Glutathionylspermidine metabolism in Escherichia coli

Unique thiol metabolism in trypanosomatids: Redox homeostasis and drug resistance

Trypanosomatids are mainly responsible for heterogeneous parasitic diseases: Leishmaniasis, Sleeping sickness, and Chagas disease and control of these diseases implicates serious challenges due to the emergence of drug resistance. Redox-active biomolecules are the endogenous substances in organisms, which play important role in the regulation of redox homeostasis. The redox-active substances like glutathione, trypanothione, cysteine, cysteine persulfides, etc., and other inorganic intermediates (hydrogen peroxide, nitric oxide) are very useful as defence mechanism. In the present review, the suitability of trypanothione and other essential thiol molecules of trypanosomatids as drug targets are described in Leishmania and Trypanosoma. We have explored the role of tryparedoxin, tryparedoxin peroxidase, ascorbate peroxidase, superoxide dismutase, and glutaredoxins in the anti-oxidant mechanism and drug resistance. Up-regulation of some proteins in trypanothione metabolism helps the parasites in survival against drug pressure (sodium stibogluconate, Amphotericin B, etc.) and oxidative stress. These molecules accept electrons from the reduced trypanothione and donate their electrons to other proteins, and these proteins reduce toxic molecules, neutralize reactive oxygen, or nitrogen species; and help parasites to cope with oxidative stress. Thus, a better understanding of the role of these molecules in drug resistance and redox homeostasis will help to target metabolic pathway proteins to combat Leishmaniasis and trypanosomiases.

Physiological Studies of Chlorobiaceae Suggest that Bacillithiol Derivatives Are the Most Widespread Thiols in Bacteria

Low-molecular-weight thiols are key metabolites that participate in many basic cellular processes: central metabolism, detoxification, and oxidative stress resistance. Here we describe a new thiol, N-methyl-bacillithiol, found in an anaerobic phototrophic bacterium and identify a gene that is responsible for its synthesis from bacillithiol, the main thiol metabolite in many Gram-positive bacteria. We show that the presence or absence of this gene in a sequenced genome accurately predicts thiol content in distantly related bacteria. On the basis of these results, we analyzed genome data and predict that bacillithiol and its derivatives are the most widely distributed thiol metabolites in biology.

Low-molecular-weight (LMW) thiols mediate redox homeostasis and the detoxification of chemical stressors. Despite their essential functions, the distribution of LMW thiols across cellular life has not yet been defined. LMW thiols are also thought to play a central role in sulfur oxidation pathways in phototrophic bacteria, including the Chlorobiaceae. Here we show that Chlorobaculum tepidum synthesizes a novel LMW thiol with a mass of 412 ± 1 Da corresponding to a molecular formula of C₁₄H₂₄N₂O₁₀S, which suggests that the new LMW thiol is closely related to bacillithiol (BSH), the major LMW thiol of low-G+C Gram-positive bacteria. The Cba. tepidum LMW thiol structure was N-methyl-bacillithiol (N-Me-BSH), methylated on the cysteine nitrogen, the fourth instance of this modification in metabolism. Orthologs of bacillithiol biosynthetic genes in the Cba. tepidum genome and the CT1040 gene product, N-Me-BSH synthase, were required for N-Me-BSH synthesis. N-Me-BSH was found in all Chlorobiaceae examined as well as Polaribacter sp. strain MED152, a member of the Bacteroidetes. A comparative genomic analysis indicated that BSH/N-Me-BSH is synthesized not only by members of the Chlorobiaceae, Bacteroidetes, Deinococcus-Thermus, and Firmicutes but also by Acidobacteria, Chlamydiae, Gemmatimonadetes, and Proteobacteria. Thus, BSH and derivatives appear to be the most broadly distributed LMW thiols in biology.

Polyamine-Based Thiols in Trypanosomatids: Evolution, Protein Structural Adaptations, and Biological Functions

SIGNIFICANCE

Major pathogenic enterobacteria and protozoan parasites from the phylum Euglenozoa, such as trypanosomatids, are endowed with glutathione (GSH)-spermidine (Sp) derivatives that play important roles in signaling and metal and thiol-redox homeostasis. For some Euglenozoa lineages, the GSH-Sp conjugates represent the main redox cosubstrates around which entire new redox systems have evolved. Several proteins underwent molecular adaptations to synthesize and utilize the new polyamine-based thiols. Recent Advances: The genomes of closely related organisms have recently been sequenced, which allows mining and analysis of gene sequences that belong to these peculiar redox systems. Similarly, the three-dimensional structures of several of these proteins have been solved, which allows for comparison with their counterparts in classical redox systems that rely on GSH/glutaredoxin and thioredoxin.

CRITICAL ISSUES

The evolutionary and structural aspects related to the emergence and use of GSH-Sp conjugates in Euglenozoa are reviewed focusing on unique structural specializations that proteins developed to use N¹,N⁸-bisglutathionylspermidine (trypanothione) as redox cosubstrate. An updated overview on the biochemical and biological significance of the major enzymatic activities is also provided.

FUTURE DIRECTIONS

A thiol-redox system strictly dependent on trypanothione is a feature unique to trypanosomatids. The physicochemical properties of the polyamine-GSH conjugates were a major driving force for structural adaptation of proteins that use these thiols as ligand and redox cofactor. In fact, the structural differences of indispensable components of this system can be exploited toward selective drug development. Future research should clarify whether additional cellular processes are regulated by the trypanothione system. Antioxid. Redox Signal. 28, 463-486.

