Structure determination and refinement of human alpha class glutathione transferase A1-1, and a comparison with the Mu and Pi class enzymes

A surface mutant (G82R) of a human alpha-glutathione S-transferase shows decreased thermal stability and a new mode of molecular association in the crystal

A chimeric enzyme (GST121) of the human alpha-glutathione S-transferases GST1-1 and GST2-2, which has improved catalytic efficiency and thermostability from its wild-type parent proteins, has been crystallized in a space group that is isomorphous with that reported for crystals of GST1-1. However, a single-site (G82R) mutant of GST121, which exhibits a significant reduction both in vitro and in vivo in protein thermostability, forms crystals that are not isomorphous with GST1-1. The mutant protein crystallizes in space group P2(1)2(1)2(1), with cell dimensions a = 49.5, b = 92.9, c = 115.9 A, and one dimer per asymmetric unit. Preliminary crystallographic results show that a mutation of the surface residue Gly 82 from a neutral to a charged residue causes new salt bridges to be formed among the GST dimers, suggesting that the G82R mutant might aggregate more readily than does GST121 in solution, resulting in a change of its solution properties.

Contribution of amino acid residue 208 in the hydrophobic binding site to the catalytic mechanism of human glutathione transferase A1-1

Glutathione transferases (GSTs) catalyze the nucleophilic attack of the thiolate of glutathione on a variety of noxious, often hydrophobic, electrophiles. The interactions responsible for the binding of glutathione have been deduced in great detail from the 3-dimensional structures that have been solved for three different GSTs, each a member of a distinct structural class. However, the interactions of the electrophilic substrates with these enzymes are still largely unexplored. The contribution of the active-site Met208 to aromatic and benzylic chloride substitution reactions catalyzed by human class Alpha GST A1-1 has been evaluated by comparison of wild-type enzyme with variants mutated in position 208. The results show that the amino acid residue at position 208 primarily affects the aromatic substitution reaction, tested with 1-chloro-2,4-dinitrobenzene as substrate, possibly by interacting with the delocalized negative charge of the substituted ring structure in the transition state.

Isolation and characterization of octopus hepatopancreatic glutathione S-transferase. Comparison of digestive gland enzyme with lens S-crystallin

Glutathione S-transferase from Octopus vulgaris hepatopancreas was purified to apparent homogeneity by single glutathione-Sepharose-4B affinity chromatography with overall yield 46% and purification 249-fold. The enzyme was a homodimer with subunit M(r) 24,000, which was smaller than that of the octopus lens S-crystallin (M(r) 27,000) with glutathione-S-transferase-like structure. Both proteins showed substrate specificities similar to alpha/pi-type isozyme of glutathione S-transferase. Under native conditions, both proteins exhibited multiple forms upon polyacrylamide gel electrophoresis or isoelectric focusing, albeit with distinct mobilities; however, only one kind of N-terminal amino acid sequence was determined for the multiple forms of each protein. The hepatopancreatic GST, with pI value 6.6-7.3, dissociated into two monomers in an acidic or alkaline environment. Two amino acid residues, with pKa values 5.69 +/- 0.14 and 9.03 +/- 0.11 were involved in the subunit interactions of the hepatopancreatic enzyme.

Naturally occurring human glutathione S-transferase GSTP1-1 isoforms with isoleucine and valine in position 104 differ in enzymic properties

Glutathione S-transferase P1-1 isoforms, differing in a single amino acid residue (Ile104 or Val104), have been previously identified in human placenta [Ahmad, H., Wilson, D. E., Fritz, R. R., Singh, S. V., Medh, R. D., Nagle, G. T., Awasthi, Y. C. & Kurosky, A. (1990) Arch. Biochem. Biophys. 278, 398-408]. In the present report, the enzymic properties of these two proteins are compared. [I104]glutathione S-transferase P1-1 has been expressed from its cDNA in Escherichia coli and purified to homogeneity by affinity chromatography; the cDNA has been mutated to replace Ile104 by Val104, and [V104]glutathione S-transferase P1-1 was expressed and isolated as described for [I104]glutathione S-transferase P1-1. The two enzymes differed in their specific activity and affinity for electrophilic substrates (KM values for 1-chloro-2,4-dinitrobenzene were 0.8 mM and 3.0 mM for [I-104]glutathione S-transferase P1-1 and [V-104]glutathione S-transferase P1-1, respectively), but were identical in their affinity for glutathione. In addition, the two enzymes were distinguishable by their heat stability, with half-lives at 45 degrees C of 19 min and 51 min, respectively. The resistance to heat denaturation was differentially modulated by the presence of substrates. These data, in conjunction with molecular modeling, indicate that the residue in position 104 helps to define the geometry of the hydrophobic substrate-binding site, and may also influence activity by interacting with residues directly involved in substrate binding.

