Thioesters linkage

Step 2, another priming reaction, involves a further exchange of thioester linkages by another nucleophilic acyl substitution and results in covalent bonding of the acetyl group to a cysteine residue in the synthase complex that will catalyze the upcoming condensation step. 

Williams and Ibrahim, 1981). Other nucleophiles also are reactive. Sulfhydryl groups may attack the active species and form thioester linkages, although these are not as stable as the bond formed with an amine. 

Native chemical ligation has been used successfully to couple two unprotected peptides together during solid phase synthesis, wherein one of the peptides is attached to the resin using a thioester linkage and the other peptide is introduced containing a cysteine at its N-terminal... 

LeClaire, J. Vial, L. Otto, S. Sanders, J. K. M. Expanding diversity in dynamic combinatorial libraries Simultaneous exchauge of disulfide and thioester linkages. Chem. Commun. 2005,1959-1961. 

The obvious product of the aldol reaction would be the thioester citryl-CoA. However, the enzyme citrate synthase also carries out hydrolysis of the thioester linkage, so that the product is citric acid hence the terminology. The hydrolysis of the thioester is actually responsible for disturbing the equilibrium and driving the reaction to completion. 

The reaction catalysed by citrate synthase in the Krebs cycle (see Section 15.3) is primarily an aldol reaction, but the subsequent step, hydrolysis of a thioester linkage, is also catalysed by the same enzyme. This is shown below. 

Since the thioester linkage is susceptible to nucleophilic attack but stable to TEA treatment during solid-phase peptide synthesis (using standard Boc protection), Weigel and colleagues " have envisioned that hydroxylamine derivatives could directly cleave resin-bound peptide thioesters 201 or 202 to form the corresponding peptide hydroxamates 203 (Scheme 88). 

Fig. 11 Copolymerization of CL with aliphatic co-mercapto carboxylic acids (top) and transesterification of pCL with aliphatic co-mercapto carboxylic acids (bottom) to give random copolymers with thioester linkages [30]...

Proteins with an isoprenoid modification possess either a C15-famesyl residue or a C20-geranyl-geranyl residue. Both residues are bound via a thioester linkage to a cysteine residue. As with myristoylation, these are constitutive, stable modification performed by farnesyl or geranyl transferases. 

Ei catalyzes first the decarboxylation of pyruvate, producing hydroxyethyl-TPP, and then the oxidation of the hydroxyethyl group to an acetyl group. The electrons from this oxidation reduce the disulfide of lipoate bound to E2, and the acetyl group is transferred into thioester linkage with one —SH group of reduced lipoate. 

The thioester linkage in acetyl-CoA activates the methyl hydrogens, and Asp375 abstracts a proton from the methyl group, forming an enolate intermediate. 

Fig. 1. A schematic image of w.-t. PYP chromophore linked to protein by thioester linkage and hydrogen bonded with surrounding amino acid residues in PNS.

The most hydrophobic integral membrane proteins can be extracted into organic solvents such as mixtures of chloroform and methanol. One such proteolipid protein, the 23.5-kDa lipophilin, accounts for over half the protein of myelin.57 182 The purified protein from rat brain contains 66% of nonpolar amino acids and six molecules of covalently bound palmitic acid and other fatty acids per peptide chain in thioester linkage to cysteine side chains. This protein evidently has four transmembrane helical segments with the six fatty acid chains incorporated into the membrane bilayer. It also has cytoplasmic and extracellular loops, one of which binds inositol hexakisphosphate (Ins P-6). (Fig. 11-9).183 The myelin proteolipid is an essential component of the myelin sheath and defects in this protein are associated with some demyelinating diseases57 which are discussed in Chapter 30. 

Both C3b and C4b bind tightly to cell surfaces, a feature that helps to direct the complement system s attack to the surfaces of invading organisms. This tight binding also involves covalent attachment of macromolecules by reaction with a preformed thioester just as with a2-macroglobulin.hd In fact, the thioester linkage was first discovered in the complement proteins. Both C3 and C4 contain the thioester... 

Polycarboxylic acid synthases. Several enzymes, including citrate synthase, the key enzyme which catalyzes the first step of the citric acid cycle, promote condensations of acetyl-CoA with ketones (Eq. 13-38). An a-oxo acid is most often the second substrate, and a thioester intermediate (Eq. 13-38) undergoes hydrolysis to release coenzyme A.199 Because the substrate acetyl-CoA is a thioester, the reaction is often described as a Claisen condensation. The same enzyme that catalyzes the condensation of acetyl-CoA with a ketone also catalyzes the second step, the hydrolysis of the CoA thioester. These polycarboxylic acid synthases are important in biosynthesis. They carry out the initial steps in a general chain elongation process (Fig. 17-18). While one function of the thioester group in acetyl-CoA is to activate the methyl hydrogens toward the aldol condensation, the subsequent hydrolysis of the thioester linkage provides for overall irreversibility and "drives" the synthetic reaction. 

The mechanism of the cleavage of the pyruvate in Eq. 15-37 is not obvious. Thiamin diphosphate is not involved, and free C02 is not formed. The first identified intermediate is an acetyl-enzyme containing a thioester linkage to a cysteine side chain. This is cleaved by reaction with CoA-SH to give the final product. A clue came when it was found by Knappe and coworkers that the active enzyme, which is rapidly inactivated by oxygen, contains a long-lived free radical.326 Under anaerobic conditions cells convert the inactive form E to the active form Ea by an enzymatic reaction with S-adenosylmethionine and reduced flavodoxin Fd(red) as shown in Eq. 15-38.327-329 A deactivase reverses the process.330... 

The second five-carbon branched unit, in which the branch is one carbon further down the chain, is an intermediate in the biosynthesis of polyprenyl (isoprenoid) compounds and steroids. Three two-carbon units are used as the starting material with decarboxylation of one unit. Two acetyl units are first condensed to form acetoacetyl-CoA. Then a third acetyl unit, which has been transferred from acetyl-CoA onto an SH group of the enzyme, is combined with the acetoacetyl-CoA through an ester condensation. The thioester linkage to the enzyme is hydrolyzed to free the product 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). This sequence is illustrated in Eq. 17-5. The thioester group of HMG-CoA is reduced to the... 

Phosphopantetheine coenzymes form thioester linkages with acyl groups, which they activate for group transfer reactions. 

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