Discovering the primary structure of phytochelatins as compounds with repeating y-carboxyamide linkages led to a paradigm shift away from gene control of peptide synthesis on ribosomes to enzymatic biosynthesis. The constancy of in the carboxy terminus of each member of the phytochelatin family suggested a biosynthesis dependent on glutathione. This hypothesis is now supported in various ways. The situation for the other families of yGluCys peptides remains unclear. Whether specific biosyntheses are simply a dependence on the availability of monothiols (e.g. glutathione versus homoglutathione in certain Fabales) is unclear. In other plants the origins of the pertinent monothiols and yGluCysGln are unclear. Does their biosynthesis parallel that of glutathione or are they degradation products of polythiols originally derived from glu-tathione?
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