Photoaffinity labelling of the active site of the rat glutathione transferases 3-3 and 1-1 and human glutathione transferase A1-1

The glutathione transferases (GSTs) form a group of enzymes responsible for a wide range of molecular detoxications. The photoaffinity label S-(2-nitro-4-azidophenyl)glutathione was used to study the hydrophobic region of the active site of the rat liver GST 1-1 and 2-2 isoenzymes (class Alpha) as well as the rat class-Mu GST 3-3. Photoaffinity labelling was carried out using a version of S-(2-nitro-4-azidophenyl)glutathione tritiated in the arylazido ring. The labelling occurred with higher levels of radioisotope incorporation for the Mu than the Alpha families. Taking rat GST 3-3, 1.18 (+/- 0.05) mol of radiolabel from S-(2-nitro-4-azidophenyl)glutathione was incorporated per mol of dimeric enzyme, which could be blocked by the presence of the strong competitive inhibitor, S-tritylglutathione (Ki = 1.4 x 10(-7) M). Radiolabelling of the protein paralleled the loss of enzyme activity. Photoaffinity labelling by tritiated S-(2-nitro-4-azidophenyl)glutathione on a preparative scale (in the presence and absence of S-tritylglutathione) followed by tryptic digestion and purification of the labelled peptides indicated that GST 3-3 was specifically photolabelled; the labelled peptides were sequenced. Similarly, preparative photoaffinity labelling by S-(2-nitro-4-azidophenyl)glutathione of the rat liver 1-1 isoenzyme, the human GST A1-1 and the human-rat chimaeric GST, H1R1/1, was carried out with subsequent sequencing of radiolabelled h.p.l.c.-purified tryptic peptides. The results were interpreted by means of molecular-graphics analysis to locate photoaffinity-labelled peptides using the X-ray-crystallographic co-ordinates of rat GST 3-3 and human GST A1-1. The molecular-graphical analysis indicated that the labelled peptides are located within the immediate vicinity of the region occupied by S-substituted glutathione derivatives bound in the active-site cavity of the GSTs investigated.

Isolation and characterization of the Methylophilus sp. strain DM11 gene encoding dichloromethane dehalogenase/glutathione S-transferase

The restricted facultative methylotroph Methylophilus sp. strain DM11 utilizes dichloromethane as the sole carbon and energy source. It differs from other dichloromethane-utilizing methylotrophs by faster growth on this substrate and by possession of a group B dichloromethane dehalogenase catalyzing dechlorination at a fivefold-higher rate than the group A enzymes of slow-growing strains. We isolated dcmA, the structural gene of the strain DM11 dichloromethane dehalogenase, to elucidate its relationship to the previously characterized dcmA gene of Methylobacterium sp. strain DM4, which encodes a group A enzyme. Nucleotide sequence determination of dcmA from strain DM11 predicts a protein of 267 amino acids, corresponding to a molecular mass of 31,197 Da. The 5' terminus of in vivo dcmA transcripts was determined by primer extension to be 70 bp upstream of the translation initiation codon. It was preceded by a putative promoter sequence with high resemblance to the Escherichia coli sigma 70 consensus promoter sequence. dcmA and 130 bp of its upstream sequence were brought under control of the tac promoter and expressed in E. coli to approximately 20% of the total cellular protein by induction with isopropylthiogalactopyranoside (IPTG) and growth at 25 degrees C. Expression at 37 degrees C led to massive formation of inclusion bodies. Comparison of the strain DM11 and strain DM4 dichloromethane dehalogenase sequences revealed 59% identity at the DNA level and 56% identity at the protein level, thus indicating an ancient divergence of the two enzymes. Both dehalogenases are more closely related to eukaryotic class theta glutathione S-transferases than to a number of bacterial glutathione S-transferases.