The orphan protein bis-γ-glutamylcystine reductase joins the pyridine nucleotide disulfide reductase family

Facile DNA sequencing became possible decades after many enzymes had been purified and characterized. Consequently, there are still "orphan" enyzmes for which activities are known but for which encoding genes have not been identified. Identification of the genes encoding orphan enzymes is important because it allows correct annotation of genes of unknown function or with misassigned function. Bis-γ-glutamylcystine reductase (GCR) is an orphan protein that was purified in 1988. This enzyme catalyzes the reduction of bis-γ-glutamylcystine. γ-Glutamylcysteine is the major low-molecular weight thiol in halobacteria. We purified GCR from Halobacterium sp. NRC-1 and identified the sequence of 23 tryptic peptides by nano-liquid chromatography electrospray ionization tandem mass spectrometry. These peptides cover 62% of the protein predicted to be encoded by a gene in Halobacterium sp. NRC-1 that is annotated as mercuric reductase. GCR and mercuric reductase activities were assayed using enzyme that was expressed in Escherichia coli and refolded from inclusion bodies. The enzyme had robust GCR activity but no mercuric reductase activity. The genomes of most, but not all, halobacteria for which whole genome sequences are available have close homologues of GCR, suggesting that there is more to be learned about the low-molecular weight thiols used in halobacteria.

Shotgun proteomics study of early biofilm formation process of Acidithiobacillus ferrooxidans ATCC 23270 on pyrite

Acidithiobacillus ferrooxidans is a chemolithoautotrophic, mesophilic Gram-negative bacterium able to oxidize ferrous iron, sulfur, and metal sulfides. It forms monolayer biofilms where extracellular polymeric substances are essential for cell attachment and metal sulfide leaching. High-throughput proteomics has been applied to study the early process of biofilm formation on pyrite by At. ferrooxidans ATCC 23270. After 24 h contact with the mineral, planktonic and sessile (biofilm) cell subpopulations were separated and proteins extracted. In total, 1319 proteins were detected in both samples. Sixty-two of these were found to be increased in biofilms. Additionally, 25 proteins were found to be decreased in the biofilm cell subpopulation. Three transcriptional factors were found to be increased or decreased among both cell subpopulations, suggesting their potential involvement in the regulation of these processes. Although no significant differences were observed for the known proteins related to ferrous iron and sulfur oxidation pathways among both cell subpopulations, the results presented here show that the early steps of At. ferrooxidans biofilm formation consist of a set of metabolic adaptations following cell attachment to the mineral surface. Functions such as extracellular polymeric substances biosynthesis seem to be pivotal. This first high-throughput proteomic study may also contribute to the annotation of several unknown At. ferrooxidans proteins found.

Escherichia coli glutathionylspermidine synthetase/amidase: phylogeny and effect on regulation of gene expression

Glutathionylspermidine synthetase/amidase (Gss) and the encoding gene (gss) have only been studied in Escherichia coli and several members of the Kinetoplastida phyla. In the present article, we have studied the phylogenetic distribution of Gss and have found that Gss sequences are largely limited to certain bacteria and Kinetoplastids and are absent in a variety of invertebrate and vertebrate species, Archea, plants, and some Eubacteria. It is striking that almost all of the 75 Enterobacteria species that have been sequenced contain sequences with very high degree of homology to the E. coli Gss protein. To find out the physiological significance of glutathionylspermidine in E. coli, we have performed global transcriptome analyses. The microarray studies comparing gss(+) and Δgss strains of E. coli show that a large number of genes are either up-regulated (76 genes more than threefold) or down-regulated (35 genes more than threefold) by the loss of the gss gene. Most significant categories of up-regulated genes include sulfur utilization, glutamine and succinate metabolism, polyamine and arginine metabolism, and purine and pyrimidine metabolism.

Glutathione analogs in prokaryotes

BACKGROUND

Oxygen is both essential and toxic to all forms of aerobic life and the chemical versatility and reactivity of thiols play a key role in both aspects. Cysteine thiol groups have key catalytic functions in enzymes but are readily damaged by reactive oxygen species (ROS). Low-molecular-weight thiols provide protective buffers against the hazards of ROS toxicity. Glutathione is the small protective thiol in nearly all eukaryotes but in prokaryotes the situation is far more complex.

SCOPE OF REVIEW

This review provides an introduction to the diversity of low-molecular-weight thiol protective systems in bacteria. The topics covered include the limitations of cysteine as a protector, the multiple origins and distribution of glutathione biosynthesis, mycothiol biosynthesis and function in Actinobacteria, recent discoveries involving bacillithiol found in Firmicutes, new insights on the biosynthesis and distribution of ergothioneine, and the potential protective roles played by coenzyme A and other thiols.

MAJOR CONCLUSIONS

Bacteria have evolved a diverse collection of low-molecular-weight protective thiols to deal with oxygen toxicity and environmental challenges. Our understanding of how many of these thiols are produced and utilized is still at an early stage.

