植物络合素和植物络合素合酶的研究

Research advances in Phytochelatins and Phytochelatin synthase

植物络合素 ( Phytochelatins,PCs)是由于重金属离子诱导而在植物体内合成的一类小分子多肽 ,其结构式为 ( γ- Glu- Cys) n- Gly,( n=2～ 1 1 ) ;PCs能够螯合重金属 ,从而起到对重金属解毒的作用。PCs并非基因的直接产物 ,而是由植物络合素合酶 ( phytochelatin syn-thase,PCS) ,以 GSH为底物催化合成的 ;植物络合素合酶基因的表达是组成型的 ,重金属离子能够活化 PCS,诱导 PCs的合成。 1 989年 ,人们首次报道得到了部分纯化的 PCS,1 0年后 ,3个研究小组分别于 1 999年同时克隆和鉴定了编码 PCS的基因 ;这些结果不仅对于研究PCs的合成途径和模型的建立及植物抗重金属机制的探讨有重要意义 。

Phytochelatins,a class of posttranslationally synthesized peptides,play a pivotal role in heavy metal tolerance in plants by chelating heavy metal ions and decreasing their concentrations in living bodies.Phytochelatins,which Derived from glutathione and related thiols by the action of γ glutamylcysteine dipeptidyl transpeptidase (EC.2.3.2.15,phytochelatin synthase,PCS),consist of repeating units of γ glutamylcysteine followed by a C terminal Gly [(γ Glu Cys) n X aa ,(n=2~11)].In 1989,the PCS was partially purified for the first time.Ten years later,the genes encoding the PCS had been simultaneously cloned and characterized independently by three groups in 1999.The PCS protein deduced from its gene sequences of Arabidopsis thaliana ,wheat and yeast ( Schizosaccharomyces pombe ) had a molecular weight of 50~55 kDa.Homological analysis showed that PCS contains a highly conserved N terminal region that is presumed to be the catalytic domain,and a variable C terminal region containing multiple Cys residues,which is proposed to be involved in activation of the enzyme by metal ions directly binding to it.However,another model of PCs biosynthesis has been established,GSH like peptides containing blocked thiol groups (possibly blocked by metals),but not metal binding per se are required by PCs for catalytic activation.Interestingly,a similar gene was identified in the nematode ( Caenorhabditis elegans ),suggesting that PCs may also be expressed in some animal species.All these advances will improve our knowledge about the phytochelatin biosynthesis and the heavy metal detoxification in plants.The development of phytoremediation will also gain considerable benefit from them.