Termination of the phytochelatin synthase reaction through sequestration of heavy metals by the reaction product

The newly discovered enzyme γ-glutamylcysteine dipeptidyl transpeptidase was purified to apparent homogeneity from cell suspension cultures of Silene cucubalus. The enzyme catalyzes, in the presence of heavy metal ions, the formation of metal chelating peptides, the phytochelatins, from glutathione. The stoichiometry of the transfer reaction for the first phytochelatin member was determined to be: 2 (γ-Glu-Cys)-Gly → (γ-Glu-Cys)₂-Gly + Gly. The enzyme is self regulated in that the reaction products, the phytochelatins, chelate the enzyme activating metal, thus terminating the enzymatic reaction. The higher order phytochelatins have a higher relative complexing affinity than the lower ones, as judged from their ability to terminate the enzymatic reaction.