GENERAL SIGNIFICANCE

Extensive diversity existed among prokaryotes prior to evolution of the cyanobacteria and the development of an oxidizing atmosphere. Bacteria that managed to adapt to life under oxygen evolved, or acquired, the ability to produce a variety of small thiols for protection against the hazards of aerobic metabolism. Many pathogenic prokaryotes depend upon novel thiol protection systems that may provide targets for new antibacterial agents. This article is part of a Special Issue entitled Cellular functions of glutathione.

Comparison of the functions of glutathionylspermidine synthetase/amidase from E. coli and its predicted homologues YgiC and YjfC

Protein function prediction is very important in establishing the roles of various proteins in bacteria; however, some proteins in the E. coli genome have their function assigned based on low percent sequence homology that does not provide reliable assignments. We have made an attempt to verify the prediction that E. coli genes ygiC and yjfC encode proteins with the same function as glutathionylspermidine synthetase/amidase (GspSA). GspSA is a bifunctional enzyme that catalyzes the ATP-dependent formation and hydrolysis of glutathionylspermidine (G-Sp), a conjugate of glutathione (GSH) and spermidine. YgiC and YjfC proteins show 51% identity between themselves and 28% identity to the synthetase domain of the GspSA enzyme. YgiC and YjfC proteins were expressed and purified, and the properties of GspSA, YgiC, and YjfC were compared. In contrast to GspSA, proteins YgiC and YjfC did not bind to G-Sp immobilized on the affinity matrix. We demonstrated that all three proteins (GspSA, YgiC and YjfC) catalyze the hydrolysis of ATP; however, YgiC and YjfC cannot synthesize G-Sp, GSH, or GSH intermediates. gsp, ygiC, and yjfC genes were eliminated from the E. coli genome to test the ability of mutant strains to synthesize G-Sp conjugate. E. coli cells deficient in GspSA do not produce G-Sp while synthesis of the conjugate is not affected in ΔygiC and ΔyjfC mutants. All together our results indicate that YgiC and YjfC are not glutathionylspermidine synthetases as predicted from the amino acid sequence analysis.

Ni2+-activated glyoxalase I from Escherichia coli: substrate specificity, kinetic isotope effects and evolution within the βαβββ superfamily

The Escherichia coli glyoxalase system consists of the metalloenzymes glyoxalase I and glyoxalase II. Little is known regarding Ni(2+)-activated E. coli glyoxalase I substrate specificity, its thiol cofactor preference, the presence or absence of any substrate kinetic isotope effects on the enzyme mechanism, or whether glyoxalase I might catalyze additional reactions similar to those exhibited by related βαβββ structural superfamily members. The current investigation has shown that this two-enzyme system is capable of utilizing the thiol cofactors glutathionylspermidine and trypanothione, in addition to the known tripeptide glutathione, to convert substrate methylglyoxal to non-toxic D-lactate in the presence of Ni(2+) ion. E. coli glyoxalase I, reconstituted with either Ni(2+) or Cd(2+), was observed to efficiently process deuterated and non-deuterated phenylglyoxal utilizing glutathione as cofactor. Interestingly, a substrate kinetic isotope effect for the Ni(2+)-substituted enzyme was not detected; however, the proton transfer step was observed to be partially rate limiting for the Cd(2+)-substituted enzyme. This is the first non-Zn(2+)-activated GlxI where a metal ion-dependent kinetic isotope effect using deuterium-labelled substrate has been observed. Attempts to detect a glutathione conjugation reaction with the antibiotic fosfomycin, similar to the reaction catalyzed by the related superfamily member FosA, were unsuccessful when utilizing the E. coli glyoxalase I E56A mutein.

Structure and function of YghU, a nu-class glutathione transferase related to YfcG from Escherichia coli

The crystal structure (1.50 Å resolution) and biochemical properties of the GSH transferase homologue, YghU, from Escherichia coli reveal that the protein is unusual in that it binds two molecules of GSH in each active site. The crystallographic observation is consistent with biphasic equilibrium binding data that indicate one tight (K(d1) = 0.07 ± 0.03 mM) and one weak (K(d2) = 1.3 ± 0.2 mM) binding site for GSH. YghU exhibits little or no GSH transferase activity with most typical electrophilic substrates but does possess a modest catalytic activity toward several organic hydroperoxides. Most notably, the enzyme also exhibits disulfide-bond reductase activity toward 2-hydroxyethyl disulfide [k(cat) = 74 ± 6 s(-1), and k(cat)/K(M)(GSH) = (6.6 ± 1.3) × 10(4) M(-1) s(-1)] that is comparable to that previously determined for YfcG. A superposition of the structures of the YghU·2GSH and YfcG·GSSG complexes reveals a remarkable structural similarity of the active sites and the 2GSH and GSSG molecules in each. We conclude that the two structures represent reduced and oxidized forms of GSH-dependent disulfide-bond oxidoreductases that are distantly related to glutaredoxin 2. The structures and properties of YghU and YfcG indicate that they are members of the same, but previously unidentified, subfamily of GSH transferase homologues, which we suggest be called the nu-class GSH transferases.

Interplay between drug efflux and antioxidants in Escherichia coli resistance to antibiotics

We investigated possible cross talk between endogenous antioxidants glutathione, spermidine, and glutathionylspermidine and drug efflux in Escherichia coli. We found that cells lacking either spermidine or glutathione are less susceptible than the wild type to novobiocin and certain aminoglycosides. In contrast, exogenous glutathione protects against both bactericidal and bacteriostatic antibiotics. The glutathione protection does not require the AcrAB efflux pump but fails in cells lacking TolC because exogenous glutathione is toxic to these cells.

