Ester linkages, covalent

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. 

The hemicelluloses are soluble in alkali, and can therefore be readily separated from the cellulose component by alkali extraction. However, this can only be done when the wood has first been delignified. This is because they are probably linked to lignin via covalent ester linkages (see Chapter 3) which need to be cleaved... 

In addition to ester-linkages, hydroxycinnamic acids can also be covalently linked to cell wall components via phenyl ether bonds. While this has only been demonstrated for wheat (Triticum aestivum L.) straw lignin... 

Scheme 15.2 Covalent imprinting of a-phenyl-D-mannopyranoside in a divinyl benzene/4-vinylphenylboronic acid matrix, via the formation of covalent boionic ester linkages between the 4-vinylphenylboronic acid and the carbohydrate. Adapted from Wulff, Vesper, et al. (1977). Copyright 1977 Wiley InterScience.

Chymotrypsin enhances the rate of peptide bond hydrolysis by a factor of at least 109. It does not catalyze a direct attack of water on the peptide bond instead, a transient covalent acyl-enzyme intermediate is formed. The reaction thus has two distinct phases. In the acylation phase, the peptide bond is cleaved and an ester linkage is formed between the peptide carbonyl carbon and the enzyme. In the deacylation phase, the ester linkage is hydrolyzed and the nonacylated enzyme regenerated. 

Amino acid attachment site Each tRNA molecule has an attachment site for a specific amino acid at its 3 -end (Figure 31.6). The carboxyl group of the amino acid is in an ester linkage with the 3-hydroxyl of the ribose moiety of the adenosine nucleotide at the 3 -end of the tRNA. [Note When a tRNA has a covalently attached amino acid, it is said to be charged when tRNA is not bound to an amino acid, it is described as being uncharged.] The amino acid that is attached to the tRNA molecule is said to be activated. 

Solid-phase synthesis is usually done on a silica support with a covalently attached succinamide as shown in Eq. 5-25. The first nucleotide at the 3 end of the chain to be synthesized is attached by an ester linkage to the bound succinamide (step a, Eq. 5-25). The 5 -protecting group is removed in step b and the 5 -OH reacts with the activated phosphine of the second nucleotide (step c, Eq. 5-25). Steps b and c are then repeated as often as necessary to complete the chain. The finished polynucleotide can be removed from the solid support, the cyanoethyl groups removed... 

In standard aqueous media, hydrolases are enzymes which are able to hydrolyse covalent bonds (Fig. 1). Three classes of hydrolases are used industrially osidases (glycosidic linkage hydrolysis), proteinoses (peptidic linkage hydrolysis) and esterases (ester linkage hydrolysis). 

In 1994 Shea et al. reported the preparation of gel-like imprinted polymers with enantioselective esterolytic activity toward the Boc-D-phenyl-alanine p-nitrophenol ester (28) [19]. The polymers were prepared using a covalent approach, rather than metal complexes or non-covalent interactions, by attaching the catalytic phenol-imidazole unit to the TSA phosphonate via ester linkage (29). The imprinted polymer, containing the catalytic unit (30), showed little selectivity toward the D-enantiomer used for the imprinting. 

Functionalisation with bulky hydrophobic carboxylic acids/DCC was studied for the synthesis of amphiphilic polymers based on dextran. Bile acid is covalently bound to dextran (Fig. 27) through ester linkage in the presence of DCC/DMAP (added in dichloromethane) as coupling reagent. 

The oxidative introduction of carboxylic functions to nanotubes provides a large number of CNT-functional exploitations and permits covalent functionalization by the formation of amide and ester linkages and other carboxyl derivatives [24]. Bifunctional molecules (diamines, diols, etc.) are often utilized as linkers. More illustrative examples are nanotubes decorated with amino-functionalized dendrimers, nucleic acids, enzymes, etc., and the formation of bioconjugates of CNTs [96]. 

Simultaneous noncovalent and covalent functionalizations occur in the inter-tubulary dimers and excimers already depicted in Scheme 1.10. While the pyrene tether is covalently bound through an ester linkage with one individual SWCNT, an SWCNT dimer is formed by jt-jt interactions of the polyaromatic pyrene system with a neighboring CNT [138]. 

Figure 12.3 (A) Mechanism of inactivation of the 20.S proteasome by NPI-0052. (B) Superposition of NPI-0052 (SalA) and salinosporamide B (SalB) in the P5 subunit of the yeast 20.S proteasome. Each inhibitor is covalently bound to the N-terminal threonine (Tl) via an ester linkage between Thr lOy and the carbonyl derived from the P-lactone ring. In the case of NPI-0052, the chlorine atom is displaced to form a five-membered cyclic ether ring.15...

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