|Family type peptidase||C83.001 - gamma-glutamylcysteine dipeptidyltranspeptidase (Nostoc sp. PCC 7120), MEROPS Accession MER0081295 (peptidase unit: 1-242)|
|Content of family||The family contains exopeptidases some of which also have transferase activity.|
Identifier created: MEROPS 8.0 (8 January 2008)|
The synthesis of phytochelatins by the action of a gamma-glutamylcysteine dipeptidyl transpeptidase was described by Grill et al., 1989. Phytochelatins are the principle heavy-metal detoxifying compounds in the plant kingdom. They are linear polymers of the gamma-Glu-Cys portion of glutathione (GSH), with the general formula (Glu-Cys)n-Gly, where n= 2-11. In the related iso-phytochelatins the C-terminal Gly is replaced by Ser, Glu or Gln, and in homo-phytochelatin it is beta-Ala (Oven et al., 2002). The family also contains enzymes that hydrolyse the terminal Gly from glutathione (gamma-Glu-Cys-Gly) but lack the transferase activity necessary for the synthesis of phytochelatin. Glutathione is gamma-Glu-Cys-Gly, and its degradation by cleavage of the C-terminal Gly contrasts with the action of the gamma-glutamyltransfereses (e.g. T03.001) that remove the N-terminal residue.
|Active site residues||Q64 C70 H183 D201 |
|Active site||The active site is similar to that of papain, containing Gln, Cys, His and Asp (see the Alignment).|
|Activities and specificities||Glutathione is cleaved to gamma-Glu-Cys + Gly, either by hydrolysis or transpeptidation. The subset of enzymes in the family that catalyse the transpeptidation reaction produce phytochelatin, and are termed phytochelatin synthases. The formation of phytochelatin is stimulated by heavy metal ions.|
|Molecular structure||The papain-like crystal structure of the gamma-glutamylcysteine dipeptidyl transpeptidase NsPCS from Nostoc places family C83 in clan CA (Vivares et al., 2005).|