Protein S-thiolation by Glutathionylspermidine (Gsp): the role of Escherichia coli Gsp synthetASE/amidase in redox regulation

Certain bacteria synthesize glutathionylspermidine (Gsp), from GSH and spermidine. Escherichia coli Gsp synthetase/amidase (GspSA) catalyzes both the synthesis and hydrolysis of Gsp. Prior to the work reported herein, the physiological role(s) of Gsp or how the two opposing GspSA activities are regulated had not been elucidated. We report that Gsp-modified proteins from E. coli contain mixed disulfides of Gsp and protein thiols, representing a new type of post-translational modification formerly undocumented. The level of these proteins is increased by oxidative stress. We attribute the accumulation of such proteins to the selective inactivation of GspSA amidase activity. X-ray crystallography and a chemical modification study indicated that the catalytic cysteine thiol of the GspSA amidase domain is transiently inactivated by H(2)O(2) oxidation to sulfenic acid, which is stabilized by a very short hydrogen bond with a water molecule. We propose a set of reactions that explains how the levels of Gsp and Gsp S-thiolated proteins are modulated in response to oxidative stress. The hypersensitivities of GspSA and GspSA/glutaredoxin null mutants to H(2)O(2) support the idea that GspSA and glutaredoxin act synergistically to regulate the redox environment of E. coli.

Metabolic shutdown in Escherichia coli cells lacking the outer membrane channel TolC

The outer membrane channel TolC is a key component of multidrug efflux and type I secretion transporters in Escherichia coli. Mutational inactivation of TolC renders cells highly susceptible to antibiotics and leads to defects in secretion of protein toxins. Despite impairment of various transport functions, no growth defects were reported in cells lacking TolC. Unexpectedly, we found that the loss of TolC notably impairs cell division and growth in minimal glucose medium. The TolC-dependent phenotype was further exacerbated by the loss of ygiB and ygiC genes expressed in the same operon as tolC and their homologues yjfM and yjfC located elsewhere on the chromosome. Our results show that this growth deficiency is caused by depletion of the critical metabolite NAD(+) and high NADH/NAD(+) ratios. The increased amounts of PspA and decreased rates of NADH oxidation in Delta tolC membranes indicated stress on the membrane and dissipation of a proton motive force. We conclude that inactivation of TolC triggers metabolic shutdown in E. coli cells grown in minimal glucose medium. The Delta tolC phenotype is partially rescued by YgiBC and YjfMC, which have parallel functions independent from TolC.

Evolution and multifarious horizontal transfer of an alternative biosynthetic pathway for the alternative polyamine sym-homospermidine

Polyamines are small flexible organic polycations found in almost all cells. They likely existed in the last universal common ancestor of all extant life, and yet relatively little is understood about their biological function, especially in bacteria and archaea. Unlike eukaryotes, where the predominant polyamine is spermidine, bacteria may contain instead an alternative polyamine, sym-homospermidine. We demonstrate that homospermidine synthase (HSS) has evolved vertically, primarily in the alpha-Proteobacteria, but enzymatically active, diverse HSS orthologues have spread by horizontal gene transfer to other bacteria, bacteriophage, archaea, eukaryotes, and viruses. By expressing diverse HSS orthologues in Escherichia coli, we demonstrate in vivo the production of co-products diaminopropane and N(1)-aminobutylcadaverine, in addition to sym-homospermidine. We show that sym-homospermidine is required for normal growth of the alpha-proteobacterium Rhizobium leguminosarum. However, sym-homospermidine can be replaced, for growth restoration, by the structural analogues spermidine and sym-norspermidine, suggesting that the symmetrical or unsymmetrical form and carbon backbone length are not critical for polyamine function in growth. We found that the HSS enzyme evolved from the alternative spermidine biosynthetic pathway enzyme carboxyspermidine dehydrogenase. The structure of HSS is related to lysine metabolic enzymes, and HSS and carboxyspermidine dehydrogenase evolved from the aspartate family of pathways. Finally, we show that other bacterial phyla such as Cyanobacteria and some alpha-Proteobacteria synthesize sym-homospermidine by an HSS-independent pathway, very probably based on deoxyhypusine synthase orthologues, similar to the alternative homospermidine synthase found in some plants. Thus, bacteria can contain alternative biosynthetic pathways for both spermidine and sym-norspermidine and distinct alternative pathways for sym-homospermidine.

Analysis of the structure and function of YfcG from Escherichia coli reveals an efficient and unique disulfide bond reductase

YfcG is one of eight glutathione (GSH) transferase homologues encoded in the Escherichia coli genome. The protein exhibits low or no GSH transferase activity toward a panel of electrophilic substrates. In contrast, it has a very robust disulfide-bond reductase activity toward 2-hydroxyethyldisulfide on par with mammalian and bacterial glutaredoxins. The structure of YfcG at 2.3 A-resolution from crystals grown in the presence of GSH reveals a molecule of glutathione disulfide in the active site. The crystallographic results and the lack of functional cysteine residues in the active site of YfcG suggests that the reductase activity is unique in that no sulfhydryl groups in the YfcG protein are covalently involved in the redox chemistry.

