Construction, expression, and preliminary characterization of chimeric class mu glutathione S-transferases with altered catalytic properties

Characterization of the binding of 8-anilinonaphthalene sulfonate to rat class Mu GST M1-1

Molecular docking and ANS-displacement experiments indicated that 8-anilinonaphthalene sulfonate (ANS) binds the hydrophobic site (H-site) in the active site of dimeric class Mu rGST M1-1. The naphthalene moiety provides most of the van der Waals contacts at the ANS-binding interface while the anilino group is able to sample different rotamers. The energetics of ANS binding were studied by isothermal titration calorimetry (ITC) over the temperature range of 5-30 degrees C. Binding is both enthalpically and entropically driven and displays a stoichiometry of one ANS molecule per subunit (or H-site). ANS binding is linked to the uptake of 0.5 protons at pH 6.5. Enthalpy of binding depends linearly upon temperature yielding a DeltaC(p) of -80+/-4 cal K(-1) mol(-1) indicating the burial of solvent-exposed nonpolar surface area upon ANS-protein complex formation. While ion-pair interactions between the sulfonate moiety of ANS and protein cationic groups may be significant for other ANS-binding proteins, the binding of ANS to rGST M1-1 is primarily hydrophobic in origin. The binding properties are compared with those of other GSTs and ANS-binding proteins.

Impact of domain interchange on conformational stability and equilibrium folding of chimeric class micro glutathione transferases

Rat micro class glutathione transferases M1-1 and M2-2 are homodimers that share a 78% sequence identity but display differences in stability. M1-1 is more stable at the secondary and tertiary structural levels, whereas its quaternary structure is less stable. Each subunit in these proteins consists of two structurally distinct domains with intersubunit contacts occurring between domain 1 of one subunit and domain 2 of the other subunit. The chimeric subunit variants M(12), which has domain 1 of M1 and domain 2 of M2, and its complement M(21), were used to investigate the conformational stability of the chimeric homodimers M(12)-(12) and M(21)-(21) to determine the contribution of each domain toward stability. Exchanging entire domains between class micro GSTs is accommodated by the GST fold. Urea-induced equilibrium unfolding data indicate that whereas the class micro equilibrium unfolding mechanism (i.e., N(2) <--> 2I <--> 2U) is not altered, domain exchanges impact significantly on the conformational stability of the native dimers and monomeric folding intermediates. Data for the wild-type and chimeric proteins indicate that the order of stability for the native dimer (N(2)) is M2-2 > M(12)-(12) M1-1 approximately M(21)-(21), and that the order of stability of the monomeric intermediate (I) is M1 > M2 approximately M(12) > M(21). Interactions involving Arg 77, which is topologically conserved in GSTs, appear to play an important role in the stability of both the native dimeric and folding monomeric structures.

Solution structure of the carboxyl terminus of a human class Mu glutathione S-transferase: NMR assignment strategies in large proteins

Strategies to obtain the NMR assignments for the HN, N, CO, Calpha and Cbeta resonance frequencies for the human class mu glutathione-S-transferase GSTM2-2 are reported. These assignments were obtained with deuterated protein using a combination of scalar and dipolar connectivities and various specific labeling schemes. The large size of this protein (55 kDa, homodimer) necessitated the development of a novel pulse sequence and specific labeling strategies. These aided in the identification of residue type and were essential components in determining sequence specific assignments. These assignments were utilized in this study to characterize the structure and dynamics of the carboxy-terminal residues in the unliganded protein. Previous crystallographic studies of this enzyme in complex with glutathione suggested that this region may be disordered, and that this disorder may be essential for catalysis. Furthermore, in the related class alpha protein extensive changes in conformation of the C terminus are observed upon ligand binding. On the basis of the results presented here, the time-averaged conformation of the carboxyl terminus of unliganded GSTM2-2 is similar to that seen in the crystal structure. NOE patterns and 1H-15N heteronuclear nuclear Overhauser enhancements suggest that this region of the enzyme does not undergo motion on a rapid time scale.

