The role of phytochelatins in plant and animals: A review

Increasing emissions of heavy metals such as cadmium, mercury, and arsenic into the environment pose an acute problem for all organisms. As a mass of protection, many of them, develop mechanisms of full resistance or at least exhibit partially resisting toward these effects. In this way, based on the chemical similarity of the involved metallic species, they are able, to replace them with viable metals necessary for the effective functioning of the cell. These heavy metals may be bound to the functional groups of proteins and modify their structure and through this also affect their physiological function ^{1, 2}. Higher plants, algae, certain yeasts and animals are able to respond to heavy metals by synthesizing phytochelatins (PCs) and related cysteine-rich polypeptides. Phytochelatin synthases are γ-glutamylcysteine (γ-Glu-Cys) dipeptidyl transpeptidases that catalyze the synthesis of heavy metal-binding PCs ^{3, 4}. PCs, cysteine-rich peptides, are produced from glutamine, cysteine and glycine. Unlike commonmetal-binding structures, MT and GSH, PCs are not gene-encoded, but enzymatically synthesized peptides ⁵. PCs have been identified in a wide variety of plant species, microorganisms and some invertebrates ^6-10. They are structurally related to glutathione (GSH) and were presumed to be the products of a biosynthetic pathway. Numerous physiological, biochemical and genetic studies have confirmed GSH as the substrate for PCs biosynthesis ^{11, 12}. The general structure of PCs is (c-Glu-Cys)n-Gly, with increasing repetitions of the dipeptide Glu-Cys linked through a c-carboxylamide bond (Fig 1), where n varies from 2 to 11, but typically reaching not further than five ¹³. Except glycine, also other amino acid residues can be found on C-terminal end of (γ-Glu-Cys)n peptides. Examples of which, like Ser, Glu, Gln and Ala are often found at this position in some plant species, and they are assumed to be functionally analogous and synthesised via essentially similar biochemical pathways ^{14, 15}. In in vitro studies of PC synthase expressed in E. coli or in S. cerevisiae, the enzyme was activated to varying extents by Cd, Cu, Ag, Hg, Zn and Pb ions ^16-18. PC synthase genes were also isolated in A.thaliana ¹⁶ and T.aestivum ¹⁸. Genes homologous to those from A.thaliana and T.aestivum were also found in S.pombe and C.elegans, suggesting the existence of PC synthase genes in more species ¹⁹.

Figure 1. a) The general structure of PCs. b) Biosynthesis of phytochelatins.

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