Fatty acids thioesters

Dimerized fatty acid thioesters (with a dithiol) in combination with fatty amines are sulfur-containing corrosion inhibitors [888]. The corrosion inhibitor solvent is preferably a hydrocarbon. 

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. 

The pheromone biosynthetic pathway of T. ni is well defined and is relatively simple compared to those of many other moth species (5) (Figure 2). The initial substrate is the common 16-carbon saturated fatty acid thioester of Coenzyme A (palmitoylrCoA), which is derived from the combined actions of acetyl-CoA carboxylase and fatty acid synthase. An acyl-CoA All desaturase acts upon palmitoykCoA to produce a Z double bond between carbon atoms 11 and 12 (Zll-16 CoA). The multienzyme -oxidation complex subsequently acts on this compound in two successive rounds of j3-oxidation to produce Z9-14 CoA followed by Z7-12 CoA. The active component of the T. ni pheromone, Z7-12 OAc, results from the sequential action on Z7-12 CoA of a reductase and an acetyltransferase. A minor pheromone component, Z5- 12 OAc is produced by the same enzymatic steps as... 

This mixed anhydride then undergoes a carbonyl addition reaction with the sulfhydryl group of coenzyme A to form a tetrahedral carbonyl addition intermediate, which collapses to give AMP and an acyl-CoA (a fatty acid thioester of coenzyme A) ... 

Intrinsic proteins may be anchored in the membrane by several different mechanisms. Of these, the hydrophobic interaction of an amino acid sequence with the interior of the lipid bilayer is the most common. Such interactions include hydrophobic sequences at one end of the protein only as well as polypeptide chains which traverse the membrane (several times). In addition, some proteins may be attached through interactions with lipid. Thus, fatty acid acylation of the NH2-terminus or the formation of fatty acid thioesters with cysteine residues have been observed. In addition, a number of membrane proteins are believed to be covalently attached to phosphatidylinositol. 

A variety of cellular and viral proteins contain fatty acids covalently bound via ester linkages to the side chains of cysteine and sometimes to serine or threonine residues within a polypeptide chain (Figure 9.18). This type of fatty acyl chain linkage has a broader fatty acid specificity than A myristoylation. Myristate, palmitate, stearate, and oleate can all be esterified in this way, with the Cjg and Cjg chain lengths being most commonly found. Proteins anchored to membranes via fatty acyl thioesters include G-protein-coupled receptors, the surface glycoproteins of several viruses, and the transferrin receptor protein. 

FIGURE 24.8 The mechanism of the acyl-CoA synthetase reaction involves fatty acid carboxylate attack on ATP to form an acyl-adenylate intermediate. The fatty acyl CoA thioester product is formed by CoA attack on this intermediate. 

FIGURE 25.13 Double bonds are introduced into the growing fatty acid chain in E. coli by specific dehydrases. Palmitoleoyl-ACP is synthesized by a sequence of reactions involving four rounds of chain elongation, followed by double bond insertion by /3-hydroxydecanoyl thioester dehydrase and three additional elongation steps. Another elongation cycle produces cA-vaccenic acid. 

The four steps of the /3-oxidation pathway, resulting in the cleavage of an acetyl group from the end of the fatty-acid chain. The key chain-shortening step is a retro-Claisen reaction of a /3-keto thioester. Individual steps are explained in the text. 

In step 7 of fatty-acid biosynthesis (Figure 29.5), dehydration of a /3-hydroxy thioester occurs to give fm/rs-crotonyl ACP. Is the dehydration a syn elimination or an anti elimination ... 

Particularly important to the pathways of modular synthases is the incorporation of novel precursors, including nonproteinogenic amino acids in NRP systems [17] and unique CoA thioesters in PK and fatty acid synthases [18]. These building blocks expand the primary metabolism and offer practically unlimited variability applied to natural products. Noteworthy within this context is the contiguous placement of biosynthetic genes for novel precursors within the biosynthetic gene cluster in prokaryotes. Such placement has allowed relatively facile elucidation of biosynthetic pathways and rapid discovery of novel enzyme mechanisms to create such unique building blocks. These new pathways offer a continued expansion of the enzymatic toolbox available for chemical catalysis. 

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]. 

Each CHS monomer consists of two structural domains (Fig. 12.5, left). The upper domain exhibits the a-p-a-p-a pseudo-symmetric motif observed in fatty acid P-ketoacyl synthases (KASs) (Fig. 12.5, right).20 Both CHS and KAS use a cysteine as a nucleophile in the condensation reaction, and shuttle reaction intermediates via CoA thioester-linked molecules or ACPs, respectively. The conserved architecture of the upper domain maintains the three-dimensional position of the catalytic residues of each enzyme Cysl64, His303, and Asn336 in CHS correspond to a Cys, His, and His in KAS I and II. 

The second class of stable membrane anchoring motives does not rely on electrostatic interactions but supports the first (often isoprenoid) hydrophobic modification by additional thioester formation with fatty acids (eg. the H- and N-isoforms of Ras or in the a subunits of heterotrimeric G-proteins) or a second isoprenoid moiety (eg. Rab proteins).1331... 

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],... 

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).

Once the 16-carbon atom acyl chain is formed, the thioester link between the acyl group and the 4 -phosphopante-theine of the carrier protein is hydrolysed by a thioesterase and palmitate is released. Synthesis of shorter-chain fatty acids, e.g. myristic (a C-14 carbon acid), requires a specific cytosolic thioesterase (thioesterase II) which is present in liver. It hydrolyses the thioester bond when fatty acids reach lengths of less than 16 carbon atoms. 

Coenzyme A is used as the alcohol part of thioesters, which are more reactive than oxygen esters (see Section 7.9.3) and are thus exploited in biochemistry in a wide range of reactions, e.g. fatty acid biosynthesis and metabolism (see Section 15.5). 

This is a complex molecule, made up of an adenine nucleotide (ADP-3 -phosphate), pantothenic acid (vitamin B5), and cysteamine (2-mercaptoethylamine), but for mechanism purposes can be thought of as a simple thiol, HSCoA. Pre-eminent amongst the biochemical thioesters is the thioester of acetic acid, acetyl-coenzyme A (acetyl-CoA). This compound plays a key role in the biosynthesis and metabolism of fatty acids (see Sections 15.4 and 15.5), as well as being a building block for the biosynthesis of a wide range of natural products, such as phenols and macrolide antibiotics (see Box 10.4). 

Perhaps the most important example of the reverse Claisen reaction in biochemistry is that involved in the P-oxidation of fatty acids, used to optimize energy release from storage fats, or fats ingested as food (see Section 15.4). In common with most biochemical sequences, thioesters rather than oxygen esters are utilized (see Box 10.8). 

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. 

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).

---