Two conserved residues govern the salt and pH dependencies of the binding reaction of a PDZ domain

PDZ domains are protein-protein interaction modules found in hundreds of human proteins. Their binding reactions are sensitive to variations in salt and pH but the basis of the respective dependence has not been clear. We investigated the binding reaction between PSD-95 PDZ3 and a peptide corresponding to a native ligand with protein engineering in conjunction with stopped-flow and equilibrium fluorimetry and found that the two conserved residues Arg-318 and His-372 were responsible for the salt and pH dependencies, respectively. The basis of the salt-dependent variation of the affinity was explored by mutating all charged residues in and around the peptide-binding pocket. Arg-318 was found to be crucial, as mutation to alanine obliterated the effect of chloride on the binding constants. The direct interaction of chloride with Arg-318 was demonstrated by time-resolved urea denaturation experiments, where the Arg-318 --> Ala mutant was less stabilized by addition of chloride as compared with wild-type PDZ3. We also demonstrated that protonation of His-372 was responsible for the increase of the equilibrium dissociation constant at low pH. Both chloride concentration and pH (during ischemia) vary in the postsynaptic density, where PSD-95 is present, and the physiological buffer conditions may thus modulate the interaction between PSD-95 and its ligands through binding of chloride and protons to the "molecular switches" Arg-318 and His-372, respectively.

Renal cell carcinoma risk in type 2 von Hippel-Lindau disease correlates with defects in pVHL stability and HIF-1alpha interactions

The von Hippel-Lindau (VHL) tumor suppressor protein is the substrate binding subunit of the CBC(VHL) E3 ubiquitin ligase complex. Mutations in the VHL gene cause a variety of tumors with complex genotype/phenotype correlations. Type 2A and type 2B VHL disease are characterized by a low or high risk of renal cell carcinoma, respectively. To investigate the molecular basis underlying the difference between disease types 2A and 2B, we performed a detailed biochemical analysis of the two most frequent type 2A mutations, Y98 H and Y112 H, in comparison to type 2B mutations in the same residues, Y98N and Y112N. While none of these mutations affected the assembly of CBC(VHL) complexes, the type 2A mutant proteins exhibited higher stabilities at physiological temperature. Moreover, the type 2A mutant proteins possessed higher binding affinities for the key cellular substrate, hypoxia-inducible transcription factor 1 (HIF-1alpha). Consistent with these results, type 2A but not type 2B mutant VHL proteins retained significant ubiquitin ligase activity towards HIF-1alpha in vitro. We propose that this residual ubiquitin ligase activity is sufficient to suppress renal cell carcinogenesis in vivo.

Fibroblast growth factors share binding sites in heparan sulphate

HS (heparan sulphate) proteoglycans bind secreted signalling proteins, including FGFs (fibroblast growth factors) through their HS side chains. Such chains contain a wealth of differentially sulphated saccharide epitopes. Whereas specific HS structures are commonly believed to modulate FGF-binding and activity, selective binding of defined HS epitopes to FGFs has generally not been demonstrated. In the present paper, we have identified a series of sulphated HS octasaccharide epitopes, derived from authentic HS or from biosynthetic libraries that bind with graded affinities to FGF4, FGF7 and FGF8b. These HS species, along with previously identified oligosaccharides that interact with FGF1 and FGF2, constitute the first comprehensive survey of FGF-binding HS epitopes based on carbohydrate sequence analysis. Unexpectedly, our results demonstrate that selective modulation of FGF activity cannot be explained in terms of binding of individual FGFs to specific HS target epitopes. Instead, different FGFs bind to identical HS epitopes with similar relative affinities and low selectivity, such that the strength of these interactions increases with increasing saccharide charge density. We conclude that FGFs show extensive sharing of binding sites in HS. This conclusion challenges the current notion of specificity in HS-FGF interactions, and instead suggests that a set of common HS motifs mediates cellular targeting of different FGFs.