X-ray crystal structures of cytosolic glutathione S-transferases. Implications for protein architecture, substrate recognition and catalytic function

Crystal structures of cytosolic glutathione S-transferases (EC 2.5.1.18), complexed with glutathione or its analogues, are reviewed. The atomic models define protein architectural relationships between the different gene classes in the superfamily, and reveal the molecular basis for substrate binding at the two adjacent subsites of the active site. Considerable progress has been made in understanding the mechanism whereby the thiol group of glutathione is destabilized (lowering its pKa) at the active site, a rate-enhancement strategy shared by the soluble glutathione S-transferases.

Crystallization and preliminary X-ray diffraction studies of a glutathione S-transferase from the Australian sheep blowfly, Lucilia cuprina

Crystals of a glutathione S-transferase from the Australian sheep blowfly Lucilia cuprina have been grown from ammonium sulphate by the hanging drop vapour diffusion method. Successful crystallization required the presence of the inhibitor S-hexylglutathione. The crystals belong to the tetragonal space group P4(1)22 (or P4(3)22) with cell dimensions of a = b = 88.1 A and c = 66.9 A. They contain one monomer in the asymmetric unit and diffract beyond 2.8 A resolution.

Detoxication of base propenals and other alpha, beta-unsaturated aldehyde products of radical reactions and lipid peroxidation by human glutathione transferases

Radiation and chemical reactions that give rise to free radicals cause the formation of highly cytotoxic base propenals, degradation products of DNA. Human glutathione transferases (GSTs; RX:glutathione R-transferase, EC 2.5.1.18) of classes Alpha, Mu, and Pi were shown to promote the conjugation of glutathione with base propenals and related alkenes. GST P1-1 was particularly active in catalyzing the reactions with the propenal derivatives, and adenine propenal was the substrate giving the highest activity. The catalytic efficiency of GST P1-1 with adenine propenal (kcat/Km = 7.7 x 10(5) M-1.s-1) is the highest so far reported with any substrate for this enzyme. In general, GST A1-1 and GST M1-1, in contrast to GST P1-1, were more active with 4-hydroxyalkenals (products of lipid peroxidation) than with base propenals. The adduct resulting from the Michael addition of glutathione to the alkene function of one of the base propenals (adenine propenal) was identified by mass spectrometry. At the cellular level, GST P1-1 was shown to provide protection against alpha, beta-unsaturated aldehydes. GST P1-1 added to the culture medium of HeLa cells augmented the protective effect of glutathione against the toxicity of adenine propenal and thymine propenal. No protective effect of the enzyme was observed in the presence of the competitive inhibitor S-hexylglutathione. GST P1-1 introduced into Hep G2 cells by electroporation was similarly found to increase their resistance to acrolein. The results show that glutathione transferases may play an important role in cellular detoxication of electrophilic alpha, beta-unsaturated carbonyl compounds produced by radical reactions, lipid peroxidation, ionizing radiation, and drug metabolism.

Investigation of intra-domain and inter-domain interactions of glutathione transferase P1-1 by limited chymotryptic cleavage

