Plant glutathione biosynthesis: diversity in biochemical regulation and reaction products

Abstract

In plants, exposure to temperature extremes, heavy metal-contaminated soils, drought, air pollutants, and pathogens results in the generation of reactive oxygen species that alter the intracellular redox environment, which in turn influences signaling pathways and cell fate. As part of their response to these stresses, plants produce glutathione. Glutathione acts as an anti-oxidant by quenching reactive oxygen species, and is involved in the ascorbate-glutathione cycle that eliminates damaging peroxides. Plants also use glutathione for the detoxification of xenobiotics, herbicides, air pollutants (sulfur dioxide and ozone), and toxic heavy metals. Two enzymes catalyze glutathione synthesis: glutamate-cysteine ligase, and glutathione synthetase. Glutathione is a ubiquitous protective compound in plants, but the structural and functional details of the proteins that synthesize it, as well as the potential biochemical mechanisms of their regulation, have only begun to be explored. As discussed here, the core reactions of glutathione synthesis are conserved across various organisms, but plants have diversified both the regulatory mechanisms that control its synthesis and the range of products derived from this pathway. Understanding the molecular basis of glutathione biosynthesis and its regulation will expand our knowledge of this component in the plant stress response network.

Keywords: Arabidopsis; biosynthesis; glutathione; metabolism; redox regulation.

Figure 1

Glutathione biosynthesis. Substrates and products of the reactions catalyzed by glutamate-cysteine ligase (GCL) and glutathione synthetase (GS) are shown.

Figure 2

Localization of glutathione biosynthesis enzymes in Arabidopsis leaf cells. (A) Localization of GCL. Immunocytochemical labeling with α-GCL and gold-labeled secondary antibody is shown. Labeled GCL within the chloroplast (CP) is indicated by black arrows. Cell wall (CW) is also indicated. (B) Localization of GS. Immunocytochemical labeling with α-GS and gold-labeled secondary antibody is shown. Labeled GS within the chloroplast (CP) is indicated by black arrows and localization in the cytosol by white arrows.

Figure 3

Overview of glutamate–cysteine ligase. (A) Oligomeric organization and redox regulation of the three types of GCL are shown. (B) Structure of the B. juncea GCL showing the positions of the two disulfide bonds (gold space-filling models). Secondary structure elements are colored as follows: α-helices in blue and β-strands in rose.

Figure 4

Diversity in Plant Glutathione Synthetases. (A) The chemical structures of glutathione analogs synthesized by various plants are shown. All share the core γ-glutamylcysteine structure with modifications to the third amino acid position as indicated. (B) Ribbon diagram of the homoglutathione (hGS) dimer (Galant et al., 2009). Monomers are colored gold and blue, respectively. The lid domain (dark blue), glycine-rich loop (cyan), and the alanine-rich loop (red) are highlighted in the gold monomer. Positions of bound ADP, sulfate, and homoglutathione are highlighted in the blue monomer with corresponding ligands colored gray in the gold monomer.