Glutamine residues carboxylate

Brignon et al. (1969) demonstrated that the maximum acid-binding capacity of /3-lactoglobulin D is the same as that of the other variants. The curves are identical at pH 4.0. At pH 6.5, one less proton is dissociated in the titration of the D variant than with the B variant, as would be predicted from the substitution of a glutamine residue for a glutamic acid residue in B. The anomalous carboxyl group observed in the other variants is also detected in the D variant. 

This was first foimd by Sanger et al. (1955) in a peptide from insulin and was observed with other peptides by Hirs et al. (1956) and Smyth et al. (1962). The reaction appears to occur when acidic buffers or dilute acids are employed for isolation of peptides. Conversion of the cyclic pyrrolidone carboxyl residue to a glutamyl residue is obtained on mild hydrolysis in dilute acids or alkalies. The cyclization reaction leads to difficulties when sequence methods are used which proceed from the amino-terminal end of a peptide. In addition, this reaction can occur when an internal glutamine residue becomes amino-terminal in the course of stepwise sequence analysis under acidic conditions, as in the Edman methods. An incorrect sequence for a peptide from ribonuclease was deduced as the result of cyclization of amino-terminal glutamine and acidic destruction of serine and threonine in the same peptide (Smyth et al., 1962). 

The hydroxylation of certain proteins occurs by the addition of the OH group to proline and lysine residues in the protein. Hydroxylation is mediated in the presence of vitamin C as a cofactor for the respective hydroxylases. Other proteins may undergo carboxylation of glutamine residue. This process requires vitamin K as a cofactor. 

A determination of the extra formaldehyde and ammonia produced in this reaction gives an estimation of the number of free a-carboxyl groups, and the residue on which they are located is identified from the nature of the amino alcohol and of the aldehyde R-CHO. Chibnall and Rees have also used this technique to determine the distribution of the protein amide groups between asparagine and glutamine. Residues of aspartic or glutamic acid which have a free oo-carboxyl group are destroyed by the above treatment, while those in amide form remain intact and can be estimated after hydrolysis of the protein. 

The general approach for carboxyl protection is esterification. The simplest solution, the use of methyl or ethyl esters, is suitable for semipermanent blocking, although the commonly applied process of unmasking, alkaline hydrolysis, is far from unequivocal. It is accompanied by racemization, partial hydrolysis of carboxamide groups in the side chain of asparagine and glutamine residues and by several other side reactions which are initiated by proton abstraction (Cf. Chapter VII). Nevertheless, perhaps because of the attractively simple esterification of amino acids... 

The alkaline treatment of collagen causes hydrolysis of carboxyamides of asparagine and glutamine residues from collagen in carboxylic groups [41]. 

The acids are not present in the oil as free carboxylic acid molecules instead they are bound to a glutamine residue as a coiyugate . Corynebacteria, which are present on the skin, break the conjugates down, releasing the carboxylic acids, so we start to smell. 

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