Thioesters of coenzyme

A number of biosynthetically important enzymes promote aldol related Claisen additions of acetyl thioesters of coenzyme A (CoA) to ketones, often to a-oxoacids. This includes the citrate synthase (EC 4.1.3.7) which is one of the key enzymes in central metabolism, and several others involved in the biosynthesis... 

Sex pheromones in the Lepidoptera are multi-component mixtures consisting mostly of olefinic compounds possessing a terminal aldehyde, alcohol, or acetate moiety. Besides functional group differences, the constituents of lepidopteran sex pheromones vary in hydrocarbon chain length and in the specific number, location, and geometry of double bonds. These chemical structures are formed in biosynthetic pathways involving a limited number of enzymatic steps believed to use fatty-acyl thioesters of coenzyme A (acyl-CoA) as substrates. Key reactions are desaturation, limited [3-oxidation, and a small number of terminal functional group modifications (reviewed in Chapter 3). 

The reactions use thiol esters rather than ordinary esters. The esterifying group is a thiol called coenzyme A, and we shall just represent this molecule as R (you can find its full structure on p. 1386). The first reaction is between a malonate half-thioester and an acetate thioester of coenzyme A Look at the mechanism and you will see how similar it is to the Claisen ester condensation. 

In Section 17.20, you saw that carboxyhc acids can be activated in biological systems by being converted to thioesters of coenzyme A. 

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... 

Plants synthesize natural rubber via what is known as activated acetic acid, the acetic thioester of coenzyme A ... 

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) ... 

Thioesters of coenzyme A (D 11) are important intermediates in carboxylic acid metabolism. They are formed by acid-tbiol bgases (thiokinases) either via acyl phosphates ... 

In a different approach, the reactivity of thiazolium salts derived acyl anion equivalents (biological active aldehyde ) toward sulfur electrophiles has been examined recently (329) and provides a model for the thioester-forming step catalyzed by the lipoic acid containing enzymes. The results suggest that the biological generation of thioesters of coenzyme A from a-keto acids occurs via the direct reductive acylation of enzyme-bound lipoic acid by the active aldehyde, as already shown on page 453. 

Tubbs, P. K., 1963, Inhibition of citrate formation of long chain acyl thioesters of Coenzyme A as a possible control mechanism in fatty acid biosynthesis, Biochim. Biophys. Acta 70 608. 

Acyl CoA s, such as acetyl CoA, are the most common thioesters in nature. Coenzyme A, abbreviated CoA, is a thiol formed by a phosphoric anhydride linkage (0 = P—O—P=0) between phosphopantetheine and adenosine 3, 5 -bisphosphate. (The prefix "bis" means "two" and indicates that adenosine 3, 5 -bisphosphate has two phosphate groups, one on C3 and one on C5. ) Reaction of coenzyme A with an acyl phosphate or acyl adenylate... 

Figure 21.9 Formation of the thioester acetyl CoA by nucleophilic acyl substitution reaction of coenzyme A (CoA with acetyl adenylate.

Step 1 of Figure 29.3 Introduction of a Double Bond The /3-oxidation pathway begins when a fait)7 acid forms a thioester with coenzyme A to give a fatty acyl Co A. Two hydrogen atoms are then removed from C2 and C3 of the fatty acyl CoA by one of a family of acyl-CoA dehydrogenases to yield an a,/3-unsaturated acyl CoA. This kind of oxidation—the introduction of a conjugated double bond into a carbonyl compound—occurs frequently jn biochemical pathways and usually involves the coenzyme flavin adenine dinucleotide (FAD). Reduced FADH2 is the by-product. 

Step 4 of Figure 29.12 Oxidative Decarboxylation The transformation of cr-ketoglutarate to succinyl CoA in step 4 is a multistep process just like the transformation of pyruvate to acetyl CoA that we saw in Figure 29.11. In both cases, an -keto acid loses C02 and is oxidized to a thioester in a series of steps catalyzed by a multienzynie dehydrogenase complex. As in the conversion of pyruvate to acetyl CoA, the reaction involves an initial nucleophilic addition reaction to a-ketoglutarate by thiamin diphosphate vlide, followed by decarboxylation, reaction with lipoamide, elimination of TPP vlide, and finally a transesterification of the dihydrolipoamide thioester with coenzyme A. 

Pathways Providing Coenzyme A Thioesters of other HASCL.104... 

PHA synthases are the key enzymes of PHA biosynthesis. These enzymes catalyze the covalent linkage between the hydroxyl group of one and the carboxyl group of another hydroxyalkanoic acid. The substrates of PHA synthases are the coenzyme A thioesters of hydroxyalkanoic acids there is no evidence that PHA synthases can utilize either free hydroxyalkanoic acids or other derivatives of hydroxyalkanoic acids. With respect to size, structure, and substrate specificity, three different types of PHA synthases (I, II, and III) can be distinguished (see below). 

In addition to the aspects mentioned above, the extent of PHA accumulation might depend on quite different features. The only physiological inhibitor of PHA synthase that has been identified is coenzyme A as pointed out in an earlier section. Since the concentration of coenzyme A in the cytoplasma will probably never rise to very high levels, it may be questionable whether this inhibition is physiologically relevant at all. In our opinion, the availability of a hy-droxyacyl coenzyme A thioester provided by the biosynthesis pathway is most important to initiate PHA biosynthesis (compare also [8] - this book). 

Thioesters are more reactive towards nucleophilic substitution than oxygen esters, and are widely employed in natural biochemical processes because of this property. Coenzyme A is a structurally complex thiol, and functions in the transfer of acetyl groups via its thioester acetyl coenzyme A (acetyl-CoA CH3CO-SC0A). 

The increased acidity associated with thioesters is one of the reasons that biochemical reactions tend to involve thioesters rather than oxygen esters. The most important thiol encountered in such thioesters is coenzyme A (see Box 7.18). 

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). 

Acetyl-CoA (see Box 10.8) is a thioester of acetic acid with coenzyme A. It is a remarkably common intermediate in many metabolic degradative and synthetic pathways, for which the reactivity of the thioester fnnction plays a critical role. There are two major sonrces of the acetyl-CoA entering the Krebs cycle glycolysis via the oxidative... 

The enzyme malate synthase also synthesises (S )-malic acid from glyoxylic acid (OHC-CO2H) and the thioester acetyl-coenzyme A (CH3CO.SC0A this may be regarded as a source of the nucleophile CH2CO.SC0A). 

Lipases are enzymes that catalyze the in vivo hydrolysis of lipids such as triacylglycerols. Lipases are not used in biological systems for ester synthesis, presumably because the large amounts of water present preclude ester formation due to the law of mass action which favors hydrolysis. A different pathway (using the coenzyme A thioester of a carboxylic acid and the enzyme synthase [Blei and Odian, 2000]) is present in biological systems for ester formation. However, lipases do catalyze the in vitro esterification reaction and have been used to synthesize polyesters. The reaction between alcohols and carboxylic acids occurs in organic solvents where the absence of water favors esterification. However, water is a by-product and must be removed efficiently to maximize conversions and molecular weights. 

RGURE 13-6 Hydrolysis of acetyl-coenzyme A Acetyl-CoA is a thioester with a large, negative, standard free energy of hydrolysis Thioesters contain a sulfur atom in the position occupied by an oxygen atom in oxygen esters. The complete structure of coenzyme A (CoA, or CoASH) is shown in Rgure 8-41. 

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