title: Tentative identification of the second substrate binding site in Arabidopsis phytochelatin synthase.
abstract: Phytochelatin synthase (PCS) uses the substrates glutathione (GSH, γGlu-Cys-Gly) and a cadmium (Cd)-bound GSH (Cd∙GS2) to produce the shortest phytochelatin product (PC2, (γGlu-Cys)2-Gly) through a ping-pong mechanism. The binding of the 2 substrates to the active site, particularly the second substrate binding site, is not well-understood. In this study, we generated a structural model of the catalytic domain of Arabidopsis AtPCS1 (residues 12-218) by using the crystal structure of the γGlu-Cys acyl-enzyme complex of the PCS of the cyanobacterium Nostoc (NsPCS) as a template. The modeled AtPCS1 revealed a cavity in proximity to the first substrate binding site, consisting of 3 loops containing several conserved amino acids including Arg152, Lys185, and Tyr55. Substitutions of these amino acids (R152K, K185R, or double mutation) resulted in the abrogation of enzyme activity, indicating that the arrangement of these 2 positive charges is crucial for the binding of the second substrate. Recombinant AtPCS1s with mutations at Tyr55 showed lower catalytic activities because of reduced affinity (3-fold for Y55W) for the Cd∙GS2, further suggesting the role of the cation-π interaction in recognition of the second substrate. Our study results indicate the mechanism for second substrate recognition in PCS. The integrated catalytic mechanism of PCS is further discussed.
authors: Department of Biochemical Science and Technology, College of Life Science, National Taiwan University, Taipei, Taiwan.
authors: Ju-Chen
authors: JC
authors: Chia
authors: Chien-Chih
authors: CC
authors: Yang
authors: Yu-Ting
authors: YT
authors: Sui
authors: Shin-Yu
authors: SY
authors: Lin
authors: Rong-Huay
authors: RH
authors: Juang
mesh_terms: D019881:Aminoacyltransferases / Q000737:chemistry* / Q000235:genetics; D017360:Arabidopsis / Q000201:enzymology* / Q000235:genetics; D029681:Arabidopsis Proteins / Q000737:chemistry* / Q000235:genetics; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics; D001665:Binding Sites; D002384:Catalysis; D046937:Nostoc / Q000201:enzymology / Q000235:genetics; D017433:Protein Structure, Secondary
chemical_list: D029681:Arabidopsis Proteins; D001426:Bacterial Proteins; D019881:Aminoacyltransferases; C502365:AT5G44070 protein, Arabidopsis; C093784:glutathione gamma-glutamylcysteinyltransferase