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Project article: Glutathione S-transferase

Glutathione S-transferase structure
Protein sequence and structure are important classification methods for GSTs: while classes from the cytosolic superfamily of GSTs possess more than 40% sequence homology, those from other classes may have less than 25%. Cytosolic GSTs are divided into 13 classes based upon their structure: alpha, beta, delta, epsilon, zeta, theta, mu, nu, pi, sigma, tau, phi, and omega. Mitochondrial GSTs are in class kappa. The MAPEG superfamily of microsomal GSTs consists of subgroups designated I-IV, between which amino acid sequences share less than 20% identity.

The glutathione binding site, or "G-site," is located in the thioredoxin-like domain of both cytosolic and mitochondrial GSTs. The region containing the greatest amount of variability between the assorted classes is that of helix α2, where one of three different amino acid residues interacts with the glycine residue of glutathione. Two subgroups of cytosolic GSTs have been characterized based upon their interaction with glutathione: the Y-GST group, which uses a tyrosine residue to activate glutathione, and the S/C-GST, which instead use serine or cysteine residues.

Bibliography of relevant research
Glutathione transferases: a structural perspective

Glutathione transferases as mediators of signaling pathways involved in cell proliferation and cell death

Assignment 4
Ideas (TEMPORARY):
 * Infobox detailing orthologous structure in human genome as per infobox guidelines for gene & protein articles, specifically GNF protein box
 * Gene information

Structure
Protein sequence and structure are important classification methods for GSTs: while classes from the cytosolic superfamily of GSTs possess more than 40% sequence homology, those from other classes may have less than 25%. Cytosolic GSTs are divided into 13 classes based upon their structure: alpha, beta, delta, epsilon, zeta, theta, mu, nu, pi, sigma, tau, phi, and omega. Mitochondrial GSTs are in class kappa. The MAPEG superfamily of microsomal GSTs consists of subgroups designated I-IV, between which amino acid sequences share less than 20% identity.

The glutathione binding site, or "G-site," is located in the thioredoxin-like domain of both cytosolic and mitochondrial GSTs. The region containing the greatest amount of variability between the assorted classes is that of helix α2, where one of three different amino acid residues interacts with the glycine residue of glutathione. Two subgroups of cytosolic GSTs have been characterized based upon their interaction with glutathione: the Y-GST group, which uses a tyrosine residue to activate glutathione, and the S/C-GST, which instead use serine or cysteine residues.

The porcine pi-class enzyme pGTSP1-1 was the first GST to have its structure determined, and it is representative of other members of the cytosolic GST superfamily, which contain a thioredoxin-like N-terminus domain as well as a C-terminus domain consisting of alpha helices.

Role in cell signaling


Although best known for their ability to conjugate GSH and thereby detoxify cellular environments, GSTs are also capable of binding nonsubstrate ligands, with important cell signaling implications. Several GST isozymes from various classes have been shown to inhibit the function of a kinase involved in the MAPK pathway that regulates cell proliferation and death, preventing the kinase from carrying out its role in facilitating the signaling cascade.

The cytosolic π-class GST composed of subunit 1 homodimers (GSTP1-1), a well-characterized isozyme of the mammalian GST family, is expressed primarily in heart, lung, and brain tissues; in fact, it is the most common GST expressed outside the liver. Based on its overexpression in a majority of human tumor cell lines and prevalence in chemotherapeutic-resistant tumors, GSTP1-1 is thought to play a role in the development of cancer and its potential resistance to drug treatment. Further evidence for this comes from the knowledge that GSTπ can selectively inhibit C-jun phosphorylation by JNK, preventing apoptosis. During times of low cellular stress, a complex forms through direct protein-protein interactions between GSTπ and the C-terminus of JNK, effectively preventing the action of JNK and thus its induction of the JNK pathway. Cellular oxidative stress causes the dissociation of the complex, oligomerization of GSTπ, and induction of the JNK pathway, resulting in apoptosis. The connection between GSTπ inhibition of the pro-apoptotic JNK pathway and the isozyme's overexpression in drug-resistant tumor cells may itself account for the tumor cells' ability to escape apoptosis mediated by drugs that are not substrates of GSTπ.

Like GSTπ, GSTμ 1 (GSTM1) is involved in regulating apoptotic pathways through direct protein-protein interactions, although it acts on ASK1, which is upstream of JNK. The mechanism and result are similar to that of GSTπ and JNK, in that GSTM1 sequesters ASK1 through complex formation and prevents its induction of the pro-apoptotic p38 and JNK portions of the MAPK signaling cascade. Like GSTπ, GSTM1 interacts with its partner in the absence of oxidative stress, although ASK1 is also involved in heat shock response, which is likewise prevented during ASK1 sequestration. The fact that high levels of GST are associated with resistance to apoptosis induced by a range of substances, including chemotherapeutic agents, supports its putative role in MAPK signaling prevention.

Implications in cancer development
There is a growing body of evidence supporting the role of GST, particularly GSTP, in cancer development and chemotherapeutic resistance. The link between GSTP and cancer is most obvious in the overexpression of GSTP in many cancers, but it is also supported by the fact that the transformed phenotype of tumor cells is associated with aberrantly regulated kinase signaling pathways and cellular addiction to overexpressed proteins. That most anti-cancer drugs are poor substrates for GSTP indicates that the role of elevated GSTP in many tumor cell lines is not to detoxify the compounds, but must have another purpose; this theory is also given credence by the common finding of GSTP overexpression in tumor cell lines that are not drug resistant.

Assignment 6 (NOT FINAL)
Ideas (TEMPORARY):
 * New section: Gene
 * Information about polymorphisms, etc. May not work because of huge variety of superfamilies, classes, isozymes, etc.
 * New section: Clinical significance
 * See Gpruett2's suggestions
 * New section: History/Discovery
 * How to do this without using primary sources?
 * Follow these guidelines for new sections.

All-new content below! Other changes to GST article documented on user page.

Function
The function of GSTs is dependent upon a steady supply of GSH from the synthetic enzymes gamma-glutamylcysteine synthetase and glutathione synthetase, as well as the action of specific transporters to remove conjugates of GSH from the cell. The primary role of GSTs is to detoxify xenobiotics by catalyzing the nucleophilic attack by GSH on electrophilic carbon, sulfur, or nitrogen atoms of said nonpolar xenobiotic substrates, thereby preventing their interaction with crucial cellular proteins and nucleic acids. Specifically, the function of GSTs in this role is twofold: (1) to bind both the substrate at the enzyme's hydrophobic H-site and GSH at the adjacent, hydrophilic G-site, which together form the active site of the enzyme; (2) and subsequently to activate the thiol group of GSH, enabling the aforementioned nucleophilic attack upon the substrate. The compounds targeted in this manner by GSTs encompass a diverse range of environmental or otherwise exogenous toxins, including chemotherapeutic agents and other drugs, pesticides, herbicides, carcinogens, and variably-derived epoxides; indeed, GSTs are responsible for the conjugation of β1-8,9-epoxide, a reactive intermediate formed from aflatoxin B1, which is a crucial means of protection against the toxin in rodents. The aforementioned detoxification reactions comprise the first four steps of mercapturic acid synthesis, with the conjugation to GSH serving to make the substrates more soluble and allowing them to be removed from the cell by transporters such as multidrug resistance-associated protein 1 (MRP1). After export, the conjugation products are converted into mercapturic acids and excreted via the urine or bile.