Glutathione transferase tags toxic molecules, making them easy to recognize and remove.
Top: glutathione transferase, with glutathione in red and a toxic molecule in purple. Bottom: glutathione is built of three amino acids.Download high quality TIFF image
Cells are filled with a confusing jumble of small molecules. Sometimes dangerous molecules get introduced in the mix, and cells need a way to find and remove them. In a first line of defense, enzymes like cytochrome p450
modify slippery carbon-rich toxins, making them more soluble. In a second line of defense, the glutathione transferases attach a convenient handle to unwanted molecules, which is then recognized by the cell’s export machinery.
The trick to making a useful molecular handle is two-fold. First, the handle needs to recognizable, and recognizably different from other molecules in the cell. Glutathione is composed of three familiar amino acids: a glutamate, a cysteine and a glycine. But it won’t be confused with normal peptides because the glutamate is attached in an unusual way, forming the covalent bond through its sidechain. Second, a molecular handle needs an easy chemical way to attach it to other molecules. Glutathione has a sulfur atom at the center (part of the cysteine) that is easily activated and attached to a variety of different molecules.
A collection of just over 20 glutathione transferases work together to scour each cell for toxins. The one shown here is one of the best studied, termed hGSTP1-1 (PDB entry 3gss
). These twenty enzymes protect us from many different perils, including toxins made by bacteria and fungi, reactive molecules formed during the cooking of food or smoking, and a variety of environmental pollutants. Because of this, each glutathione transferase typically recognizes a variety of foreign molecules, attaching them all to glutathione. This job is so important that in some cells, such as liver cells, glutathione transferase can make up 10% of the total protein content of the cell!
Mosquitoes and DDT
The process of detoxification can lead to an arms race, where defenses battle with attackers. An example has recently been studied with mosquitoes that carry malaria in Africa. DDT is widely used to control mosquitoes, but DDT-resistant mosquitoes appear in regions of heavy pesticide use. Looking inside these mosquitoes, one of the ways they become resistant is by building a mutant glutathione transferase. Ironically, the mutant replaces a single leucine with a larger phenylalanine, but the change causes a slight refolding of the protein that creates a larger active site that fits the bulky DDT molecule (PDB entries 3zml
Exploring the Structure
Glutathione transferases also play a central role in detoxification of drugs. The three structures shown here, all of the same enzyme, show some of the things that can happen. PDB entry 3pgt captures the enzyme as it is detoxifying a potent carcinogen. PDB entry 3csh shows the enzyme attaching glutathione to an anti-cancer drug, reducing its effectiveness. To counteract this and make the drug effective for a longer time, inhibitor molecules like that in PDB entry 3gss can be administered along with the drug to inhibit the action of the enzymes. To explore these structures in more detail, click on the image for an interactive JSmol.
Topics for Further Discussion
- Many structures of different glutathione transferases have been determined and deposited in the PDB archive. Try searching for “glutathione transferase” to see them.
- These enzymes typically have two domains: one that binds to glutathione and one that interacts with the target molecule. When you’re exploring the structure of the enzymes, try looking at a ribbon or backbone diagram to see the two domains.
- 3zmk, 3zml: JM Riveron, C Yunta, SS Ibrahim, R Djouaka, H Irving, BD Menze, HM Ismail, J Hemingway, H Ranson, A Albert & CS Wondji (2014) A single mutation in the GSTe2 gene allows tracking of metabolically based insecticide resistance in a major malaria vector. Genome Biology 15, R27.
- PG Board & D Menon (2013) Glutathione transferases, regulators of cellular metabolism and physiology. Biochimica et Biophysica Acta 1830, 3267-3288.
- B Wu & D Dong (2012) Human cytosolic glutathione transferases: structure, function, and drug discovery. Trends in Pharmacological Sciences 33, 656-668.
- A Oakley (2011) Glutathione transferases: a structural perspective. Drug Metabolism Reviews 43, 138-151.
- 3csh: LJ Parker, S Ciccone, LC Italiano, A Primavera, AJ Oakley, CJ Morton, NC Hancock, M Lo Bello & MW Parker (2008) The anti-cancer drug chlorambucil as a substrate for the human polymorphic enzyme glutathione transferase P1-1: kinetic properties and crystallographic characterization of allelic variants. Journal of Molecular Biology 380, 131-144.
- JD Hayes, JU Flanagan & IR Jowsey (2005) Glutathione transferases. Annual Review of Pharmacology and Toxicology 45, 51-88.
- 3pgt: X Ji, J Blaszczyk, B Xiao, R O’Donnell, X Hu, C Herzog, SV Singh & P Zimniak (1999) Structure and function of residue 104 and water molecules in the xenobiotic substrate-binding site in human glutathione S-transferase P1-1. Biochemistry 38, 10231-10238.
- 3gss: AJ Oakley, J Rossjohn, M Lo Bello, AM Caccuri, G Federici & MW Parker (1997) The three-dimensional structure of the human Pi class glutathione transferase P1-1 in complex with the inhibitor ethacrynic acid and its glutathione conjugate. Biochemistry 36, 576-585
August 2017, David Goodsell