In this chapter, the authors reported that phytochelatines (PCs) and metallothionies (MTs) are actively involved in metal binding and detoxification as observed more in hyperaccumulation plant species. Also, most reports have explained single metal/metalloid detoxification via PCs and MTs; hence, it remains to be seen how plants use these metal ligands at the time of multiple metal stress and generate at the time of defence system against heavy metal stress condition.
TopPhytochelatins
Phytochelatins (PCs) are low molecular weight cysteine rich small polypeptide with a general structure (g-Glu-Cys)nGly, where n 1/42-11 and are not only reported in plants but also have found in fungi and other organisms (Yadav et al., 2010; Mirza et al., 2014). Phytochelatins are one of the most important classes of metal chelators that respond to the harmful effects of a variety of toxic metals. Phytochelatins are known to be synthesized in the cytosol in response to the heavy metal toxicity”. Phytochelatins-metal and Phytochelatins metalloid complexes are very stable in nature and are formed and “sequestration in the vacuolar compartments where the toxic effect metals is of less concern (Shen et al., 2010; Dago et al., 2014). The biosynthesis of PCs is catalyzed by the key enzyme of phytochelatins synthase PCs (Kutrowska and Szelag, 2014).
Phytochelatins belong to a family of peptides which were first discovered as cadmium (Cd)-binding complexes in Schizosaccharomyces pombe exposed to Cd and were named as cadystins (Inouhe, 2005). The amino acids required for the synthesis of this peptide are L-glutamate (Glu), L-cysteine (Cys), and glycine (Gly). Phytochelatins are syntheisized from GSH; therefore, the biosynthetic pathway overlaps with GSH biosynthesis. The general structure of PC oligomer is (g-Glu-Cys)n-Gly where n usually range from (2-5); but has been reported as high as 11 in some species (Sharma et al., 2015; Cobbett, 2000). During the PCs synthesis enzyme of g-glutamylcysteine synthase catalyzes the formation of g-glutamylcysteine from L-glutamate and L-cysteine. Further enzymes glutathione synthase adds glycine to g-glutamylcysteine to form GSH in both these reactions the presence of adenosines triphosphates (ATP) is required (Meister, 1988; Sharma et al., 2015). After the formation of GSH PCs are synthesized in presence of enzyme PC synthase” (Figure 1). The enzyme was normally named g-Glu-Cys dipeptididyl transpeptidase due to transpeptidation of g-Glu-Cys moiety of GSH (Cobbett, 2000; Sharma et al., 2015). However, before transpeptidation, glycine is cleaved from GSH and then in the next step, during transpeptidation, the resulting g-glutamylcysteine dipeptide forms a peptide bond with either GSH to form PC2 or another PC molecule that acts as an acceptor molecule to produce in np 1 oligomer (Clemens, 2006; Sharma et al., 2016).
Figure 1. Biosynthesis pathway of phytochelatines