Phytochelatins, the heavy-metal-binding peptides of plants, are synthesized from glutathione by a specific γ-glutamylcysteine dipeptidyl transpeptidase (phytochelatin synthase)

Abstract

An enzyme has been discovered and characterized from Silene cucubalus cell suspension cultures that catalyzes the transfer of the y-glutamylcysteine dipeptide moiety of glutathione to an acceptor glutathione molecule or a growing chain of [Glu(-Cys)],-Gly oligomers, thus synthesizing phytochelatins, the metal-binding peptides of higher plants and select fungi. The enzyme was named y-glutamylcysteine dipeptidyl transpeptidase and given the trivial name phytochelatin synthase. The primary reaction catalyzed is [Glu(-Cys)]-Gly + [Glu(-Cys)],,-Gly -* [Glu(-Cys)],k+-Gly + Gly. The enzyme is isoelectric near pH 4.8 and has temperature and pH optima at 35'C and 7.9, respectively. Phytochelatin synthase is constitutively present in cell cultures of various plant species and its formation is not noticeably induced by heavy metal ions in the growth medium. The enzyme (Mr 95,000) seems to be composed of four subunits, the dimer (Mr 50,000) being also catalytically active. Cd2+ is by far the best metal activator of the enzyme followed by Ag+, Bi3+, Pb2+, Zn2+, Cu2+, Hg2+, and Au+. The Km for glutathione is 6.7 mM. The enzyme activity seems to be self-regulated in that the product of the reaction (the phytochelatins) chelates the enzyme-activating metal, thus terminating the enzyme reaction. The molar ratio of the y-glutamylcysteine dipeptide in phytochelatin to Cd2+ in the newly formed complex was 2:1.

Cadmium induced phytochelatins in marine algaPhaeodactylum tricornutum:effect of metal speciation

“…A ratio of 1.9 ± 0.4 sulfhydryl groups to cadmium in the isolated complex can be calculated. This value is similar to the molar ratio of Cd to cystein found in purificated PCs from plants and algae found by others (Gekeler et al, 1988;Grill et a/.,1985;Grill et a/.,1989;de Knecht et al, 1994).…”

Reduced translocation of cadmium from roots is associated with increased production of phytochelatins and their precursors

“…Once PCs take over the detoxification process, GSH gets involved in a secondary defense mechanism by scavenging free radicals in Cd 2+ -induced oxidative stress (Gallego et al, 2005;Ranieri et al, 2005). Thus, GSH is not only a precursor for PC synthesis (Grill et al, 1989) but also an important antioxidant in plants. In the present study, the amount of GSH was always higher in barley, probably contributing to higher chelation of Cd 2+ compared to lettuce.…”

Interactions of fungi with toxic metals

“…The similarity in structure between glutathione and y-glutamyl peptides indicates that biosynthesis of these peptides shares common features with tbat of glutathione (Steffens, 1990) and, in fact, they may be viewed as linear polymers of the y-glutamylcysteine (y-Glu-Cys) portion of glutathione (Grill et al, 1989). Because of the repetitive y-glutamic acid bonds, they cannot be regarded as primary gene products and must be formed by a ribosomeindependent enzyme reaction (Steffens, 1990).…”

“…(y-Glu-Cys) Gly + (yGlu-Cys)^Gly ( y-Glu-Cys),,^,Glŷ "^ is the best activator of the enzyme followed by Ag^ Bi^^ Vh^\ Zn•^^ Cu^^ Hg^^ and Au+ and it appears that the reaction product (y-glutamyl peptide) chelates the activating metal ions thus terminating the reaction (Grill et al, 1989;Loeffler et al, 1989). In cell-free systems of S. pombe, an additional pathway has also been discovered which involves yEC polymerization from (yEC)" and glutathione to (yEC)^^j followed by addition of glycine with glutathione synthetase (Hayashi et al, 1991).…”

“…It is now known that y-glutamyl peptides are synthesized by the enzyme y-glutamylcysteine dipeptidyl transpeptidase ('phytochelatin synthase'). This catalyses the transfer of the yglutamylcysteine dipeptide moiety of glutathione to an acceptor glutathione molecule or a growing chain of (y-Glu-Cys)^-Gly oligomers, thus synthesizing metal-binding y-glutamyl peptides (Grill et al, 1989). The primary reaction can be expressed:…”