Glutamyl Derivatives of Protein Amino Acids

The y-glutamyl derivatives of protein amino acids so far identified in plants are listed in Table 1. The table gives information on the character of the isolated material and attempts in this manner to give an indication of the reliability of the identification. In many cases no assignment of the configuration of the amino acids has been made but L-configurations are assumed. However, this cannot be taken for granted, especially 

Compound Plant species Plant family Character of Glutamic The Ref. of Y-glutamyl  

Y-L-Glutamylglycine Tulipa cjesneriana. Liliaceae Evap. residue. 53) 2 191) 

Y-L-Glutamyl-L-alanine Iris tincjitana, leaves Iridaceae Cryst. L L 231) 2 279) 

Y-L-Glutamyl-L-valine Allium cepa, bulbs Alliaceae Cryst. L L 356) 2 279) 

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Table 1. y-Glutamyl Derivatives of Protein Amino Acids Identified in Plants... 

All the y-glutamyl derivatives of protein amino acids listed in Table 1, with the exception of y-glutamylarginine, have been synthesized. Information on these syntheses has been included but is not comprehensive. 

In addition to these ubiquitous amino acids, an additional 700 amino acids that are not involved in primary metabolism have been reported. About 300 of these are found in plants (Hunt, 1991). Most nonprotein amino acids occur free, as their 4-glutamyl (or y-glutamyl) derivatives, or linked to carbohydrate moieties. There are two main types of nonprotein amino acids. A number of nonprotein amino acids are similar to primary amino acids others are unrelated to any of these. Members of the first group may arise in three principal ways. Some nonprotein amino acids arise by simple structural modifications of protein amino acids others arise by modification of the pathways leading to protein amino acids a third group arises by novel routes. 

As is apparent from the tables, most of the y-glutamyl derivatives have been found in seeds, although some have been identified in fresh plant parts. This is due partly to the fact that seeds have been more thoroughly investigated because of their availability and economic importance, but it also reflects the general tendency for non-protein amino acids and y-glutamyl derivatives to occur in high concentrations in seeds but in low concentrations or not at all in fresh plant parts (cf. Section VII. 5). 

In the case of a few y-glutamyl derivatives of non-protein amino acids the parent amino acid has not been found in natural material. Furthermore, with a number of the y-glutamyl derivatives of amines specific problems have been encountered. In these cases a detailed individual discussion is of interest. 

Allium vegetables and various other plants contain small amounts of amino acids derived from cysteine and methionine, in which sulfur is replaced by selenium. These amino acids are the main organic form of selenium and, together with Se-containing proteins, represent the main selenium source in foods (see Section 6.2.3.1). Examples of these amino acids are Se-alk(en)ylcysteines and their y-glutamyl derivatives. 

V. Lakshman, P.G. Schultz, An archaebacteria-derived glutamyl-tRNA synthetase and tRNA pair for unnatural amino acid mutagenesis of proteins in Escherichia coli, Nucleic Acids Res. 2003, 31, 6700-6709. 

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