Review
Glutathione transferases, regulators of cellular metabolism and physiology

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Abstract

Background

The cytosolic glutathione transferases (GSTs) comprise a super family of proteins that can be categorized into multiple classes with a mixture of highly specific and overlapping functions.

Scope of review

The review covers the genetics, structure and function of the human cytosolic GSTs with particular attention to their emerging roles in cellular metabolism.

Major conclusions

All the catalytically active GSTs contribute to the glutathione conjugation or glutathione dependant-biotransformation of xenobiotics and many catalyze glutathione peroxidase or thiol transferase reactions. GSTs also catalyze glutathione dependent isomerization reactions required for the synthesis of several prostaglandins and steroid hormones and the catabolism of tyrosine. An increasing body of work has implicated several GSTs in the regulation of cell signaling pathways mediated by stress-activated kinases like Jun N-terminal kinase. In addition, some members of the cytosolic GST family have been shown to form ion channels in intracellular membranes and to modulate ryanodine receptor Ca2 + channels in skeletal and cardiac muscle.

General significance

In addition to their well established roles in the conjugation and biotransformation of xenobiotics, GSTs have emerged as significant regulators of pathways determining cell proliferation and survival and as regulators of ryanodine receptors that are essential for muscle function. This article is part of a Special Issue entitled Cellular functions of glutathione.

Highlights

► Glutathione transferases are known for their capacity to conjugate xenobiotics. ► Glutathione transferases isomerize intermediates in steroid hormone synthesis. ► Zeta class glutathione transferases catalyze a required step in tyrosine catabolism. ► Several glutathione transferases regulate signaling pathways through JNK. ► Some members of the GST family form and modulate ion channels.

Introduction

It is clear that glutathione transferase (GST) activity is critically important in biological systems as it has evolved via convergent pathways in at least four structurally distinct enzyme families (the cytosolic GSTs; the Kappa class mitochondrial GSTs; the MAPEG enzymes; the fosfomycin resistance proteins) [1], [2], [3]. The cytosolic GST super family is the most extensively studied family and occurs in all cellular life forms. Mammalian cytosolic GSTs came to prominence in biomedical research because of the roles played by many family members in drug and xenobiotic metabolism. Numerous studies of plants, insects and microbes have reported related enzymes involved in the conjugation of glutathione (GSH) to a vast array of chemical entities. As discussed in this review further research has shown that members of the cytosolic GST family are involved many additional cellular processes.

The Kappa class GSTs are also soluble enzymes with some substrate specificities that are similar to the cytosolic GSTs and they were originally named and considered as a distant member of the cytosolic GST family [4]. Subsequently, sequence analysis and structural studies revealed their distinct and ancient evolutionary origin [2], [3], [5]. The Kappa class GSTs appear to be expressed in mitochondria and peroxisomes in mammals and in Caenorhabditis elegans [5], [6]. Phylogenetic sequence analysis indicates that they are widely distributed in nature but are notably absent in insects [7]. The prokaryotic 2-hydroxychromene-2-carboxylate isomerases and the disulfide-bond-forming oxidoreductases (DsbA) share similar structural motifs to the Kappa class enzymes suggesting a common ancestor and the likelihood of yet to be discovered-structurally related enzymes in a wide range of species [2]. A single Kappa class enzyme has been identified in humans and mice but its physiological role has not been clearly identified. A potential role for GSTK1-1 in the oligomerization of adiponectin has been suggested [8] but studies of GSTK knockout mice have not been supportive [9], [10]. Readers are directed to a recent review for more extensive discussion of the Kappa class GSTs [7].

The prokaryotic fosfomycin resistance proteins represent another family of soluble proteins that catalyze glutathione transferase reactions [11]. Fosfomycin ((1R,2S)-epoxypropyl phosphonic acid) is a broad-spectrum antibiotic that is inactivated by the K+ dependant FosA catalyzed addition of glutathione. Fos A is a Mn(II)-dependant metalloprotein with structural similarities to the vicinial oxygen chelate (VOC) super family of proteins which includes glyoxalase I that also uses glutathione as a co-factor [11], [12].

