Glutamic and Aspartic Acid Residues

These amino acid residues are usually esterified with methanolic HCl. There can be side reactions, such as methanolysis of amide derivatives or N,0-acyl migration in serine or threonine residues  

The nucleophilic reactivity of the reagents decreases in the series hydride arsenite and phosphite alkanethiol aminoalkanethiol thiophenol and cyanide sulfite OH p-nitrophenol thiosulfate thiocyanate. Cleavage with sodium borohydride and with thiols was covered in Section 1.2.4.3.5. Complete cleavage with sulfite requires that oxidative agents (e. g. Cu +) be present and that the pH be higher than 7  

Diazoacetamide reacts with a carboxyl group and also with the cysteine residue  

Amino acid esters or other similar nucleophilic compounds can be attached to a carboxyl group of a protein with the help of a carbodi-imide  

Amidation is also possible by activating the carboxyl group with an isooxazolium salt Woodward reagent) to an enolester and its conversion with an amine. 

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The skin is a primary target of sulphur mustard. Exposure of human callus to 14C-sulphur mustard showed that a significant part (15-20%) of the radioactivity was covalently bound to keratin (Van der Schans, G.P. et al., 2003). Approximately 80% of the bound radioactivity could be released by alkaline hydrolysis, indicating it to be bound as esters of glutamic and aspartic acid residues. 

Table 4 examines the amino acid composition of purified SCP2 [21] and compares it with the amino acid compositions of 3 other preparations of interest [47-49], The most abundant amino acid in purified SCP2 is lysine, which accounts for 14 mole%. Since the isoelectric point for SCPj is approximately 8.6, it follows that a substantial portion of the glutamic and aspartic acid residues are amidated. Also of interest is the finding that SCPj contains no arginine or tyrosine, and only small amounts (perhaps 1 residue each) of histidine and tryptophan. 

P. Rastogi, W. Kristof, and G. Zundel, Easily Polarisable Proton Transfer Hydrogen Bonds Between the Side Chains of Histidine and the Carboxylic Acid Groups of Glutamic and Aspartic Acid Residues in Proteins, Int. J. Biol Macromol 3, 154-158 (1981). 

Molecules of nitrogen-containing compounds, such as alkaloids, amines, and peptides, usually contain (under physiological conditions) positively charged N-atoms that can form ionic bonds with negatively charged amino acid residues of glutamic and aspartic acid in proteins. Both the covalent and the noncovalent interactions will modulate the three-dimensional protein structure, that is, the conformation that is so important for the bioactivities of proteins (enzymes. 

Data of Table III agree in some detail with the physical conclusion that elastin is not a collagen. The former is relatively low in residues with polar side chains hydroxyproline, glutamic and aspartic acids, arginine, lysine, serine, and threonine. It is particularly rich in glycine, alanine, the leucines and valine, residues whose side chains are nonpolar. Elastin and collagen are similar only in their moderately high proline contents and in their low proportions of histidine, cystine, tyrosine, and tryptophan. 

Later, other workers confirmed these findings which are summarized in Table 2. Thus isopeptides to date have been found to be formed during heating in keratin (40), milk proteins (41), muscle proteins (42), ribonuclease (4 )" and other proteins of nutritional interest (44). The ease of formation of the isopeptide in any specific protein should be dependent on two factors firstly, the concentration of glutamic and aspartic acids in relation to the concentration of lysine and, secondly, the proximity of the lysyl and carboxylic residues which will be determined by the conformation of the protein chain. 

Lys (K) residues exhibit similar nonlinear pKa shifts, in this case to lower pKa values, on increasing hydrophobicity by increasing the number of phenylalanine residues per 30-mer, but the magnitudes of the shifts are less than for glutamic and aspartic acids. "... 

The substrate was composed of synthetic peptides, immobilized to a mica surface. The etching enzyme was Staphylococcal serine V8 protease, an enzyme that recognizes either glutamic or aspartic acid residues in the peptide and digests the peptides carboxyl acid terminus [1]. The enzyme was immobilized to the apex of an AFM probe, and the enzyme functionalized tip was scanned across the surface, in solution (phosphate buffer). 

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