Simulation and experiment conspire to reveal cryptic intermediates and a slide from the nucleation-condensation to framework mechanism of folding

There is a change from three-state to two-state kinetics of folding across the homeodomain superfamily of proteins as the mechanism slides from framework to nucleation-condensation. The tendency for framework folding in this family correlates with inherent helical propensity. The cellular myeloblastis protein (c-Myb) falls in the mechanistic transition region. An earlier, preliminary report of protein engineering experiments and molecular dynamics simulations (MD) showed that the folding mechanism for this protein has aspects of both the nucleation-condensation and framework models. In the more in-depth analysis of the MD trajectories presented here, we find that folding may be attributed to both of these mechanisms in different regions of the protein. The folding of the loop, middle helix, and turn is best described by nucleation-condensation, whereas folding of the N and C-terminal helices may be described by the framework model. Experimentally, c-Myb folds by apparent two-state kinetics, but the MD simulations predict that the kinetics hide a high-energy intermediate. We stabilized this hypothetical folding intermediate by deleting a residue (P174) in the loop between its second and third helices, and the mutant intermediate is long-lived in the simulations. Equilibrium and kinetic experiments demonstrate that folding of the DeltaP174 mutant is indeed three-state. The presence and shape of the intermediate observed in the simulations were confirmed by small angle X-ray scattering experiments.

The kinetics of PDZ domain-ligand interactions and implications for the binding mechanism

PDZ domains are protein adapter modules present in a few hundred human proteins. They play important roles in scaffolding and signal transduction. PDZ domains usually bind to the C termini of their target proteins. To assess the binding mechanism of this interaction we have performed the first in-solution kinetic study for PDZ domains and peptides corresponding to target ligands. Both PDZ3 from postsynaptic density protein 95 and PDZ2 from protein tyrosine phosphatase L1 bind their respective target peptides through an apparent A + B --> A.B mechanism without rate-limiting conformational changes. But a mutant with a fluorescent probe (Trp) outside of the binding pocket suggests that slight changes in the structure take place upon binding in protein tyrosine phosphatase-L1 PDZ2. For PDZ3 from postsynaptic density protein 95 the pH dependence of the binding reaction is consistent with a one-step mechanism with one titratable group. The salt dependence of the interaction shows that the formation of electrostatic interactions is rate-limiting for the association reaction but not for dissociation of the complex.

The Structure of the Major Transition State for Folding of an FF Domain from Experiment and Simulation

We have analysed the transition state of folding of the four-helix FF domain from HYPA/FBP11 by high-resolution experiment and simulation as part of a continuing effort to understand the principles of folding and the refinement of predictive methods. The major transition state for folding was subjected to a Phi-value analysis utilising 50 mutants. The transition state contained a nucleus for folding centred around the end of helix 1 (H1) and the beginning of helix 2 (H2). Secondary structure in this region was fully formed (PhiF=0.9-1) and tertiary interactions were well developed. Interactions in the distal part of the native structure were weak (PhiF=0-0.2). The hydrophobic core and other parts of the protein displayed intermediate Phi-values, suggesting that interactions coalesce as the end of H1 and beginning of H2 are in the process of being formed. The distribution of Phi-values resembled that of barnase, which folds via an intermediate, rather than that of CI2 which folds by a concerted nucleation-condensation mechanism. The overall picture of the transition state structure identified in molecular dynamics simulations is in qualitative agreement, with the turn connecting H1 and H2 being formed, a loosened core, and H4 partially unfolded and detached from the core. There are some differences in the details and interpretation of specific Phi-values.