ATP-dependent ligases in trypanothione biosynthesis--kinetics of catalysis and inhibition by phosphinic acid pseudopeptides

Glutathionylspermidine is an intermediate formed in the biosynthesis of trypanothione, an essential metabolite in defence against chemical and oxidative stress in the Kinetoplastida. The kinetic mechanism for glutathionylspermidine synthetase (EC 6.3.1.8) from Crithidia fasciculata (CfGspS) obeys a rapid equilibrium random ter-ter model with kinetic constants K_GSH = 609 μm, K_Spd = 157 μm and K_ATP = 215 μm. Phosphonate and phosphinate analogues of glutathionylspermidine, previously shown to be potent inhibitors of GspS from Escherichia coli, are equally potent against CfGspS. The tetrahedral phosphonate acts as a simple ground state analogue of glutathione (GSH) (K_i ∼ 156 μm), whereas the phosphinate behaves as a stable mimic of the postulated unstable tetrahedral intermediate. Kinetic studies showed that the phosphinate behaves as a slow-binding bisubstrate inhibitor [competitive with respect to GSH and spermidine (Spd)] with rate constants k₃ (on rate) = 6.98 × 10⁴m⁻¹·s⁻¹ and k₄ (off rate) = 1.3 × 10⁻³ s⁻¹, providing a dissociation constant K_i = 18.6 nm. The phosphinate analogue also inhibited recombinant trypanothione synthetase (EC 6.3.1.9) from C. fasciculata, Leishmania major, Trypanosoma cruzi and Trypanosoma brucei with K_i^app values 20–40-fold greater than that of CfGspS. This phosphinate analogue remains the most potent enzyme inhibitor identified to date, and represents a good starting point for drug discovery for trypanosomiasis and leishmaniasis.

The multiple functions of the thiol-based electron flow pathways of Escherichia coli: Eternal concepts revisited

Electron flow via thiols is a theme with many variations in all kingdoms of life. The favourable physichochemical properties of the redox active couple of two cysteines placed in the optimised environment of the thioredoxin fold allow for two electron transfers in between top biological reductants and ultimate oxidants. The reduction of ribonucleotide reductases by thioredoxin and thioredoxin reductase of Escherichia coli (E. coli) was one of the first pathways to be elucidated. Diverse functions such as protein folding in the periplasm, maturation of respiratory enzymes, detoxification of hydrogen peroxide and prevention of oxidative damage may be based on two electron transfers via thiols. A growing field is the relation of thiol reducing pathways and the interaction of E. coli with different organisms. This concept combined with the sequencing of the genomes of different bacteria may allow for the identification of fine differences in the systems employing thiols for electron flow between pathogens and their corresponding mammalian hosts. The emerging possibility is the development of novel antibiotics.

Dependence of protective functions of Escherichia coli polyamines on strength of stress caused by superoxide radicals

Mechanisms of antioxidant effect of polyamines were studied in dependence on the strength of superoxide stress. Under conditions of weak stress, polyamines from Escherichia coli cultures were shown to function mainly as a scavenger of free superoxide radicals, whereas under conditions of strong stress they mainly acted as positive modulators of antioxidant genes. Spectrofluorimetry was used to show that both polyamine-dependent mutants and wild type cells treated with inhibitors of polyamine synthesis contained an elevated amount of free oxygen radicals, which could be decreased to the normal level by addition of exogenous polyamines. Under conditions of strong stress, polyamines positively influenced expression of the soxRS regulon genes of antioxidant defense, which was accompanied by an increase in the quantity (activity) of their gene products, such as glucose-6-phosphate dehydrogenase (Zwf) and fumarase (FumC). These effects led to an increase in the number of live cells in the cultures subjected to superoxide stress.

Biological chemistry of naturally occurring thiols of microbial and marine origin

The presence of thiols in living systems is critical for the maintenance of cellular redox potentials and protein thiol-disulfide ratios, as well as for the protection of cells from reactive oxygen species. In addition to the well-studied tripeptide glutathione (gamma-Glu-Cys-Gly), a number of compounds have been identified that contribute to these essential cellular roles. This review provides a survey of the chemistry and biochemistry of several critically important and naturally occurring intracellular thiols such as coenzyme M, trypanothione, mycothiol, ergothioneine, and the ovothiols. Coenzyme M is a key thiol required for methane production in methogenic bacteria. Trypanothione and mycothiol are very important to the biochemistry of a number of human pathogens, and the enzymes utilizing these thiols have been recognized as important novel drug targets. Ergothioneine, although synthesized by fungi and the Actinomycetales bacteria, is present at significant physiological levels in humans and may contribute to single electron redox reactions in cells. The ovothiols appear to function as important modulators of reactive oxygen toxicity and appear to serve as small molecule mimics of glutathione peroxidase, a key enzyme in the detoxification of reactive oxygen species.

