Thioester hydrolase

Specific chain length fatty acids could be produced in two ways. One is through the action of a thioester hydrolase that interacts with fatty acid synthetase to produce fatty acids shorter in length. Aphids produce myristic acid (14 carbons) and a specific thioester hydrolase releases the fatty acid from fatty acid synthetase after 6 additions of malonyl-CoA. If the hydrolase is not present then the fatty acid synthetase produces stearic acid [27]. A specific thioester hydrolase was ruled out in the biosynthesis of moth sex pheromones because labeling studies showed that longer chain length fatty acids were incorporated into shorter chain length pheromone components [22,28]. 

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).

Grunnet, I. and Knudsen, J. 1978. Medium chain acyl-thioester hydrolase activity in goat and rabbit mammary fatty acid synthetase complexes. Biochem. Biophys. Res. Commun. 80, 745-749. 

Knudsen, J., Grunnet, I. and Dils, R. 1981. Medium-chain fatty acyl-s-4 -phos-phopantetheine fatty acid synthetase thioester hydrolase from lactating rabbit and goat mammary glands. In Methods in Enzymology, Vol. 710. J.J. Lowenstein (Editor). Academic Press, New York, pp. 200-229. 

Knudsen J., Clark S. and Dils R. (1976) Purification and some properties of a medium-chain acyl-thioester hydrolase from lactating-rabbit mammary gland which terminates chain elongation in fatty acid synthesis. Biochem. J. 160, 683-691. 

Release of the finished fatty acid occurs when the chain length reaches C]6 by action of thioester hydrolase, which is specific for long-chain acyl-CoA derivatives. A thioester hydrolase of mammary gland is specific for acyl residues of Cg, Cio, or C12. 

Thioester hydrolase catalyzes the removal of palmitate from the 4 -phosphopantetheine arm of the acyl carrier site. 

A putative physiological role for this enzyme is to function together with the thioester hydrolase glyoxalase II (hydroxyacylglutathione hydrolase) to chemically remove from cells cytotoxic methylglyoxal (R = CH3) as D-lactate (104). 

Foster, R.J., Bonsall, R.F., Poulose, A.J. Kolattukudy, P.E. (1985). Interaction of S-acyl fatty add synthase thioester hydrolase with fatty add synthase. ]. Biol Chem., 260, 1386-9. 

Rogers, L., Kolattukudy, P.E. de Renobales, M. (1982). Purification and characterization of S-acyl fatty acid synthase thioester hydrolase which modifies the product specificity of fatty acid synthase in the uropygial gland of Mallard. J. Biol. Chem., 257, 880-6. 

Broustas, C.G., Larkins, L.K., Uhler, M.D. Hajra, A.K. (1996) J. Biol. Chem. 271, 10470-10476. Molecular cloning and expression of cDNA encoding rat brain cytosohc acyl-cenzyme A thioester hydrolase. 

The chemical diversity of carboxylic acid esters (R-CO-O-R ) originates in both moieties, i.e., the acyl group (R-CO-) and the alkoxy or aryloxy group (-OR7). Thus, the acyl group can be made up of aliphatic or aromatic carboxylic acids, carbamic acids, or carbonic acids, and the -OR7 moiety may be derived from an alcohol, an enol, or a phenol. When a thiol is involved, a thioester R-CO-S-R7 is formed. The model substrates to be discussed in Sect. 7.3 will, thus, be classified according to the chemical nature of the -OR7 (or -SR7) moiety, i.e., the alcohol, phenol, or thiol that is the first product to be released during the hydrolase-catalyzed reaction (see Chapt. 3). Diesters represent substrates of special interest and will be presented separately. 

Thioesters play a paramount biochemical role in the metabolism of fatty acids and lipids. Indeed, fatty acyl-coenzyme A thioesters are pivotal in fatty acid anabolism and catabolism, in protein acylation, and in the synthesis of triacylglycerols, phospholipids and cholesterol esters [145], It is in these reactions that the peculiar reactivity of thioesters is of such significance. Many hydrolases, and mainly mitochondrial thiolester hydrolases (EC 3.1.2), are able to cleave thioesters. In addition, cholinesterases and carboxylesterases show some activity, but this is not a constant property of these enzymes since, for example, carboxylesterases from human monocytes were found to be inactive toward some endogenous thioesters [35] [146], In contrast, allococaine benzoyl thioester was found to be a good substrate of pig liver esterase, human and mouse butyrylcholinesterase, and mouse acetylcholinesterase [147],... 

A number of synthetic fatty acid thioesters, notably derivatives of branched fatty acids or thioester-containing phospholipids, have been used as kinetic and catalytic probes of various hydrolases [148] [149]. However, such compounds are of limited interest in our context. 

Simple alkyl or aryl thioesters are commonly assayed as substrates of hydrolases, witness the hydrolysis of phenyl thioesters by horse serum carbox-ylesterase [150], For most substrates investigated, e.g., phenyl thioacetate, phenyl thiopropionate, and phenyl thiobutyrate (7.66, R = Me, Et, and Bu, respectively), kcat values were found, which were a few times larger than those of corresponding nitrophenyl esters, whereas the affinities were lower by approximately one order of magnitude. Methyl and phenyl esters of various linear thioacids were also found to be good substrates of mammalian liver carboxylesterases and serum cholinesterases [151]. 

The serine hydrolase family is one of the largest and most diverse classes of enzymes. They include proteases, peptidases, lipases, esterases, and amidases and play important roles in numerous physiological and pathological process including inflammation [53], angiogenesis [54], cancer [55], and diabetes [56]. This enzyme family catalyzes the hydrolysis of ester, thioester, and amide bonds in a variety of protein and nonprotein substrates. This hydrolysis chemistry is accomplished by the activation of a conserved serine residue, which then attacks the substrate carbonyl. The resulting covalent adduct is then cleaved by a water molecule, restoring the serine to its active state [57] (Scheme 1). 

Nonstereoselective hydrolysis of the acyl-CoA thioesters by one or more hydrolases to yield free S-ibuprofen [17]. 

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