The high activity of rat glutathione transferase 8-8 with alkene substrates is dependent on a glycine residue in the active site

Rat glutathione transferase (GST) 8-8 displays high catalytic activity with alpha, beta-unsaturated carbonyl compounds, including lipid peroxidation products such as 4-hydroxyalkenals. The catalytic efficiency of the related class Alpha GST 1-1 is substantially lower with the same substrates. Chimeric enzymes were prepared by replacing N-terminal subunit 8 segments of different lengths (6, 25, or 100 residues) with corresponding sequences from subunit 1 using recombinant DNA techniques. The chimeric subunit r1(25)r8, containing 25 amino acid residues from subunit 1, had the same low activity with alkenal substrates as that displayed by subunit 1. Mutation of Ala-12 into Gly in r1(25)r8 gave rise to the high alkenal activity characteristic of subunit 8, showing the importance of amino acid residue 12 for the activity. However, other structural determinants are also essential, as demonstrated by the corresponding Ala-12-->Gly mutation in subunit 1, which did not afford high alkenal activity. The results show that a single point mutation in a GST subunit may give rise to a 100-fold increase in catalytic efficiency with certain substrates. Introduction of such mutations may have contributed to the biological evolution of GST isoenzymes with altered substrate specificities and may also find use in the engineering of GSTs for novel functions.

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.

Sequence of the mRNA for a glutathione transferase Pi with a different substrate specificity in V79 Chinese hamster lung cells

The mRNA sequence for a glutathione transferase (GST) belonging to the Pi class has been determined. This was a first step towards elucidating, at the molecular level, why V79 Chinese hamster lung cells lack the capacity to conjugate the benzo[a]pyrene (BP) derivative BPDE, but nonetheless contain the GST pi gene, express GST pi mRNA and contain a protein that binds to antibodies directed against the human GST Pi enzyme. The sequencing strategy involved synthesis of a cDNA library, circularization of the GST pi cDNA for PCR amplification and subsequent DNA sequencing. The coding sequence for the GST Pi protein of V79 cells, designated CLOGSTP1, consisted of 627 bp coding for 209 amino acids (aa), corresponding to a 23-kDa protein. The cDNA sequence obtained demonstrated extensive homology to those from other species, especially rat and mouse, where this homology was 92 and 91%, respectively. Upon comparing the aa sequence predicted from CLOGSTP1 to those of rat, mouse, pig, cow and man, the most striking differences were found in aa positions 19, 39, 40, 110, 113 and 151. Consequently, the explanation for the lower capacity for GST Pi-catalyzed conjugation in V79 cells, as compared to other species, remains a matter of speculation, since none of these aa positions coincides with positions involved in the xenobiotic substrate-binding site of GST Pi from pig and human or of GST Mu from rat. The most likely candidates for causing the observed change in substrate specificity might be Lys110 and Glu113, which are the altered residues closest to this binding site and which might, thus, exclude BPDE as a substrate for the Chinese hamster enzyme.

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.

Reversible modification of rat liver glutathione S-transferase 3-3 with 1-chloro-2,4-dinitrobenzene: specific labelling of Tyr-115

Rat liver glutathione S-transferase 3-3 (GST, EC 2.5.1.18), a triple mutant with all three cysteine residues replaced with serine (CallS) and a quadruple mutant with a Tyr-115 to phenylalanine substitution on CallS (CallSY115F) were overexpressed in Escherichia coli under the control of a phoA promoter. Using this system, we obtained over 35 mg of fully active pure protein/litre of cell medium. GST 3-3 and CallS mutant were modified with 1-chloro-2,4-dinitrobenzene (CDNB), a model substrate for the enzyme, in the absence of GSH. Dinitrophenol, but not S-methylglutathione, inhibits this process. The dinitrophenyl groups are readily removed from the enzyme with GSH, but much more slowly with dithiothreitol. Results from peptide mapping and amino acid sequence analyses indicate that CDNB modifies the cysteine residues and Tyr-115 on wild-type GST 3-3, but only Tyr-115 on CallS. In addition, CDNB cannot modify the CallSY115F mutant. We propose that Tyr-115 is located at or near the H-site of GST 3-3.

Three-dimensional structure of class pi glutathione S-transferase from human placenta in complex with S-hexylglutathione at 2.8 A resolution

The three-dimensional structure of human class pi glutathione S-transferase from placenta (hGSTP1-1), a homodimeric enzyme, has been solved by Patterson search methods and refined at 2.8 A resolution to a final crystallographic R-factor of 19.6% (8.0 to 2.8 A resolution). Subunit folding topology, subunit overall structure and subunit association closely resembles the structure of porcine class pi glutathione S-transferase. The binding site of a competitive inhibitor, S-hexylglutathione, is analyzed and the locations of the binding regions for glutathione (G-site) and electrophilic substrates (H-site) are determined. The specific interactions between protein and the inhibitor's glutathione peptide are the same as those observed between glutathione sulfonate and the porcine isozyme. The H-site is located adjacent to the G-site, with the hexyl moiety lying above a segment (residues 8 to 10) connecting strand beta 1 and helix alpha A where it is in hydrophobic contact with Tyr7, Phe8, Val10, Val35 and Tyr106. Catalytic models are discussed on the basis of the molecular structure.