Sulfur-Containing Amino Acid Metabolism in Parasitic Protozoa

Sulfur-containing amino acids play indispensable roles in a wide variety of biological activities including protein synthesis, methylation, and biosynthesis of polyamines and glutathione. Biosynthesis and catabolism of these amino acids need to be carefully regulated to achieve the requirement of the above-mentioned activities and also to eliminate toxicity attributable to the amino acids. Genome-wide analyses of enzymes involved in the metabolic pathways of sulfur-containing amino acids, including transsulfuration, sulfur assimilatory de novo cysteine biosynthesis, methionine cycle, and degradation, using genome databases available from a variety of parasitic protozoa, reveal remarkable diversity between protozoan parasites and their mammalian hosts. Thus, the sulfur-containing amino acid metabolic pathways are a rational target for the development of novel chemotherapeutic and prophylactic agents against diseases caused by protozoan parasites. These pathways also demonstrate notable heterogeneity among parasites, suggesting that the metabolism of sulfur-containing amino acids reflects the diversity of parasitism among parasite species, and probably influences their biology and pathophysiology such as virulence competence and stress defense.

Trypanothione biosynthesis in Leishmania major

Trypanothione plays a crucial role in regulation of intracellular thiol redox balance and in defence against chemical and oxidant stress. Crithidia fasciculata requires two enzymes for the formation of trypanothione, namely glutathionylspermidine synthetase (GspS; EC 6.3.1.8) and a glutathionylspermidine-dependent trypanothione synthetase (TryS; EC 6.3.1.9), whereas Trypanosoma cruzi and Trypanosoma brucei use a broad-specificity trypanothione synthetase to make trypanothione from glutathione (GSH) and spermidine. Here, we report the identification of two genes in Leishmania major with similarity to previously identified GSPS and TRYS. GSPS is an apparent pseudogene containing two frame shift mutations and two stop codons, whereas TRYS is in a single open-reading frame. The enzyme encoded by TRYS was expressed and found to catalyse formation of trypanothione with GSH and either spermidine or glutathionylspermidine. When GSH is varied as substrate the enzyme displays substrate inhibition (apparent Km=89 microM, Ki(s)=1mM, k(cat)=2s-1). At a fixed GSH concentration, the enzyme obeys simple hyperbolic kinetics with the other substrates with apparent Km values for spermidine, glutathionylspermidine and MgATP of 940, 40 and 63 microM, respectively. Immunofluorescence and sub-cellular fractionation studies indicate that TryS localises to the cytosol of L. major promastigotes. Phylogenetic analysis of the GspS and TryS amino acid sequences suggest that in the trypanosomatids, TryS has evolved to replace the GspS/TryS complex in C. fasciculata. It also appears that the L. major still harbours a redundant GSPS pseudogene that may be currently in the process of being lost from its genome.

A trypanothione-dependent glyoxalase I with a prokaryotic ancestry in Leishmania major

Glyoxalase I forms part of the glyoxalase pathway that detoxifies reactive aldehydes such as methylglyoxal, using the spontaneously formed glutathione hemithioacetal as substrate. All known eukaryotic enzymes contain zinc as their metal cofactor, whereas the Escherichia coli glyoxalase I contains nickel. Database mining and sequence analysis identified putative glyoxalase I genes in the eukaryotic human parasites Leishmania major, Leishmania infantum, and Trypanosoma cruzi, with highest similarity to the cyanobacterial enzymes. Characterization of recombinant L. major glyoxalase I showed it to be unique among the eukaryotic enzymes in sharing the dependence of the E. coli enzyme on nickel. The parasite enzyme showed little activity with glutathione hemithioacetal substrates but was 200-fold more active with hemithioacetals formed from the unique trypanosomatid thiol trypanothione. L. major glyoxalase I also was insensitive to glutathione derivatives that are potent inhibitors of all other characterized glyoxalase I enzymes. This substrate specificity is distinct from that of the human enzyme and is reflected in the modification in the L. major sequence of a region of the human protein that interacts with the glycyl-carboxyl moiety of glutathione, a group that is conjugated to spermidine in trypanothione. This trypanothione-dependent glyoxalase I is therefore an attractive focus for additional biochemical and genetic investigation as a possible target for rational drug design.

Conserved structural elements in glutathione transferase homologues encoded in the genome ofEscherichia coli

Multiple sequence alignments of the eight glutathione (GSH) transferase homologues encoded in the genome of Escherichia coli were used to define a consensus sequence for the proteins. The consensus sequence was analyzed in the context of the three-dimensional structure of the gst gene product (EGST) obtained from two different crystal forms of the enzyme. The enzyme consists of two domains. The N-terminal region (domain I) has a thioredoxin-like alpha/beta-fold, while the C-terminal domain (domain II) is all alpha-helical. The majority of the consensus residues (12/17) reside in the N-terminal domain. Fifteen of the 17 residues are involved in hydrophobic core interactions, turns, or electrostatic interactions between the two domains. The results suggest that all of the homologues retain a well-defined group of structural elements both in and between the N-terminal alpha/beta domain and the C-terminal domain. The conservation of two key residues for the recognition motif for the gamma-glutamyl-portion of GSH indicates that the homologues may interact with GSH or GSH analogues such as glutathionylspermidine or alpha-amino acids. The genome context of two of the homologues forms the basis for a hypothesis that the b2989 and yibF gene products are involved in glutathionylspermidine and selenium biochemistry, respectively.