Demonstration of a low-energy on-pathway intermediate in a fast-folding protein by kinetics, protein engineering, and simulation

It is controversial whether fast-folding proteins can form productive on-pathway intermediates that are more stable than the denatured state because noncovalent intermediates are usually evanescent. Here, we apply the classical criteria for the existence of intermediates: namely, the intermediates form and react rapidly enough to be on pathway and they can be isolated and characterized. The folding of the 71-residue, mainly alpha-helical FF domain from human HYPA/FBP11 fulfills these classical criteria, as was found for Im7. The FF domain folds in two phases, one on the micros and the other on the ms time scale. An engineered mutant folds only to a partly folded state, with some 20-40% of the native helical content. The kinetic properties of the mutant are identical to those found for the fast phase of the wild-type protein, and it is likely that the mutant folds just to the intermediate state. A full kinetic analysis of the folding of wild-type protein, using the amplitudes of its native and denatured states and the observed values for the mutant, rules out an off-pathway scheme but fits an on-pathway scheme, with a low energy intermediate that is modeled by the mutant. The experimental proof benchmarks a molecular dynamics method that identifies an obligatory intermediate observed in multiple simulations. The conformational space defining this intermediate is visited several times in the simulations, leading to high populations consistent with the presence of a low energy intermediate.

Processing of macromolecular heparin by heparanase

Heparanase is an endo-glucuronidase expressed in a variety of tissues and cells that selectively cleaves extracellular and cell-surface heparan sulfate. Here we propose that this enzyme is involved also in the processing of serglycin heparin proteoglycan in mouse mast cells. In this process, newly synthesized heparin chains (60-100 kDa) are degraded to fragments (10-20 kDa) similar in size to commercially available heparin (Jacobsson, K. G., and Lindahl, U. (1987) Biochem. J. 246, 409-415). A fraction of these fragments contains the specific pentasaccharide sequence required for high affinity binding to antithrombin implicated with anticoagulant activity. Rat skin heparin, which escapes processing in vivo, was used as a substrate in reaction with recombinant human heparanase. An incubation product of commercial heparin size retained the specific pentasaccharide sequence, although oligosaccharides (3-4 kDa) containing this sequence could be degraded by the same enzyme. Commercial heparin was found to be a powerful inhibitor (I50 approximately 20 nM expressed as disaccharide unit, approximately 0.7 nM polysaccharide) of heparanase action toward antithrombin-binding oligosaccharides. Cells derived from a serglycin-processing mouse mastocytoma expressed a protein highly similar to other mammalian heparanases. These findings strongly suggest that the intracellular processing of the heparin proteoglycan polysaccharide chains is catalyzed by heparanase, which primarily cleaves target structures distinct from the antithrombin-binding sequence.

Oligosaccharide library-based assessment of heparan sulfate 6-O-sulfotransferase substrate specificity

Heparan sulfate mediates numerous complex biological processes. Its action critically depends on the amount and the positions of O-sulfate groups (iduronyl 2-O-sulfates, glucosaminyl 6-O- and 3-O-sulfates) that form binding sites for proteins. The structures and distribution of these protein-binding domains are influenced by the expression and substrate specificity of heparan sulfate biosynthetic enzymes. We describe a general approach to assess substrate specificities of enzymes involved in glycosaminoglycan metabolism, here applied to 6-O-sulfotransferases involved in heparan sulfate biosynthesis. To understand how 2-O-sulfation affects subsequent 6-O-sulfation reactions, the substrate specificity of 6-O-sulfotransferase 3 was probed using substrates from a heparin-based octasaccharide library. Purified 3H-labeled N-sulfated octasaccharides from a library designed to sample 2-O-sulfated motifs were used as sulfate acceptors, 3'-phosphoadenosine 5'-phosphosulfate as sulfate donor, and cell extract from 6-O-sulfotransferase 3-overexpressing 293 cells as enzyme source in the 6-O-sulfotransferase-catalyzed reactions. The first 6-O-sulfate group was preferentially incorporated at the internal glucosamine unit of the octasaccharide substrate. As the reaction proceeded, the octasaccharides acquired three 6-O-sulfate groups. The specificities toward competing octasaccharide substrates, for 6-O-sulfotransferase 2 and 6-O-sulfotransferase 3, were determined using overexpressing 293 cell extracts and purified octasaccharides. Both 6-O-sulfotransferases showed a preference for 2-O-sulfated substrates. The specificity toward substrates with two to three 2-O-sulfate groups was three to five times higher as compared with octasaccharides with no or one 2-O-sulfate group.

