The Carboxyl Groups of Aspartic and Glutamic Acids

FIGURE 6.13 Activation of Na-substituted aspartic or glutamic acid in the presence or absence of amine nucleophile gives the anhydride. Aminolysis of the anhydride gives a mixture of two peptides. [Melville 1935 Bergmann et al., 1936], 

FIGURE 6.14 Peptide-bond formation by aminolysis of the A-carboxyanhydride of aspartic or glutamic acid, followed by release of carbon dioxide.48 

the best compromises for Boc and Fmoc chemistries seem to be cyclohexyl and 2,4-dimethylpent-3-yl (Dmpn), which is of intermediate stability, and the removal of which by trifluoromethanesulfonic acid with the aid of thioanisole (see Section 6.22) leads to minimal imide formation (see Section 6.13). Points to note are that acidolysis of esters by hydrogen fluoride can lead to fission at the oxy-car-bonyl bond instead of the alkyl-oxy bond, thus generating acylium ions that can react with nucleophiles (see Sections 6.16 and 6.22), and that benzyl esters may undergo transesterification if left in methanol. The side reactions of cyclization (see Section 6.16) and acylation of anisole (see Section 6.22) caused by acylium ion formation do not occur at the side chain of aspartic acid.47-51 

WJ Le Quesne, GT Young. Amino acids and peptides. Part I. An examination of the use of carbobenzoxyglutamic anhydride in the synthesis of glutamyl peptides. J Chem Soc 1954, 1950. 

RG Denkewalter, H Schwam, RG Strachan, TE Beesley, DF Veber, EF Schoenewaldt, H Barkemeyer, WJ Paleveda, TA Jacob, R Hirschmann. The controlled synthesis of peptides in aqueous solution. I. The use of a-amino acid /V-carboxyanhydrides. J Am Chem Soc 88, 3164, 1966. 

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The best evidence that protein O-methyltransferase methylates the carboxyl groups of aspartic and glutamic acid residues in proteins comes from treating the enzyme methylation product with lithium borohydride followed by hydrolysis in 6N HC1. Two new hydroxy amino acids, y-hydroxy-a-aminobutyric acid and S-hydroxy-a-aminovaleric acid, were obtained (183). 

The maximum acid-combining capacity of keratin fibers, from reaction with simple acids such as hydrochloric, phosphoric, or ethyl sulfuric acid, is approximately 0.75mmol/g for unaltered human hair and about 0.82mmol/g for wool fiber [104]. This value approximates the number of dibasic amino acid residues in the fibers [105] (i.e., the combined amounts of arginine, lysine, and histidine) see Table 5-15.The primary sites for interaction with acid (protons) are probably the carboxylate groups of aspartic and glutamic acids (ionized by interaction with the dibasic amino acid residues) and the dibasic amino acid groups themselves. 

Heating of foods rich in proteins may lead to formation of crosslinking isopeptide bonds between the S-NH2 group of lysine and the p- and y-carboxyl groups of aspartic and glutamic acid residues or their amides. 

The other nitrogen nucleophile available to enzymes is the versatile imidazole ring of histidine. This group is used more often for acid/base chemistry, but it is used occasionally as a nucleophile in, for example, phosphotransfer reactions. The serine proteases, such as a-chymotrypsin, which is illustrated in Figs. 2a and (2)b, are classic examples of the participation of serine as a nucleophile. Additional examples exist of nucleophilic mechanisms that employ the hydroxyl groups of threonine and tyrosine and the carboxylate groups of aspartate and glutamate. 

Several other amino acid side chains may act as nucleophiles. The sulfhydryl group of cysteine, the carboxylate groups of aspartate and glutamate, and the imidazole group of histidine can play this role. 

The side chains of aspartic and glutamic acids carry negatively charged carboxylate groups at pH 7 while those of lysine and arginine carry the positively charged -NH3+ and guanidinium ions, respectively. 

Carboxyl group protection. The a -esters of aspartic and glutamic acids are formed readily using the DCC method, after the geminal functionalities are sequestered (by reaction with EtiB). The acid can be regenerated under conditions lO.l M BU4NF) that do not affect an N-Boc group. 

It is easier to extend wool fibers in acid or alkaline solutions than in neutral solutions [289]. In acid solutions, the carboxylic acid groups of aspartic and glutamic acid residue side-chains lose their negative charges, and in alkaline solution the side-chains of lysine and other basic amino acid residues lose their positive charges. In both cases, there are fewer interactions between positive and negative side-chains ( salt links ) to oppose the extension of the fiber. 

In addition to the 0-methylation of aspartate and glutamate residues, the C-terminal carboxyl group of GTPases are methylated. In the G-protein family C-terminal cysteines are prenylated at the sequence CZZX, where Z is a hydrophobic amino acid and X represents any residue. Once the ZZX sequence is cleaved by a special protease, the isoprenylcysteine carboxymethyltransferase (Icmt) methylates the C-terminal carboxyl group and effectively creates a more hydrophobic enzyme. ... 

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