Thioesters 3-phosphate

Laser desorption/ionisation FT-MS and FAB-MS have been used to determine nonvolatile polymer components such as thioester, phosphate, phosphonate and hindered-amine type polymer additives [33]. Laser desorption/FT ion cyclotron resonance MS has been used to identify and determine the following types of polymer additives [34] UV absorbents, e.g., Tinuvin [28], antioxidants, e.g., Irganox MD-1024, and amide wax anti-slip additives. 

Process 5, the conversion of hydroperoxides to alkoxy and hydroxyl radicals, can be interrupted by incorporation of a secondary antioxidant such as phosphites (e.g. Irgafos 168) or thioesters (e.g. Evanstab 12). These materials act as reducing agents, converting hydroperoxides to alcohols and themselves being converted to phosphates or sulfoxides, respectively (see Fig. 16). 

As shown in Figure 16.10, this reaction mechanism involves nucleophilic attack by —SH on the substrate glyceraldehyde-3-P to form a covalent acylcysteine (or hemithioaeetal) intermediate. Hydride transfer to NAD generates a thioester intermediate. Nucleophilic attack by phosphate yields the desired mixed carboxylic-phosphoric anhydride product, 1,3-bisphosphoglycerate. Several examples of covalent catalysis will be discussed in detail in later chapters. 

The mechanism of succinyl-CoA synthetase is postulated to involve displacement of CoA by phosphate, forming succinyl phosphate at the active site, followed by transfer of the phosphoryl group to an active-site histidine (making a phosphohistidine intermediate) and release of succinate. The phosphoryl moiety is then transferred to GDP to form GTP (Figure 20.13). This sequence of steps preserves the energy of the thioester bond of succinyl-CoA in a series of high-energy intermediates that lead to a molecule of ATP ... 

Closely related to the carboxylic acids and nitriles discussed in the previous chapter are the carboxylic acid derivatives, compounds in which an acyl group is bonded to an electronegative atom or substituent that can net as a leaving group in a substitution reaction. Many kinds of acid derivatives are known, but we ll be concerned primarily with four of the more common ones acid halides, acid anhydrides, esters, and amides. Esters and amides are common in both laboratory and biological chemistry, while acid halides and acid anhydrides are used only in the laboratory. Thioesters and acyl phosphates are encountered primarily in biological chemistry. Note the structural similarity between acid anhydrides and acy) phosphates. 

Electronically, we find that strongly polarized acyl compounds react more readily than less polar ones. Thus, acid chlorides are the most reactive because the electronegative chlorine atom withdraws electrons from the carbonyl carbon, whereas amides are the least reactive. Although subtle, electrostatic potential maps of various carboxylic add derivatives indicate the differences by the relative blueness on the C-O carbons. Acyl phosphates are hard to place on this scale because they are not used in the laboratory, but in biological systems they appear to be somewhat more reactive than thioesters. 

As a consequence of these reactivity differences, it s usually possible to convert a more reactive acid derivative into a less reactive one. Acid chlorides, foi instance, can be directly converted into anhydrides, thioesters, esters, and amides, but amides can t be directly converted into esters, thioesters, anhydrides, or acid chlorides. Remembering the reactivity order is therefore a way tc keep track of a large number of reactions (Figure 21.2). Another consequence, a noted previously, is that only acyl phosphates, thioesters, esters, and amides are... 

The aldehyde intermediate can be isolated if 1 equivalent of diisobutvl-aluminum hydride (D1BAH) is used as the reducing agent instead of LiAlH4. The reaction has to be carried out at -78 °C to avoid further reduction to the alcohol. Such partial reductions of carboxylic acid derivatives to aldehydes also occur in numerous biological pathways, although the substrate is either a thioester or acyl phosphate rather than an ester. 

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... 

Chemistry of Thioesters and Acyl Phosphates Biological Carboxylic Acid Derivatives 817... 