Biosynthetic oligosaccharide libraries for identification of protein-binding heparan sulfate motifs. Exploring the structural diversity by screening for fibroblast growth factor (FGF)1 and FGF2 binding

Heparan sulfate is crucial for vital reactions in the body because of its ability to bind various proteins. The identification of protein-binding heparan sulfate sequences is essential to our understanding of heparan sulfate biology and raises the possibility to develop drugs against diseases such as cancer and inflammatory conditions. We present proof-of-principle that in vitro generated heparan sulfate oligosaccharide libraries can be used to explore interactions between heparan sulfate and proteins, and that the libraries expand the available heparan sulfate sequence space. Oligosaccharide libraries mimicking highly 6-O-sulfated domains of heparan sulfate were constructed by enzymatic O-sulfation of O-desulfated, end-group (3)H-labeled heparin octasaccharides. Acceptor oligosaccharides that were 6-O-desulfated but only partially 2-O-desulfated yielded oligosaccharide arrays with increased ratio of iduronyl 2-O-sulfate/glucosaminyl 6-O-sulfate. The products were probed by affinity chromatography on immobilized growth factors, fibroblast growth factor-1 (FGF1) and FGF2, followed by sequence analysis of trapped oligosaccharides. An N-sulfated octasaccharide, devoid of 2-O-sulfate but with three 6-O-sulfate groups, was unexpectedly found to bind FGF1 as well as FGF2 at physiological ionic strength. However, a single 2-O-sulfate group in the absence of 6-O-sulfation gave higher affinity for FGF2. FGF1 binding was also augmented by 2-O-sulfation, preferentially in combination with an adjacent upstream 6-O-sulfate group. These results demonstrate the potential of the enzymatically generated oligosaccharide libraries.

A semisynthetic glutathione peroxidase with high catalytic efficiency. Selenoglutathione transferase

Glutathione peroxidase (GPX) protects cells against oxidative damage by catalyzing the reduction of hydroperoxides by glutathione (GSH). GPX therefore has potential therapeutic value as an antioxidant, but its pharmacological development has been limited because GPX uses a selenocysteine as its catalytic group and it is difficult to generate selenium-containing proteins with traditional recombinant DNA technology. Here, we show that naturally occurring proteins can be modified to generate GPX activity. The rat theta-class glutathione transferase T2-2 (rGST T2-2) presents an ideal scaffold for the design of a novel GPX catalyst because it already binds GSH and contains a serine close to the substrate binding site, which can be chemically modified to bind selenium. The modified Se-rGST T2-2 efficiently catalyzes the reduction of hydrogen peroxide, and the GPX activity surpasses the activities of some natural GPXs.

An ensemble of theta class glutathione transferases with novel catalytic properties generated by stochastic recombination of fragments of two mammalian enzymes

The correlation between sequence diversity and enzymatic function was studied in a library of Theta class glutathione transferases (GSTs) obtained by stochastic recombination of fragments of cDNA encoding human GST T1-1 and rat GST T2-2. In all, 94 randomly picked clones were characterized with respect to sequence, expression level, and catalytic activity in the conjugation reactions between glutathione and six alternative electrophilic substrates. Out of these six different compounds, dichloromethane is a selective substrate for human GST T1-1, whereas 1-menaphthyl sulfate and 1-chloro-2,4-dinitrobenzene are substrates for rat GST T2-2. The other three substances serve as substrates for both enzymes. Through this broad characterization, we have identified enzyme variants that have acquired novel activity profiles that differ substantially from those of the original GSTs. In addition, the expression levels of many clones were improved in comparison to the parental enzyme. A library of mutants can thus display a distribution of properties from which highly divergent evolutionary pathways may emerge, resembling natural evolutionary processes. From the GST library, a clone was identified that, by the point mutation N49D in the rat GST T2-2 sequence, has a 1700% increased activity with 1-menaphthyl sulfate and a 60% decreased activity with 4-nitrophenethyl bromide. Through the N49D mutation, the ratio of these activities has thus been altered 40-fold. An extensive characterization of a population of stochastically mutated enzymes can accordingly be used to find variants with novel substrate-activity profiles and altered catalytic properties. Recursive recombination of selected sequences displaying optimized properties is a strategy for the engineering of proteins for medical and biochemical applications. Such sequential design is combinatorial protein chemistry based on remodeling of existing structural scaffolds and has similarities to evolutionary processes in nature.

