Chemical modification of amino acid

As described for lEC, elution is done by a stepwise or a continuous change of buffer composition. The mildest elution buffer is an aqueous buffer with low ionic strength, e.g., 20 mm Tris-HCl. If it is not successful, desorb with a chaotropic solvent, e.g., 2 M potassium rhodanide (thiocyanate), 2.5 M guanidinium hydrochloride, up to 7 M urea, or with increasing concentrations of methanol or acetonitrile. Especially the use of rhodanide or urea may be accompanied by a chemical modification of amino acid side chains, which disturbs amino acid analysis. 

Lundblad, R. (1996) Chemical modification of amino acid side-chains, in Proteins Labfax (Pace,... 

Although aggregation is the predominant means by which proteins become inactivated during refolding, several other inactivation pathways have also been observed. Proteins can be inactivated by thiol-disulfide exchange or alteration of the primary structure by chemical modification of amino acid side chains. In addition, refolded proteins may be inactivate due to the absence of prosthetic groups and metals or because of improper association of the subunits in multimeric proteins (79). 

Many Proteins Undergo Chemical Modification of Amino Acid Residues... 

TABLE 11.9 Commonly used chemical modifications of amino acid functional groups... 

Attempts have been made to study the active site by chemical modification of amino acid side chains (Messner e/ aL, 1970 Thrasher / a/., 1975 Thrasher and Cohen, 1971). No attempts have been made to separate the various products of the modification reactions and to study the individual homogeneous populations of modified proteins. Collectively, however, the results of these studies would appear to implicate an amino group in the cytophilic site. The data of Ciccimarra et al. (1975) suggest that there are two lysine residues in a decapeptide containing the site, but the positions of the modified amino groups have not been ascertained, nor has the effect of these reagents on other side chains and on conformations been studied. 

PR Gratzer, J.P Santerre, and J.M. Lee, Modulation of collagen proteolysis by chemical modification of amino acid side-chains in acellularized arteries. Biomaterials, 25 (11), 2081-2094, 2004. 

Although the AalT molecule has several characteristics in common with other toxins isolated from A. australis, such as a conserved hydrophobic surface conducive to binding to its receptor, AalT is unique in that it possesses an atypical disulfide bridge and an unusually long C-terminus (42). It has also been demonstrated that chemical modification of amino acid residues of the AalT molecule results in a decreased toxicity level to insects (47). 

From studies of chemical modification of amino acid residues studied by many investigators, it was shown that the ones located in loop B are essential for neurotoxicity. For instance, the Arg-31, Arg-34, Trp-37, Tyr-23, Lys-24, and Lys-25 are known to be related to neurotoxicity. It is logical to assume that loop B is most likely to bind to the AChR. 

Chemical modification of amino acid side chain functionalities will also serve to cleave specific peptide bonds selectively. Chemical cleavage of a polypeptide chain exploits the unique reactivity of chemically modified side chains of particular amino acids in the labilization of adjacent peptide bonds by neighbouring group participation (68). The residues investigated so far for this purpose have been methionine, cysteine and the aromatic amino acids including tryptophan (438-440, 443). 

Chemical modification of amino acid side chains in enzymes has been carried out to induce changes in the biological activity, in the conformation and the physical properties of an enzyme or to introduce special-purpose groups into a protein ( 7, 82, 146, 169, 183, 230, 375, 413). 

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