Characterization of recombinant glutathionylspermidine synthetase/amidase from Crithidia fasciculata

Trypanothione [N1,N8-bis(glutathionyl)spermidine] is a unique metabolite found only in trypanosomatids, where it subsumes many of the functions of GSH in other organisms. In Crithidia fasciculata, two distinct ATP-dependent ligases, glutathionylspermidine synthetase (GspS; EC 6.3.1.8) and trypanothione synthetase (TryS; EC 6.3.1.9), are involved in the synthesis of trypanothione from GSH and spermidine. Both enzymes have been cloned previously, but expression in Escherichia coli produced insoluble and inactive protein. Here we report on the successful expression of soluble (His)6-tagged C. fasciculata GspS in E. coli. Following purification using nickel-chelating affinity chromatography, the tag sequence was removed and the enzyme purified to homogeneity by anion-exchange chromatography. The kinetic parameters of the recombinant enzyme have been determined using a coupled enzyme assay and also by HPLC analysis of end-product formation. Under optimal conditions (0.1 M K+-Hepes, pH 7.3) GspS has synthetase activity with apparent K(m) values for GSH, spermidine and MgATP of 242, 59 and 114 microM respectively, and a k(cat) of 15.5 s(-1). Glutathionylspermidine is formed as end product and the enzyme lacks TryS activity. Like E. coli GspS, the recombinant enzyme also possesses amidase activity (EC 3.5.1.78), hydrolysing glutathionylspermidine to GSH and spermidine with a k(cat) of 0.38 s(-1) and a K(m) of 500 microM. GspS can also hydrolyse trypanothione at about 1.5% of the rate with glutathionylspermidine. A single amino acid mutation (Cys-79-->Ala) is shown to ablate the amidase activity without affecting the synthetase activity.

Structural basis for the thioredoxin-like activity profile of the glutaredoxin-like NrdH-redoxin from Escherichia coli

NrdH-redoxin is a representative of a class of small redox proteins that contain a conserved CXXC motif and are characterized by a glutaredoxin-like amino acid sequence and thioredoxin-like activity profile. The crystal structure of recombinant Escherichia coli NrdH-redoxin in the oxidized state has been determined at 1.7 A resolution by multiwavelength anomalous diffraction. NrdH-redoxin belongs to the thioredoxin superfamily and is structurally most similar to E. coli glutaredoxin 3 and phage T4 glutaredoxin. The angle between the C-terminal helix alpha3 and strand beta4, which differs between thioredoxin and glutaredoxin, has an intermediate value in NrdH-redoxin. The orientation of this helix is to a large extent determined by an extended hydrogen-bond network involving the highly conserved sequence motif (61)WSGFRP(D/E)(67), which is unique to this subclass of the thioredoxin superfamily. Residues that bind glutathione in glutaredoxins are in general not conserved in NrdH-redoxin, and no glutathione-binding cleft is present. Instead, NrdH-redoxin contains a wide hydrophobic pocket at the surface, similar to thioredoxin. Modeling studies suggest that NrdH-redoxin can interact with E. coli thioredoxin reductase at this pocket and also via a loop that is complementary to a crevice in the reductase in a similar manner as observed in the E. coli thioredoxin-thioredoxin reductase complex.

Cloning and characterization of the two enzymes responsible for trypanothione biosynthesis in Crithidia fasciculata

Protozoa of the order Kinetoplastida differ from other organisms in their ability to conjugate glutathione (gamma-Glu-Cys-Gly) and spermidine to form trypanothione (N1,N8-bis(glutathionyl)spermidine), which is involved in maintaining intracellular thiol redox and in defense against oxidants. In this study, the genes from Crithidia fasciculata, Cf-GSS and Cf-TRS, which encode, respectively, glutathionylspermidine synthetase (EC 6.3.1.8) and trypanothione synthetase (EC 6.3.1.9) have been cloned and expressed. The deduced amino acid sequence of both Cf-GSS and Cf-TRS share 50% sequence similarity with the Escherichia coli glutathionylspermidine synthetase/amidase. Both genes are present as single copies in the C. fasciculata genome. When expressed in E. coli and Saccharomyces cerevisiae, neither protein was present in an active soluble form. However, thiol analysis of S. cerevisiae demonstrated that cells transformed with the Cf-GSS gene contained substantial amounts of glutathionylspermidine, whereas cells expressing both the Cf-GSS and Cf-TRS genes contained glutathionylspermidine and trypanothione, confirming that these genes encode the functional glutathionylspermidine and trypanothione synthetases from C. fasciculata. The translation products of Cf-GSS and Cf-TRS show significant homology to the amidase domain present in E. coli glutathionylspermidine synthetase, which can catalyze both synthesis and degradation of glutathionylspermidine. Glutathionylspermidine synthetase isolated from C. fasciculata was found to possess a similar amidase activity.