Role of the glutamyl alpha-carboxylate of the substrate glutathione in the catalytic mechanism of human glutathione transferase A1-1

The Glu alpha-carboxylate of glutathione contributes to the catalytic function of the glutathione transferases. The catalytic efficiency of human glutathione transferase A1-1 (GST A1-1) in the conjugation reaction with 1-chloro-2,4-dinitrobenzene is reduced 15 000-fold if the decarboxylated analogue of glutathione, dGSH (GABA-Cys-Gly), is used as an alternative thiol substrate. The decrease is partially due to an inability of the enzyme to promote ionization of dGSH. The pK(a) value of the thiol group of the natural substrate glutathione decreases from 9.2 to 6.7 upon binding to GST A1-1. However, the lack of the Glu alpha-carboxylate in dGSH raised the pK(a) value of the thiol in the enzymatic reaction to that of the nonenzymatic reaction. Furthermore, K(M)(dGSH) was 100-fold higher than K(M)(GSH). The active-site residue Thr68 forms a hydrogen bond to the Glu alpha-carboxylate of glutathione. Introduction of a carboxylate into GST A1-1 by a T68E mutation increased the catalytic efficiency with dGSH 10-fold and reduced the pK(a) value of the active site bound dGSH by approximately 1 pH unit. The altered pK(a) value is consistent with a catalytic mechanism where the carboxylate contributes to ionization of the glutathione thiol group. With Delta(5)-androstene-3,17-dione as substrate the efficiency of the enzyme is decreased 24 000-fold while with 4-nitrocinnamaldehyde (NCA) the decrease is less than 150-fold. In the latter reaction NCA accepts a proton and, unlike the other reactions studied, may not be dependent on the Glu alpha-carboxylate for deprotonation of the thiol group. An additional function of the Glu alpha-carboxylate may be productive orientation of glutathione within the active site.

Yeast glyoxalase I is a monomeric enzyme with two active sites

The tertiary structure of the monomeric yeast glyoxalase I has been modeled based on the crystal structure of the dimeric human glyoxalase I and a sequence alignment of the two enzymes. The model suggests that yeast glyoxalase I has two active sites contained in a single polypeptide. To investigate this, a recombinant expression clone of yeast glyoxalase I was constructed for overproduction of the enzyme in Escherichia coli. Each putative active site was inactivated by site-directed mutagenesis. According to the alignment, glutamate 163 and glutamate 318 in yeast glyoxalase I correspond to glutamate 172 in human glyoxalase I, a Zn(II) ligand and proposed general base in the catalytic mechanism. The residues were each replaced by glutamine and a double mutant containing both mutations was also constructed. Steady-state kinetics and metal analyses of the recombinant enzymes corroborate that yeast glyoxalase I has two functional active sites. The activities of the catalytic sites seem to be somewhat different. The metal ions bound in the active sites are probably one Fe(II) and one Zn(II), but Mn(II) may replace Zn(II). Yeast glyoxalase I appears to be one of the few enzymes that are present as a single polypeptide with two active sites that catalyze the same reaction.

Measurement of glutathione transferases

There are multiple glutathione transferase genes, the proteins for which have different substrate specificities. The various genes are differentially expressed such that species and organs and tissues differ qualitatively and quantitatively for cytosolic and membrane-bound forms. This unit provides protocols for analysis of transferase activity in a continuous spectrophotometric assay and an assay with dichloromethane as the substrate.

