Thioredoxin glutathione reductase: its role in redox biology and potential as a target for drugs against neglected diseases

Abstract

Background: There are two, largely autonomous antioxidant pathways in many organisms, one based on thioredoxin and one based on glutathione, with each pathway having a unique flavoprotein oxidoreductase to maintain them in a reduced state. A recently discovered protein, thioredoxin glutathione reductase (TGR) potentially connects these two pathways. In a large group of parasitic worms, responsible for hundreds of millions of infections in humans and animals, untold morbidity and significant mortality, TGR is the sole enzyme present to maintain redox balance.

Scope of review: In this review, the current understanding of the biochemical properties of TGR enzymes is compared to the related enzymes thioredoxin reductase and glutathione reductase. The role of the rare amino acid selenocysteine is discussed. An overview of the potential to target TGR for drug development against a range of parasitic worms and preliminary results to identify TGR inhibitors for schistosomiasis treatment is presented.

Major conclusions: TGR has properties that are both unique and common to other flavoprotein oxidoreductases. TGR plays a fundamentally different and essential role in the redox biology of parasitic flatworms. Therefore, TGR is a promising target for drug development for schistosomiasis and other trematode and cestode infections.

General significance: TGR may have differing functions in host organisms, but through analyses to understand its ability to reduce both glutathione and thioredoxin we can better understand the reaction mechanisms of an important class of enzymes. The unique properties of TGR in parasitic flatworms provide promising routes to develop new treatments for diseases.

Figure 1

Model for the translational insertion of selenocysteine. A secondary structure present in the 3’ untranslated region of the mRNA called the selenocysteine insertion sequence (SECIS) element interacts with the SECIS binding protein-2 (SBP2). This complex then recruits other factors required for the re-interpretation of the codon TGA, normally a termination codon, for selenocysteine incorporation including selenocysteine elongation factor (EFSec), SECp43, ribosomal protein L30, and the specialized tRNA^sec. Other factors, known and unknown, are involved in the process [111].

Figure 2

Comparison of the domain structure of pyridine nucleotide disulfide oxidoreductases. Glutathione reductase (GR), thioredoxin reductase (TrxR) and thioredoxin glutathione reductase (TGR) all contain a common domain characteristic of pyridine nucleotide disulfide oxidoreductases (yellow) with N-terminal redox active cysteines (CVNVGC), FAD- and NADPH-binding pockets and a interface domain that is involved in protein dimerization. TrxR and TGR proteins both contain a C-terminal extension (blue) with redox active cysteine and selenocysteine (U) (or cysteine). TGR proteins have an additional N-terminal domain with sequence and structural similarities to glutaredoxin (purple). This domain has a redox active cysteine pair (CPYC) as in Schistosoma mansoni TGR or single cysteine (CPHS) as in human TGR.

Figure 3

Comparison of the proposed reaction mechanisms of pyridine nucleotide disulfide oxidoreductases. The active forms of all proteins are dimmers of identical subunits. (A) and (B) Schistosoma mansoni thioredoxin glutathione reductase. In (A) reducing equivalents are shown being transferred from NADPH to the FAD and then to the adjacent cysteine pair in the sequence (CVNVGC). This reduced cysteine pair then transfers the hydride to the C-terminal cysteineselenocysteine in the other peptide chain. (B) Reduction of the C-terminal cysteine-selenocysteine pair produces a structural change in the flexible C-terminal arm allowing it to be repositioned to either react with the glutaredoxin domain active site (CPYC) or directly with oxidized thioredoxin (and potentially other substrates). The reduced glutaredoxin domain can then interact with its substrates, glutathione disulfide (shown) or other substrates such as glutathionylated peptides. In (C), electrons from NADPH pass to the flavin and then to the redox active cysteine pair (CVNVGC) and subsequently to glutathione disulfide in glutathione reductase proteins. In (D), electrons from NADPH pass to the flavin, then to the redox active cysteine pair (CVNVGC) and subsequently to the C-terminal cysteine-selenocysteine couple in the other subunit in thioredoxin reductases. The reduced C-terminal active site can then transfer reducing equivalents to substrates (e.g., oxidized thioredoxin).

Figure 4

The oxadiazole-oxide pharmacophore. In furoxan (4-phenyl-1,2,5-oxadiazole-3-carbonitrile-2-oxide) R₁ = carbonitrile (C≡N) and R₂ = phenyl.

Scheme 1