Design of two chimaeric human-rat class alpha glutathione transferases for probing the contribution of C-terminal segments of protein structure to the catalytic properties

Two chimaeric human-rat class Alpha glutathione transferases were constructed by fusion of DNA segments derived from the plasmids pTGT2-AT and pGTB38 and expression of the corresponding proteins in Escherichia coli. The recombinant proteins H1R1/1 and H1R1/2 encoded by plasmids pH1R1/1 and pH1R1/2 are composed of a segment of the human class Alpha subunit 1 from the N-terminus to His-143 and Pro-207 respectively, followed by the complementary C-terminal portion of the rat class Alpha subunit 1 sequence. Compared with the parental human enzyme, H1R1/1 is altered in 20 positions due to the introduction of 79 residues from the rat enzyme, while H1R1/2 is altered in five positions out of 15 in the C-terminal region. The design of mutant H1R1/1 is equivalent to introduction of exons 6 and 7 of the rat subunit 1 gene in place of the homologous human nucleotide sequence. The two chimaeric proteins are enzymatically active with several substrates, even though the activity in most cases is somewhat decreased in comparison with the wild-type human enzyme. Inhibition studies show that the kinetic properties mimic those of the human enzyme, indicating that the N-terminal two-thirds of the primary structure plays the major role in governing the catalytic properties. The results of this study demonstrate that recombination of segments of primary structure between homologous enzymes may serve as a useful cassette technique for design of novel catalytically active proteins.

Glutathione transferases and cancer

The glutathione transferases, a family of multifunctional proteins, catalyze the glutathione conjugation reaction with electrophilic compounds biotransformed from xenobiotics, including carcinogens. In preneoplastic cells as well as neoplastic cells, specific molecular forms of glutathione transferase are known to be expressed and have been known to participate in the mechanisms of their resistance to drugs. In this article, following a brief description of recently identified molecular forms, we review new findings regarding the respective molecular forms involved in carcinogenesis and anticancer drug resistance, with particular emphasis on Pi class forms in preneoplastic tissues. The rat Pi class form, GST-P (GST 7-7), is strongly expressed not only in hepatic foci and hepatomas, but also in initiated cells that occur at the very early stages of chemical hepatocarcinogenesis, and is regarded as one of the most reliable markers for preneoplastic lesions in the rat liver. 12-O-Tetradecanoylphorbol-13-acetate (TPA)-responsive element-like sequences have been identified in upstream regions of the GST-P gene, and oncogene products c-jun and c-fos are suggested to activate the gene. The Pi-class forms possess unique enzymatic properties, including broad substrate specificity, glutathione peroxidase activity toward lipid hydroperoxides, low sensitivity to organic anion inhibitors, and high sensitivity to active oxygen species. The possible functions of Pi class glutathione transferases in neoplastic tissues and drug-resistant cells are discussed.

Heterologous expression of the allelic variant mu-class glutathione transferases mu and psi

cDNA encoding the more acidic form, glutathione transferase (GST) psi, of the polymorphic Mu-class GSTs discovered in liver, was mutated in the 5'-end to create an NcoI site, facilitating cloning into the expression plasmid pKK233-2. The protein expressed from this construct has a point mutation Pro-2----Ala-2, but gives a catalytically functional protein. Back-mutation of the codon for amino acid residue 2 gave rise to a plasmid expressing the wild-type enzyme GST psi, or GST Mu1b-1b. A variant cDNA, differing only in specifying lysine rather than asparagine in position 173 of the coding region, was generated by site-directed mutagenesis. The variant sequence corresponds to another cDNA clone isolated from a human liver cDNA library and expresses the near-neutral GST mu, or GST Mu1a-1a. The two recombinant proteins GST Mu1a-1a and GST Mu1b-1b, by physicochemical as well as kinetic criteria, were found to be indistinguishable from GST mu and GST psi respectively, isolated from human liver. It is therefore concluded that the recombinant proteins correspond to the allelic variants observed in the human population. The two forms have different isoelectric points and correspond to the allelic variants observed in the human population. The two forms have different isoelectric points and their protein subunits can be separated by h.p.l.c. on a reverse-phase column. With standard substrates and inhibitors no differences in kinetic parameters between the two variants were detected. The mutated GST Mu1b-1b (Pro-2----Ala) was not significantly different in catalytic properties from the wild-type enzyme, even though Pro-2 is a well conserved amino acid residue in the known Mu-class GSTs.