The MAPEG proteins (membrane associated proteins in eicosanoid and glutathione metabolism) are the fourth protein family with members exhibiting glutathione transferase activity [13]. Microsomal glutathione transferase 1 (MGST1) is the most extensively characterized GST within the MAPEG family and constitutes 3% of the endoplasmic reticulum protein in rat liver and 5% of the outer mitochondrial membrane [14]. Although MGST1 is a membrane bound trimer that is structurally distinct it shares the same broad and overlapping substrate specificity as the cytosolic GSTs. The structure and function of MGST1 and the other MAPEG enzymes have been the subject of several reviews and are not discussed further here [13], [14], [15].

Section snippets

The cytosolic GST super family

This review is focused primarily on the cytosolic GSTs (EC2.5.1.18) that represent a super family of enzymes that are found in all cellular life forms. The cytosolic GSTs have been extensively studied in humans, mice, rats as well as in some plant, insect and microbial species. There is a vast literature describing the cytosolic GSTs and a single comprehensive review of all aspects of this broad field is no longer feasible. There have been many reviews of the cytosolic GSTs [16], [17], [18],

Phylogenetic classification of the cytosolic GSTs

Early studies indicated that there were multiple cytosolic GSTs with differing properties [31] but the genetic interrelationships of these isoenzymes were not clear. The first genetic study of the GSTs used non-denaturing electrophoresis to define several human GST loci and associated polymorphisms [35]. The nomenclature has subsequently changed and the locus termed GST1 is now known as GSTM1 and GST2 is now known as GSTA1 and GSTA2. Historically the GSTs have been classified on the basis of

Genetic diversity of the GSTs

In addition to the variation between GST classes there is considerable genetic heterogeneity within classes resulting from gene duplications and deletions, as well as single nucleotide polymorphisms (SNPs) in coding and non-coding regions. Many of these variations have significant functional effects and a survey of the biomedical literature has revealed more than one thousand publications evaluating potential associations between GST polymorphisms and clinical disorders. Here we review the

Glutathione transferase structure

The ease of recombinant expression in E. coli has allowed the determination of the crystal structures of almost all known human GSTs and numerous structures for GSTs from other species. PDB files containing the coordinates of each human GST are listed in Table 2. These structural studies have shown that the cytosolic GSTs are typically dimeric proteins composed of 25–30 kDa subunits that can be further divided into two domains (Fig. 1). The N-terminal domain includes the glutathione binding site

The active site and catalysis

Although the position of the active site is conserved in all catalytically active cytosolic GSTs, there are significant differences between classes that reflect the different reactions that are characteristically catalyzed. The active site was originally conceptually subdivided into the “G” site that is primarily responsible for binding glutathione and the “H” site that is less well defined and binds the hydrophobic substrate [16]. The G-site is typically formed by residues from the N-terminal

Functions of the glutathione transferases

GSTs have been extensively studied for their catalytic role in the conjugation and elimination of electrophilic xenobiotic compounds including anti-cancer drugs and carcinogens. There is also considerable evidence that up regulation of GST expression in tumors can generate drug resistance. In addition, the roles of GSTs in the removal of endogenously produced free radicals via their selenium independent glutathione peroxidase activity and their capacity to conjugate the products of lipid

GST-deficient mice

Although most GSTs have been shown to catalyze glutathione dependent reactions with xenobiotic or exogenous substrates, many GSTs have been found to participate in a wide range of non-enzymatic processes that were not previously predicted. The generation and characterization of GST deficient mice is providing new insights into the role of GSTs in drug and carcinogen metabolism but also into our emerging understanding of the roles of GSTs in endogenous cellular metabolism. The GSTP knockout

Acknowledgements

The continuing support of the National Health and Medical Research Council through grant APP1008477 is gratefully acknowledged.

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