group bound to acyl earner protein... 

Acetyl and malonyl groups bound to acyl carrier protein... 

FIGURE 25.7 The pathway of palmhate synthesis from acetyl-CoA and malonyl-CoA. Acetyl and malonyl building blocks are introduced as acyl carrier protein conjugates. Decarboxylation drives the /3-ketoacyl-ACP synthase and results in the addition of two-carbon units to the growing chain. Concentrations of free fatty acids are extremely low in most cells, and newly synthesized fatty acids exist primarily as acyl-CoA esters. 

Steps 1-2 of Figure 29.5 Acyl Transfers The starting material for fatty-acid synthesis is the thioesteT acetyl CoA, the ultimate product of carbohydrate breakdown, as we ll see in Section 29.6. The synthetic pathway begins with several priming reactions, which transport acetyl CoA and convert it into more reactive species. The first priming reaction is a nucleophilic acyl substitution reaction that converts acetyl CoA into acetyl ACP (acyl carrier protein). The reaction is catalyzed by ACP transacyla.se. 

Bicarbonate as a source of CO2 is required in the initial reaction for the carboxylation of acetyl-CoA to mal-onyl-CoA in the presence of ATP and acetyl-CoA carboxylase. Acetyl-CoA carboxylase has a requirement for the vitamin biotin (Figure 21-1). The enzyme is a multienzyme protein containing a variable number of identical subunits, each containing biotin, biotin carboxylase, biotin carboxyl carrier protein, and transcarboxylase, as well as a regulatory allosteric site. The reaction takes place in two steps (1) carboxylation of biotin involving ATP and (2) transfer of the carboxyl to acetyl-CoA to form malonyl-CoA. 

All polyketides use the same general mechanism for chain elongation. Acetyl coenzyme A provides acetate (C2) units, which are condensed by a ketosynthase (KS). This in turn catalyzes condensation of the growing chain onto an acyl carrier protein (ACP), as generalized in Fig. 1.4. Enzymes such as ketoreductase (KR), enoyl reductase (ER), and dehydratase (DH) establish the oxidation state of caibon during translation, imparting structural diversity. Successive translation of each module leads to a chain of the required length that is eventually passed to thioeste-rase (TE), which releases the chain as a free acid or lactone. 

Fatty add synthase is a large multienzyme complex in the cytoplasm that is rapidly induced in the liver after a meal by high carbohydrate and the concomitant rise in insulin levels. It contains an acyl carrier protein (AGP) that requires the vitamin pantothenic add. Althoi malonyl CoA is the substrate used by fetty acid synthase, only the carbons from the acetyl CoA portion are actually incorporated into the fatty acid produced. Therefore, the fetty add is derived entirely from acetyl CoA. 

Sugar The hydrolysis of sucrose in the intestine produces both glucose and fructose, which are transported across the epithelial cells by specific carrier proteins. The fructose is taken up solely by the liver. Fructose is metabolised in the liver to the triose phosphates, dihydroxy-acetone and glycer-aldehyde phosphates. These can be converted either to glucose or to acetyl-CoA for lipid synthesis. In addition, they can be converted to glycerol 3-phosphate which is required for, and stimulates, esterification of fatty acids. The resulting triacylglycerol is incorporated into the VLDL which is then secreted. In this way, fructose increases the blood level of VLDL (Chapter 11). 

The fatty acid synthase protein is known to contain an acyl carrier protein (ACP) binding site, and also an active-site cysteine residue in the P-ketoacyl synthase domain. Acetyl and malonyl gronps are successively transferred from coenzyme A esters and attached to the thiol groups of Cys and ACP. 

Each subunit of the enzyme binds acetyl residues as thioesters at two different SH groups at one peripheral cysteine residue (CysSH) and one central 4-phosphopante-theine group (Pan-SH). Pan-SH, which is very similar to coenzyme A (see p. 12), is covalently bound to a protein segment of the synthase known as the acyl-carrier protein (ACP). This part functions like a long arm that passes the substrate from one reaction center to the next. The two subunits of fatty acid synthase cooperate in this process the enzyme is therefore only capable of functioning as a dimer. 

Selected entries from Methods in Enzymology [vol, page(s)] Assay, 1, 611 3, 935-938 63, 33 separation by HPLC, 72, 45 extraction from tissues, 13, 439 formation of, 1, 486, 518, 585 5, 466 free energy of hydrolysis, 1, 694 substrate for the following enzymes [acetyl-coenzyme A acyl carrier protein transacylase, 14, 50 acetyl-coenzyme A carboxylase, 14, 3, 9 acetyl-coenzyme A synthetase, 13, 375 N-acetyltransferase, 17B, 805 aminoacetone... 

