“Metallothionein‐like” metal complexes in angiosperms; their structure and function

Evidence for the presence of the metal‐binding protein metallothionein, MT, in higher plants is equivocal. Although a number of MT‐like metal complexes have been isolated from plants, the chemical structures of most of these compounds have not been fully elucidated. Recently a novel class of plant peptides, poly (γ‐glutamylcysteinyl) glycines, (γEC) n G, have been discovered. These peptides bind metal ions, and in the presence of such ions the amount of (γEC), G in plant cells increases. The presence of peptide bonds through the γ‐carboxyl group of glutamate, rather than the α‐carboxyl group, suggests that these peptides are not encoded by structural genes but are the products of biosynthetic pathways. Cells which are resistant to supra‐optimal concentrations of certain metal ions over‐produce (γEC) n G. (γEC) n G. may be functional analogues of MT. Whether or not some plants also produce MT is an important question which remains to be answered.