Carboxylic acids can be transformed into a variety of carboxylic acid derivatives in which the carboxyl -OH group has been replaced by another substituent. Acid halides, acid anhydrides, esters, and amides arc the most common such derivatives in the laboratory thioesters and acyl phosphates are common in biological molecules. 

However, there are also biogenesis models which do not require phosphate, such as the inorganic hypothesis of the origin of life proposed by Cairns-Smith (see Sect. 7.1), the thioester world proposed by de Duve (see Sect. 7.4) or the sulphur-iron world suggested by Wachtershauser (see Sect. 7.3). The RNA world (see Chap. 6), however, cannot exist without phosphate. 

L. Weber, then at the Salk Institute in San Diego, was able to form high-energy thioesters from glycerinealdehyde and A-acetylcysteine. The reaction occurred under anaerobic conditions, at pH 7, in an aqueous solution of sodium phosphate. Of the aldehyde, 0.3% was converted to the lactoyl thioester per day of reaction (Weber, 1984). 

The thioester hypothesis can be summed up as follows the formation of thiols was possible, for example, in volcanic environments (either above ground or submarine). Carboxylic acids and their derivatives were either formed in abiotic syntheses or arrived on Earth from outer space. The carboxylic acids reacted under favourable conditions with thiols (i.e., Fe redox processes due to the sun s influence, at optimal temperatures and pH values) to give energy-rich thioesters, from which polymers were formed these in turn (in part) formed membranes. Some of the thioesters then reacted with inorganic phosphate (Pi) to give diphosphate (PPi). Transphosphorylations led to various phosphate esters. AMP and other nucleoside monophosphates reacted with diphosphate to give the nucleoside triphosphates, and thus the RNA world (de Duve, 1998). In contrast to Gilbert s RNA world, the de Duve model represents an RNA world which was either supported by the thioester world, or even only made possible by it. 

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). 

When we investigate this substrate-level phosphorylation reaction in detail, we find it also involves a molecule of phosphate. Phosphate reacts initially with succinyl-CoA, converting the thioester into an acyl phosphate, which is, of course, a mixed anhydride (see Box 7.27). It is actually hydrolysis... 

The natural substrates of lipases are triglycerides and, in an aqueous environment, lipases catalyze their hydrolysis into fatty acids and glycerol. In anhydrous media, lipases can be active in the reverse reaction [19]. In fact, in the acylation step, acids, lactones, (cyclic) carbonates [20, 21], cyclic amides [22, 23], (cyclic) thioesters [24, 25], and cyclic phosphates [26] have been found to act as suitable acyl donors. In the deacylation step, apart from water, lipases also accept alcohols [27], amines [28, 29], and thiols [30] as nucleophiles although the specificity of lipases is lower for amines and thiols than for water and alcohols [31]. 

The above reagents serve as condensing reagents and have different reactivities for peptides 279, p-lactams 281, esters, thioesters, and mixed phosphates, as well as for the direct preparation of 3-acyl-2(3F/)-oxazolones. The bis(2-oxo-3-oxazohnyl)phosphinate 282 is useful for Zr(IV)-catalyzed phosphorylation of alcohols, leading to the general synthesis of acid- and base-labile mixed phosphate esters 284 (Fig. 5.67). ... 

The linear peptide thioester was dissolved in 0.2 M phosphate buffer (pH 7.2, 0.1-0.6 mM soln) containing 50% DMF and TCEP (6 equiv). After completion of the reaction (3-24 h, monitored by HPLC) the cyclic peptide was purified by preparative HPLC (small amounts of byproducts result from hydrolysis of the peptide thioester). 

FIGURE 16-3 Coenzyme A (CoA). A hydroxyl group of pantothenic acid is joined to a modified ADP moiety by a phosphate ester bond, and its carboxyl group is attached to /3-mercaptoethylamine in amide linkage. The hydroxyl group at the 3 position of the ADP moiety has a phosphoryl group not present in free ADP. The —SH group of the mercaptoethylamine moiety forms a thioester with acetate in acetyl-coenzyme A (acetyl-CoA) (lower left). 

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