Naturally-occurring amino acid derivatives in proteins

Naturally-occurring amino acid derivatives in proteins 

An ever-increasing number of amino acid derivatives is found to occur naturally in proteins. These include s-N-methyllysines, e-N-acetyl-lysine, a-N-acetyl derivatives of various residues, N-methylhistidines, N-phosphohistidines, N-methylarginines, phosphoserine, phospho-threonine, hydroxyproline, hydroxyJysine, trimethylhydroxylysine, tyrosine sulfate, iodotyrosines, carbohydrate moieties and coenzymes 

To the investigator studying chemical modification of proteins, derivatives like those listed above are of interest for two primary reasons (1) some derivatives are sites of modification and (2) complete modification of a particular class of amino acid functional groups with a reagent may not be possible because of the presence of these derivatives. For any protein being investigated by chemical modification and containing these types of derivatives, any results and conclusions must account for the effects of these derivatives. 

---