The active-site residue tyr-175 in human glyoxalase II contributes to binding of glutathione derivatives

Tyrosine-175 located in the active site of human glyoxalase II was replaced by phenylalanine in order to study the contribution of this residue to catalysis. The mutation had a marginal effect on the k(cat) value determined using S-D-lactoylglutathione as substrate. However, the Y175F mutant had an 8-fold higher K(m) value than the wild-type enzyme. The competitive inhibitor S-(N-hydroxy-N-bromophenylcarbamoyl)glutathione had a 30-fold higher K(i) value towards the mutant, than that of the wild-type. Pre-equilibrium fluorescence studies with the inhibitor showed that this was due to a significantly increased off-rate for the mutant enzyme. The phenolic hydroxyl group of tyrosine-175 is within hydrogen bonding distance of the amide nitrogen of the glycine in the glutathione moiety and the present study shows that this interaction makes a significant contribution to the binding of the active-site ligand.

Active site serine promotes stabilization of the reactive glutathione thiolate in rat glutathione transferase T2-2. Evidence against proposed sulfatase activity of the corresponding human enzyme

Ser(11) in rat glutathione transferase T2-2 is important for stabilization of the reactive enzyme-bound glutathione thiolate in the reaction with 1-menaphthyl sulfate. The S11A mutation increased the pK(a) value for the pH dependence of the rate constant for pre-steady-state product formation, from 5.7 to 7.9. This pH dependence is proposed to reflect titration of enzyme-bound glutathione thiol. Further, the mutation lowered the k(cat) value but not because of the impaired stabilization of the glutathione thiolate. In fact, several steps on the reaction pathway were affected by the S11A mutation, and the cause of the decreased k(cat) for the mutant was found to be a slower product release. The data presented here contradict the hypothesis that glutathione transferase T2-2 could act as a sulfatase that is not dependent on Ser(11) for the catalytic activity, as proposed for the corresponding human enzyme (Tan, K.-L., Chelvanayagam, G., Parker, M. W., and Board, P. G. (1996) Biochem. J. 319, 315-321; Rossjohn, J., McKinstry, W. J., Oakley, A. J., Verger, D., Flanagan, J., Chelvanayagam, G., Tan, K.-L., Board, P. G., and Parker, M. W. (1998) Structure 6, 309-322). On the contrary, Ser(11) governs both chemical and physical steps of the catalyzed reaction.

The C-terminal region of human glutathione transferase A1-1 affects the rate of glutathione binding and the ionization of the active-site Tyr9

In human glutathione transferase (GST) A1-1, the C-terminal region covers the active site and contributes to substrate binding. This region is flexible, but upon binding of an active-site ligand, it is stabilized as an amphipatic alpha-helix. The stabilization has implications for the catalytic activity of the enzyme. In the present study, residue M208 in GST A1-1 has been mutated to Lys and Glu, and residue F220 to Ala and Thr. These mutations are likely to destabilize the C-terminal region due to loss of hydrophobic interactions with the rest of the hydrophobic binding site. The rate constant for binding of glutathione to wild-type GST A1-1 is 450 mM(-)(1) s(-)(1) at 5 degrees C and pH 7.0, which is less than for an association limited by diffusion. However, the M208 and the F220 mutations increase the apparent on-rate constant for glutathione binding to 640-1170 mM(-)(1) s(-)(1). The binding data can be explained by a rapid reversible transition between different enzyme conformations occurring prior to glutathione binding, and restriction of the access to the active site by the C-terminal region. The effect of the mutations appears to be promotion of a less closed conformation, thereby facilitating the association of glutathione and enzyme. Both the M208 and F220 mutants display a lowered pK(a) value ( approximately 0.3 log unit) of the catalytically important Tyr9. Residue 208 does not interact directly with Tyr9 in the active site, and the shift in pK(a) value is therefore ascribed to the proposed dislocation of the C-terminal region caused by the mutation.

