Glutathione transferases: new functions

doi:10.1016/j.sbi.2005.10.005

Current Opinion in Structural Biology

Volume 15, Issue 6, December 2005, Pages 716-723

https://doi.org/10.1016/j.sbi.2005.10.005 Get rights and content

Well known as detoxification enzymes, the glutathione transferases also function in prostaglandin and steroid hormone synthesis. New uses for the canonical glutathione transferase fold are becoming apparent; the bacterial stringent starvation protein SspA and the yeast prion protein Ure2p (both transcription factors) were found to adopt this fold, but their roles remain unclear. The intracellular chloride ion channel CLIC1 adopts the canonical glutathione transferase fold in its soluble form and appears to undergo radical structural modification as part of its membrane insertion process. The structures of rat and human mitochondrial glutathione transferases have been solved: they adopt a topology similar to that of bacterial disulfide bond isomerases, leading to the suggestion that they have evolved independently of the canonical enzymes. Recent structural studies of integral membrane glutathione S-transferases from microsomes have revealed common patterns of tertiary and quaternary structure.

Introduction

The glutathione transferases (formerly known as glutathione S-transferases, from which the common abbreviation GST is derived) catalyse the conjugation of non-polar compounds that contain an electrophilic carbon, nitrogen or sulfur atom to reduced glutathione (GSH) [1]. In this way, GSTs contribute to the metabolism of drugs, pesticides and other xenobiotics. Some products of oxidative stress, such as peroxides, are also substrates. GST activities are upregulated in several types of tumours and are implicated in chemotherapy resistance. Polymorphisms of human GST genes are implicated in response to cancer therapy [2, 3, 4]. There has been interest in the development of GST inhibitors to enhance the therapeutic index of anticancer drugs; however, first-generation GST inhibitors (e.g. ethacrynic acid [EA]) were unsuccessful in clinical trials.

There are three major groups of GSTs: canonical (or cytosolic) GSTs (cGSTs), mitochondrial GSTs and microsomal GSTs. (The bacterial fosfomycin resistance proteins FosA and FosB [5] represent a distinct family of GSTs: they will not be considered here.) Microsomal GSTs are now known as ‘membrane-associated proteins in eicosanoid and glutathione metabolism’ (MAPEGs). The cytosolic and mitochondrial GSTs share structural and evolutionary relationships that will be discussed below.

The 1990s saw the kinetic and structural characterisation of cGSTs from mammalian, plant, bacterial and insect sources. These enzymes are divided into classes, with greater than 40% amino acid sequence identity among members of each class. Between classes, proteins have less than 25% sequence identity. Currently recognised classes include alpha, beta, delta, epsilon, theta, zeta, mu, pi, sigma, phi, tau and omega. The enzymes are dimers of 25 kDa subunits. GST monomers of the same class can form heterodimers if multiple genes are present [1].

Recent developments discussed in this review include: the determination of the structure of mitochondrial kappa class GSTs, providing insights into the evolution of these and the cGST enzymes; work on cGST-like proteins demonstrating that this ancient protein fold has been ‘recycled’ by nature for new functions as, for example, transcription and elongation factors, and even ion channels; and progress in extracting structural data on the integral membrane MAPEG enzymes.

Section snippets

Cytosolic and mitochondrial GSTs

The cGSTs are related by evolution to glutaredoxin, containing a thioredoxin domain with a unique βαβαββα topology (Figure 1). This domain binds GSH in a topologically conserved location (the G-site). The residues involved in binding GSH vary in number and kind across the glutaredoxins and cGSTs (Figure 2), but there are a few conserved features: a conserved residue before the third β strand provides mainchain hydrogen-bond donors and acceptors for GSH; catalytic residue(s) are located in a

Intracellular chloride ion channels

The intracellular chloride ion channels (CLICs) are an unusual family of proteins that exist in both soluble and integral membrane forms. Homologues so far identified include p64, parchorin and CLICs 1 to 5 [15]. CLICs are found in organelle and plasma membranes. The various homologues share a common approximately 240-residue domain that, remarkably, adopts the cGST fold [16, 17], demonstrating, for the first time, that the GST fold is stable as a monomer. A glutaredoxin-like ‘active site’ is

MAPEGs

Four MAPEG subgroups (I–IV) have been described. Proteins within a subgroup share greater than 20% sequence identity. MAPEG proteins have pro-inflammatory activities and some have general detoxification activities. Being integral membrane proteins, their structural characterisation has been slow and difficult. Nevertheless, recent work on groups I and IV has revealed critical new structural data.

The best-studied MAPEG enzyme is MGST1, which forms two-dimensional crystals. The projection

Conclusions

The GSTs are a highly diverse family of enzymes with functions ranging from detoxification to biosynthesis. The GSTs involved in prostaglandin biosynthesis are an example of this latter category. Among GST classes with well-established detoxification activities, certain isozymes may have activities unrelated to detoxification. These observations are important in the context of the large number of GST-like sequences found through genome sequencing projects. Many of these GST-like proteins may

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

• of special interest
•• of outstanding interest

Acknowledgements

I thank Philip Board for comments and suggestions regarding this paper.

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