Limited proteolysis of glutathione transferase P1-1 (GSTP1-1) by chymotrypsin performed at 20 degrees and 30 degrees C mainly generates two complementary peptides of 17 kDa and 6 kDa molecular mass with concomitant loss of catalytic capacity. Sequence analysis of these peptides showed that the peptide bond between Tyr47 and Gly48 was cleaved. The analysis of the recently resolved three-dimensional structure of GSTP1-1 [Reinemer, P., Dirr, H. W., Ladenstein, R., Huber, R., Lo Bello, M., Federici, G. & Parker, M. W. (1992) J. Mol. Biol. 227, 214-226] suggests that the proteolytically cleaved bond results located in a portion of the polypeptide chain lining the G-site which has been demonstrated to be part of an exposed and flexible region of the N-terminal domain (structural elements alpha B1 and alpha B2) [Aceto, A., Caccuri, A. M., Sacchetta, P., Bucciarelli, T., Dragani, B., Rosato, N., Federici, G. & Di Ilio, C. (1992) Biochem. J. 285, 241-245]. The fragments which are generated by proteolysis at 20 degrees C, remain linked by noncovalent interaction in a complex (nicked GSTP1-1) which is dissociated by incubation at higher temperatures. As shown by circular dichroic analysis, although inactive, nicked GSTP1-1 retains an overall secondary structure closely resembling that of the parent enzyme. However, the fluorescence data of the nicked GSTP1-1 indicate that the Trp38, which is near the chymotrypsin-cleavable bond, becomes exposed in a more polar environment. This indicates that, in the nicked enzyme, the polypeptide portion containing the structural elements alpha B1 and alpha B2 has more freedom of fluctuation. The fact that this polypeptide chain portion contains two essential amino acid residues of the G-site (Trp38 and Lys42) might account for the loss of ability to bind glutathione by the nicked enzyme which is consequently catalytically inactive. Proteolysis performed at 30 degrees C generated a homodimeric 17-kDa fragment. The structural analysis of this fragment suggests that the GSTP1-1 alpha C helix, which is located in the domain I and is thought to be involved in the inter-domain interaction, could exert a critical role in maintaining the native folding of domain II.

Structure and organization of the human alpha class glutathione S-transferase genes and related pseudogenes

We have isolated and characterized genomic DNA encoding several human Alpha class glutathione S-transferase genes and pseudogenes. All the genes are composed of seven exons with boundaries identical to those of the Alpha class genes in rats. The GSTA1 gene is approximately 12 kb in length and is closely flanked by other Alpha class gene sequences. The complete sequence of the 1.7-kb intergenic region between exon 7 of an upstream pseudogene and exon 1 of the GSTA1 gene has been determined. An additional gene that encodes an uncharacterized Alpha class glutathione S-transferase has been identified. The protein derived from this gene would have 19 amino acid substitutions compared with the GSTA1 isoenzyme. Several pseudogenes with single-base and/or complete exon deletions have been identified, but no reverse-transcribed pseudogenes have been detected. The occurrence of multiple genes and pseudogenes on a single fragment of cloned genomic DNA and the prior identification of a single chromosomal region (6p12) of hybridization (Board and Webb, 1987, Proc. Natl. Acad. Sci. USA 84:2377-2381) suggest that all the Alpha class genes are members of a closely linked gene family that has evolved by duplication and gene conversion events.

Fluorescence characterization of Trp 21 in rat glutathione S-transferase 1-1: microconformational changes induced by S-hexyl glutathione

The glutathione S-transferase (GST) isoenzyme A1-1 from rat contains a single tryptophan, Trp 21, which is expected to lie within alpha-helix 1 based on comparison with the X-ray crystal structures of the pi- and mu-class enzymes. Steady-state and multifrequency phase/modulation fluorescence studies have been performed in order to characterize the fluorescence parameters of this tryptophan and to document ligand-induced conformational changes in this region of the protein. Addition of S-hexyl glutathione to GST isoenzyme A1-1 causes an increase in the steady-state fluorescence intensity, whereas addition of the substrate glutathione has no effect. Frequency-domain excited-state lifetime measurements indicate that Trp 21 exhibits three exponential decays in substrate-free GST. In the presence of S-hexyl glutathione, the data are also best described by the sum of three exponential decays, but the recovered lifetime values change. For the substrate-free protein, the short lifetime component contributes 9-16% of the total intensity at four wavelengths spanning the emission. The fractional intensity of this lifetime component is decreased to less than 3% in the presence of S-hexyl glutathione. Steady-state quenching experiments indicate that Trp 21 is insensitive to quenching by iodide, but it is readily quenched by acrylamide. Acrylamide-quenching experiments at several emission wavelengths indicate that the long-wavelength components become quenched more easily in the presence of S-hexyl glutathione. Differential fluorescence polarization measurements also have been performed, and the data describe the sum of two anisotropy decay rates. The recovered rotational correlation times for this model are 26 ns and 0.81 ns, which can be attributed to global motion of the protein dimer, and fast local motion of the tryptophan side chain. These results demonstrate that regions of GST that are not in direct contact with bound substrates are mobile and undergo microconformational rearrangement when the "H-site" is occupied.