Convenient isolation and kinetic mechanism of glutathionylspermidine synthetase from Crithidia fasciculata

Trypanothione, the essential metabolite in the oxidant defense system of trypanosomatids, is synthesized by two distinct proteins, glutathionylspermidine synthetase and trypanothione synthetase. Glutathionylspermidine synthetase was purified to homogeneity from the trypanosomatid Crithidia fasciculata by aqueous two-phase systems and chromatography. The enzyme showed a specific activity of 38 micromol of glutathionylspermidine formed per min per mg of protein. Its molecular mass was 78 kDa in SDS-polyacrylamide gel electrophoresis, and it appeared predominantly monomeric in native polyacrylamide gel electrophoresis and gel filtration. The isoelectric point was at pH 4.6, and the pH optimum was near 7.6. Partial amino acid sequencing revealed homology with, but low similarity to, the glutathionylspermidine synthetase/amidase of Escherichia coli, and amidase activity was not detected in glutathionylspermidine synthetase of C. fasciculata. The kinetics of trypanosomatid glutathionylspermidine synthetase revealed a rapid equilibrium random mechanism with limiting Km values for Mg2+-ATP, GSH, and spermidine of 0.25 +/- 0.02, 2.51 +/- 0.33, and 0.47 +/- 0. 09 mM, respectively, and a kcat of 415 +/- 78 min-1. Partial reactions at restricted cosubstrate supply were not detected by 31P NMR, supporting the necessity of a quarternary complex formation for catalysis. ADP inhibited competitively with respect to ATP (Ki = 0. 08 mM) and trypanothione exerted a feedback inhibition competitive with GSH (Ki = 0.48 mM).

Characterization of the peptide substrate specificity of glutathionylspermidine synthetase from Crithidia fasciculata

Trypanothione, a metabolite specific to trypanosomatid parasites, is enzymatically synthesized from spermidine and glutathione by the consecutive action of the ATP-dependent carbon-nitrogen ligases, glutathionylspermidine synthetase and trypanothione synthetase. As part of our programme aimed at developing inhibitors of these enzymes, we have synthesized a series of analogues of glutathione (gamma-L-Glu-L-Cys-Gly) and tested them as substrates or inhibitors of glutathionylspermidine synthetase. Recognition at the gamma-glutamyl moiety appears to be essential, as any modification of this part of glutathione results in a total loss of activity as a substrate. Alkylation of the thiol side chain of cysteine with methyl, ethyl or propyl groups yields analogues with catalytic efficiencies (kcat/Km) as substrates equivalent to or better than glutathione. In contrast, the bulkier S-butyl analogue was a much poorer substrate. Substitution of L-Cys by amino acids with an alkyl side-chain is also well tolerated giving relative catalytic efficiencies of 1.1 and 1.5 for peptide analogues containing L-Val and L-Ile respectively. Other analogues, where the bulk of the alkyl chain is increased further (as in L-Leu or L-Phe) or where the glycine moiety is replaced with L-Ala, are inhibitors rather than substrates.

Charge is the major discriminating factor for glutathione reductase versus trypanothione reductase inhibitors

Benson et al. (Biochem. J. 1992, 286, 9) reported three novel competitive inhibitors of trypanothione reductase (TR), which were selected to complement a hydrophobic region identified on the TR structure which was not present on human glutathione reductase (hGR). Benson et al. also noted that chlorpromazine, a tricyclic antidepressant known to have trypanocidal activity, was an inhibitor of TR. Here we show that chlorpromazine is a competitive inhibitor of TRs from Crithidia fasciculata (Ki = 14 microM) and Trypanosoma cruzi (Ki = 10 microM), but the drug binds > 50-fold more weakly (Ki = 762 microM) to hGR. Analogues of chlorpromazine differing in the length of the side chain carrying the positively charged R-group are also selective TR inhibitors whereas, a tricyclic structure carrying a negatively charged side chain is a competitive inhibitor with selectivity for hGR (K(hGR)i = 165 microM vs. K(TR)i = 1400 microM). This finding suggests that simple charge characteristics, rather than differences in hydrophobicity, may account for a significant portion of the selectivity of this series of inhibitors for these two enzymes. Electrostatic analysis of the structures of TR and hGR thus provides a rationale for these results, and offers a new principle for inhibitor design. The principle gains further support from the observation that all known tricyclic competitive inhibitors of TR are positively charged. In order to investigate the in vivo relevance of our findings we have examined the effect of chlorpromazine and its negatively charged analogue on the growth of C. fasciculata parasites. Consistent with our kinetics, chlorpromazine (50 microM) inhibited the growth of parasites by 50%, while no measurable decrease in parasite growth rate was noted in the presence of the negatively charged inhibitor (400 microM). Furthermore, the highly similar inhibitory profiles of C. fasciculata TR and T. cruzi TR suggest that drug-design studies using the structurally better-studied C. fasciculata TR are also relevant to the human pathogen T. cruzi.

The crystal structure of trypanothione reductase from the human pathogen Trypanosoma cruzi at 2.3 A resolution

Trypanothione reductase (TR) is an NADPH-dependent flavoprotein unique to protozoan parasites from the genera Trypanosoma and Leishmania and is an important target for the design of improved trypanocidal drugs. We present details of the structure of TR from the human pathogen Trypanosoma cruzi, the agent responsible for Chagas' disease or South American trypanosomiasis. The structure has been solved by molecular replacement, using as the starting model the structure of the enzyme from the nonpathogenic Crithidia fasciculata, and refined to an R-factor of 18.9% for 53,868 reflections with F > or = sigma F between 8.0 and 2.3 A resolution. The model comprises two subunits (968 residues), two FAD prosthetic groups, two maleate ions, and 419 water molecules. The accuracy and geometry of the enzyme model is improved with respect to the C. fasciculata enzyme model. The new structure is described and specific features of the enzyme involved in substrate interactions are compared with previous models of TR and related glutathione reductases from human and Escherichia coli. Structural differences at the edge of the active sites suggest an explanation for the differing specificities toward glutathionylspermidine disulfide.