Fast product formation and slow product release are important features in a hysteretic reaction mechanism of glutathione transferase T2-2

The reaction mechanism of rat glutathione transferase T2-2 has been studied using pre-steady-state and steady-state kinetics. Several parts of the catalytic cycle including binding of substrates, product formation, and product release were investigated. Under saturating conditions, a two-step product release was found to be rate limiting in the enzyme-catalyzed reactions between the nucleophilic substrate glutathione and either of the two electrophilic substrates 1-menaphthyl sulfate and 4-nitrobenzyl chloride. The rate constant for pre-steady-state product formation on rat glutathione transferase T2-2 has an observed pK(a) value of 5.7 apparently due to ionization of the sulfhydryl group of glutathione. This rate constant is approximately 2 orders of magnitude higher than k(cat) at pH values of >6. It can be predicted from the pH dependence that product formation would be the sole rate-limiting step at pH values of <3. A hysteretic mechanism of rGST T2-2 is proposed based on a slow conformational transition detected in pre-steady-state displacement experiments.

Phospholipid hydroperoxide glutathione peroxidase activity of human glutathione transferases

Human glutathione transferases (GSTs) from Alpha (A), Mu (M) and Theta (T) classes exhibited glutathione peroxidase activity towards phospholipid hydroperoxide. The specific activities are in the order: GST A1-1>GST T1-1>GST M1-1>GST A2-2>GST A4-4. Using a specific and sensitive HPLC method, specific activities towards the phospholipid hydroperoxide,1-palmitoyl-2-(13-hydroper oxy-cis-9, trans-11 -octadecadienoyl)-l-3-phosphatidylcholine (PLPC-OOH) were determined to be in the range of 0.8-20 nmol/min per mg of protein. Two human class Pi (P) enzymes (GST P1-1 with Ile or Val at position 105) displayed no activity towards the phospholipid hydroperoxide. Michaelis-Menten kinetics were followed only for glutathione, whereas there was a linear dependence of rate with PLPC-OOH concentration. Unlike the selenium-dependent phospholipid hydroperoxide glutathione peroxidase (Se-PHGPx), the presence of detergent inhibited the activity of GST A1-1 on PLPC-OOH. Also, in contrast with Se-PHGPx, only glutathione could act as the reducing agent for GST A1-1. A GST A1-1 mutant (Arg15Lys), which retains the positive charge between the GSH- and hydrophobic binding sites, exhibited a decreased kcat for PLPC-OOH but not for CDNB, suggesting that the correct topography of the GSH site is more critical for the phospholipid substrate. A Met208Ala mutation, which gives a modified hydrophobic site, decreased the kcat for CDNB and PLPC-OOH by comparable amounts. These results indicate that Alpha, Mu and Theta class human GSTs provide protection against accumulation of cellular phospholipid hydroperoxides.

An evolutionary approach to the design of glutathione-linked enzymes

Studies of protein structure provide information about principles of protein design that have come into play in natural evolution. This information can be exploited in the redesign of enzymes for novel functions. The glutathione-binding domain of glutathione transferases has similarities with structures in other glutathione-linked proteins, such as glutathione peroxidases and thioredoxin (glutaredoxin), suggesting divergent evolution from a common ancestral protein fold. In contrast, the binding site for glutathione in human glyoxalase I is located at the interface between the two identical subunits of the protein. Comparison with the homologous, but monomeric, yeast glyoxalase I suggests that new domains have originated through gene duplications, and that the oligomeric structure of the mammalian glyoxalase I has arisen by 'domain swapping'. Recombinant DNA techniques are being used for the redesign of glutathione-linked proteins in attempts to create binding proteins with novel functions and catalysts with tailored specificities. Enzymes with desired properties are selected from libraries of variant structures by use of phage display and functional assays.

