Dehydratase fatty acid

Tsukamoto, Y., and Wakil, S.J. (1988) Isolation and mapping of the b-hydroxyacyl dehydratase activity of chicken liver fatty acid synthase./. Biol. Chem. 263, 16225-16229. 

Figure 11.5 Reactions of the fatty acid synthase complex. A single multi-subunit enzyme is responsible for the conversion of acetyl-CoA to palmitate. The subunits in the enzyme are (i) acetyltransferase, (ii) malonyltransferase, (iii) oxoacyl synthase, (iv) oxoacyl reductase, (v) hydroxyacyl dehydratase, (vi) enoyl reductase. Finally, a separate enzyme, thioester hydrolase, hydrolyses palmitoyl-CoA to produce palmitate (vii).

This enzyme, officially known as 3-hydroxypalmitoyl-[acyl-carrier protein] dehydratase [EC 4.2.1.61], is the fatty-acid synthase component that catalyzes the conversion of (3 i )-3-hydroxypalmitoyl-[acyl-carrier protein] to form 2-hexadecenoyl-[acyl-carrier protein] and water. This enzyme displays specificity toward 3-hydroxyacyl-[acyl-carrier protein] derivatives (with chain lengths from Ci2 to Cie), with highest activity on the palmitoyl derivative. See also Fatty Acid Synthetase... 

Figure 1 Polyketide biosynthesis. Polyketide backbones are formed via condensations from acyl-CoA thioesters of carboxylic acids. The (3-ketone which results from each condensation can undergo a series of reductive steps analogous to fatty acid biosynthesis. However, either none or only some of the reductive activities may occur in a given cycle. This allows PKSs to generate diversity through selection of priming and extender units, variation of the reductive cycle, and stereoselectivity. (ACP, acyl carrier protein AT, acyl transferase KS, ketosynthase DH, dehydratase ER, enoylreductase KR, ketoreductase TE, thioesterase.) The structure depicted in the lower right-hand corner is representative of the possible structural variations that can arise during polyketide biosynthesis.

Figure 3 The fatty acid biosynthetic cycle (ACP, acyl carrier protein KS, P-ketoacyl synthase KR, P-ketoacyl reductase DH, dehydratase ER, enoyl reductase TE, thioes-terase).

Heath, R.J., Rock, C.O. Roles of the FabA and FabZ beta-hydroxyacyl-acyl carrier protein dehydratases in Escherichia coli fatty acid biosynthesis. J Biol Chem 271 (1996) 27795-27801. 

Each of the enzymatic activities located in a single polypeptide chain of the mammalian fatty acid synthetase exists as a distinct protein in E. coli. The acyl-carrier protein (ACP) of E. coli has an Mr = 8,847 and contains 4-phosphopantotheine. The dehydratase has a molecular weight of 28,000 and catalyzes either trans 2-3 or cis 3-4 dehydration of the hydroxy acid intermediates in the biosynthesis of palmitic acid. When the chain length of the hydroxy fatty acid is C[ the synthesis of palmitoleic acid is achieved as follows ... 

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]. 

The seven activities of animal FASs are encoded as separate domains of a single 250 kDa polypeptide (Fig. 2) [30, 31]. These include a -ketoacyl synthase, malonyl/acetyl transferase, -ketoreductase, dehydratase, enoyl reductase, and an ACP domain with a phosphopantetheinylated serine. In addition to these activities, the animal FAS also includes a thioesterase domain which cleaves the product fatty acid from the enzyme. Proteolytic mapping of the polypeptide and genetic analysis have defined the location of the various domains in the primary sequence [30,31]. 

Leesong M, Henderson BS, Gillig JR, Schwab JM, Smith JL. Structure of a dehydratase-isomerase from the bacterial pathway for biosynthesis of unsaturated fatty acids two catalytic activities in one active site. Structure 1996 4 253-264. 

DH Dehydratase, found in fatty acid syntheases and polyketide syntheases, dehydrates the 3-OH of acyl thioester... 

Figure 5 Fatty acid biosynthesis catalyzed by fatty acid synthases. The growing acyl chain is tethered to the phosphopantetheinylated ACP domain, which enabies it to undergo cycles of condensation, ketone reduction, dehydration, and enol reduction catalyzed by different domains. AT, acyltransferase ACP, acyi-carrier protein KS, ketosynthase KR, ketoreductase DH, dehydratase ER, enoyireductase.

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.]...

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.]... 

The answer is e. (Murray, pp 230-267. Scriver, pp 2297-2326. Sack, pp 121-138. Wilson, pp 287-320.) The fatty acid synthase complex of mammals is composed of two identical subunits. Each of the subunits is a multienzyme complex of seven enzymes and the acyl carrier protein component. All the components are covalently linked together thus, all the components are on a single polypeptide chain, which functions in the presence of another identical polypeptide chain. Each cycle of fatty acid synthesis employs the acyl carrier protein and six enzymes acetyl transferase, malonyl transferase, p-ketoacyl synthase, p-ketoacyl reductase, dehydratase, and enoyl reductase. When the final fatty acid length is reached (usually C16), thioesterase hydrolyzes the fatty acid off of the synthase complex. 

Figure 3. Relationship between polyketide and fatty acid biosynthesis. The simplest ( minimaV) PKSs possess ketosynthase activity and produce linear polyketide products. In contrast, FASs also catalyze successive ketoreduction-dehydration-enoyl reduction reactions following each condensation. Diverse PKSs may perform none, part, or all of this reductive sequence. KS, ketosynthase KR, ketoreductase DH, dehydratase ER, enoyl reductase.

FATTY ACID CHAIN EXTENSION CYCLE KS = Ketoacyl Synthase ACP = Acyl Carrier Protein KR = Ketoreductase DHi Dehydratase ER Enoylraductase AT = Acyttransferase TE = TNoesterase... 

Despite the fact that numerous enzymes have been characterized that catalyze the addition of water to unsaturated fatty acids that are coupled to CoA or ACP, such as methylglucatonyl-CoA hydratase (E.C. 4.2.1.18), lactoyl-CoA dehydratase (E.C. 4.2.1.54), 3-hydroxybutyryl-CoA dehydratase (E.C. 4.2.1.55), itaconyl-CoA dehydratase (E. C. 4.2.1.56), isohexenylglutaconyl-CoA hydratase (E. C. 4.2.1.57), farnesyl-CoA dehydratase (E.C. 4.2.1.57), long-chain enoyl-CoA hydratase (E.C. 4.2.1.74), 3-hydroxydecanoyl-ACP dehydratase (E.C. 4.2.1.60) and 3-hydroxypalmitoyl-ACP dehydratase (E.C. 4.2.1.61), these enzymes are seldomly applied in organic synthesis. 

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