This enzyme [EC 2.3.1.38], also referred to as acetyl-CoA [acyl-carrier protein] 5-acetyltransferase, transfers an acetyl group from one acetyl-CoA to an acyl-carrier-protein (ACP) to form free coenzyme A and the acetyl-[acyl-carrier-protein]. See also Fatty Add Synthase... 

This enzyme [EC 4.1.3.25] catalyzes the conversion of (35)-citramalyl-CoA to acetyl-CoA and pyruvate. The (35)-citramalyl thioacyl-carrier protein can also be utilized as a substrate. This enzyme has been reported to be a component of citramalate lyase [EC 4.1.3.22]. 

Figure 8-2. Pathway for synthesis of palmitate by the fatty acid synthase (FAS) complex. Schematic representation of a single cycle adding two carbons to the growing acyl chain. Formation of the initial acetyl thioester with a cysteine residue of the enzyme preceded the first step shown. Acyl carrier protein (ACP) is a component of the FAS complex that carries the malonate covalently attached to a sulfhydryl group on its phosphopantatheine coenzyme (-SH in the scheme).

The sex pheromone is interesting from a biosynthetic perspective (see Fig. 4.3) because it is closely connected with primary metabohsm. That is, the monomer 4 is an intermediate in fatty acid biosynthesis. Condensation of acetyl-ACP (8 ACP, acyl carrier protein) with malonyl-CoA (9 CoA, coenzyme A) yields acetoacyl-ACP (10). Enantioselective reduction with NADPH leads to (R)-3-hydroxybutyryl-ACP (11). Two units of this precursor could then be condensed to form the pheromone 5, which then degrades to 4 and 6 as described above. Alternatively, 4 can also be formed by direct hydrolysis of intermediate 11. 

Figure 3.8 One complete cycle and the first step in the next cycle of the events during the synthesis of fatty acids. ACP = acyl carrier protein, a complex of six enzymes i.e. acetyl CoA-ACP transacetylase (AT) malonyl CoA-ACP transferase (MT) /3-keto-ACP synthase (KS) /J-ketoacyl-ACP reductase (KR) / - hydroxyacyl-ACP-dehydrase (HD) enoyl-ACP reductase (ER).

FIGURE 21-1 The acetyl-CoA carboxylase reaction. Acetyl-CoA carboxylase has three functional regions biotin carrier protein (gray) biotin carboxylase, which activates C02 by attaching it to a nitrogen in the biotin ring in an ATP-dependent reaction (see Fig. 16-16) and transcarboxylase, which transfers activated C02 (shaded green) from... 

The core of the E. coli fatty acid synthase system consists of seven separate polypeptides (Table 21-1), and at least three others act at some stage of the process. The proteins act together to catalyze the formation of fatty acids from acetyl-CoA and malonyl-CoA. Throughout the process, the intermediates remain covalently attached as thioesters to one of two thiol groups of the synthase complex. One point of attachment is the —SH group of a Cys residue in one of the seven synthase proteins (j3-ketoacyl-ACP synthase) the other is the —SH group of acyl carrier protein. 

FIGURE 21-5 Sequence of events during synthesis of a fatty acid. The fatty acid synthase complex is shown schematically. Each segment of the disk represents one of the six enzymatic activities of the complex. At the center is acyl carrier protein (ACP), with its phosphopantetheine arm ending in an —SH. The enzyme shown in blue is the one that will act in the next step. As in Figure 21-3, the initial acetyl group is shaded yellow, C-1 and C-2 of malonate are shaded pink, and the carbon released as C02 is shaded green. Steps (T) to (7) are described in the text. 

The remaining series of reactions of fatty acid synthesis in eukary-l otes is catalyzed by the multifunctional, dimeric enzyme, fatty acid synthase. Each fatty acid synthase monomer is a multicatalytic polypeptide with seven different enzymic activities plus a domain that covalently binds a molecule of 4 -phosphopantetheine. [Note 4-Phosphopantetheine, a derivative of the vitamin pantothenic add (see p. 379), carries acetyl and acyl units on its terminal thiol (-SH)j group during fatty acid synthesis. It also is a component of 00-enzyme A.] In prokaryotes, fatty acid synthase is a multienzyme complex, and the 4 -phosphopantetheine domain is a separate protein, referred to as the acyl carrier protein (ACP). ACP is used below to refer to the phosphopantetheine-binding domain of the eukaryotic fatty acid synthase molecule. The reaction numbers in1 brackets below refer to Figure 16.9. [Note The enzyme activities listed are actually separate catalytic domains present in each mulf-1 catalytic fatty acid synthase monomer.]... 

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