Kinetic characterization of recombinant human glutathione transferase T1-1, a polymorphic detoxication enzyme

Recombinant human theta class glutathione transferase T1-1 has been heterologously expressed in Escherichia coli and a simple purification method involving immobilized ferric ion affinity chromatography and Orange A dye chromatography is described. The catalytic properties of the enzyme differ significantly from those of other glutathione transferases, also within the theta class, with respect to both substrate selectivity and kinetic parameters. In addition to 1,2-epoxy-3-(4-nitrophenoxy)propane, the substrate used previously to monitor the enzyme, human glutathione transferase T1-1 has activity with the naturally occurring phenethylisothiocyanate and also displays glutathione peroxidase activity with cumene hydroperoxide. Further, the enzyme is active with 4-nitrobenzyl chloride and 4-nitrophenethyl bromide, but shows no detectable activity with the more chemically reactive 1-chloro-2,4-dinitrobenzene. The Michaelis constant for glutathione, K(m)GSH, with 1,2-epoxy-3-(4-nitrophenoxy)propane as second substrate, is high at low pH values but decreases at higher pH values. This is mirrored in kcat/K(m)GSH which increases with an apparent pKa value of 9.0, reflecting the ionization of the thiol group of glutathione in solution. The same results are obtained with 4-nitrophenethyl bromide as electrophilic substrate, although the K(m)GSH value (0.72 mM at pH 7.5), as well as the pKa (8.1) derived from the pH dependence of kcat/K(m)GSH, are lower with this substrate. In contrast, kcat and kcat/K(m)electrophile display either a maximum or a plateau at pH 7.0-7.5, and an apparent pKa value of 5.7 was determined for the pH dependence of kcat with both 4-nitrophenethyl bromide and 1,2-epoxy-3-(4-nitrophenoxy)propane as electrophilic substrates. This pKa value reflects an ionization of enzyme-bound GSH, most probably involving the sulfhydryl group, whose pKa value thus is lowered by the enzyme. Three differences in the cDNA as compared to the sequence previously published were found. One of these differences causes a change in the deduced amino acid sequence and involves the nucleotide triplet encoding amino acid 126, which was determined as GAG (Glu), instead of the published GGG (Gly).

Reduction of thymine hydroperoxide by phospholipid hydroperoxide glutathione peroxidase and glutathione transferases

Thymine hydroperoxide (5-hydroperoxymethyluracil), a model compound representing products of oxidative damage to DNA, is a substrate for glutathione peroxidase and some isoforms of glutathione transferase. In this paper, we show that selenium-dependent human phospholipid hydroperoxide glutathione peroxidase (Se-PHGPx) exhibits about four orders of magnitude higher activity on thymine hydroperoxide than that of other human enzymes such as selenium-dependent glutathione peroxidase and various representatives of glutathione transferases. The results indicate that Se-PHGPx may be an important enzyme in repairing oxidatively damaged DNA.

Heterologous expression, purification and characterization of rat class theta glutathione transferase T2-2

Rat glutathione transferase (GST) T2-2 of class Theta (rGST T2-2), previously known as GST 12-12 and GST Yrs-Yrs, has been heterologously expressed in Escherichia coli XLI-Blue. The corresponding cDNA was isolated from a rat hepatoma cDNA library, ligated into and expressed from the plasmid pKK-D. The sequence is the same as that of the previously reported cDNA of GST Yrs-Yrs. The enzyme was purified using ion-exchange chromatography followed by affinity chromatography with immobilized ferric ions, and the yield was approx. 200 mg from a 1 litre bacterial culture. The availability of a stable recombinant rGST T2-2 has paved the way for a more accurate characterization of the enzyme. The functional properties of the recombinant rGST T2-2 differ significantly from those reported earlier for the enzyme isolated from rat tissues. These differences probably reflect the difficulties in obtaining fully active enzyme from sources where it occurs in relatively low concentrations, which has been the case in previous studies. 1-Chloro-2,4-dinitrobenzene, a substrate often used with GSTs of classes Alpha, Mu and Pi, is a substrate also for rGST T2-2, but the specific activity is relatively low. The Km value for glutathione was determined with four different electrophiles and was found to be in the range 0.3 mM-0.8 mM. The Km values for some electrophilic substrates were found to be in the micromolar range, which is low compared with those determined for GSTs of other classes. The highest catalytic efficiency was obtained with menaphthyl sulphate, which gave a Kcat/Km value of 2.3 x 10(6) s-1.M-1 and a rate enhancement over the uncatalysed reaction of 3 x 10(10).