Mutagenesis of residue 157 in the active site of human glyoxalase I

Quantum Mechanics/Molecular Mechanics Study of the Reaction Mechanism of Glyoxalase I

Glyoxalase I (GlxI) is a member of the glyoxalase system, which is important in cell detoxification and converts hemithioacetals of methylglyoxal (a cytotoxic byproduct of sugar metabolism that may react with DNA or proteins and introduce nucleic acid strand breaks, elevated mutation frequencies, and structural or functional changes of the proteins) and glutathione into d-lactate. GlxI accepts both the S and R enantiomers of hemithioacetal, but converts them to only the S-d enantiomer of lactoylglutathione. Interestingly, the enzyme shows this unusual specificity with a rather symmetric active site (a Zn ion coordinated to two glutamate residues; Glu-99 and Glu-172), making the investigation of its reaction mechanism challenging. Herein, we have performed a series of combined quantum mechanics and molecular mechanics calculations to study the reaction mechanism of GlxI. The substrate can bind to the enzyme in two different modes, depending on the direction of its alcoholic proton (H2; toward Glu-99 or Glu-172). Our results show that the S substrate can react only if H2 is directed toward Glu-99 and the R substrate only if H2 is directed toward Glu-172. In both cases, the reactions lead to the experimentally observed S-d enantiomer of the product. In addition, the results do not show any low-energy paths to the wrong enantiomer of the product from neither the S nor the R substrate. Previous studies have presented several opposing mechanisms for the conversion of R and S enantiomers of the substrate to the correct enantiomer of the product. Our results confirm one of them for the S substrate, but propose a new one for the R substrate.

Aromatization of natural products by a specialized detoxification enzyme

In plants, lineage-specific metabolites can be created by activities derived from the catalytic promiscuity of ancestral proteins, although examples of recruiting detoxification systems to biosynthetic pathways are scarce. The ubiquitous glyoxalase (GLX) system scavenges the cytotoxic methylglyoxal, in which GLXI isomerizes the α-hydroxy carbonyl in the methylglyoxal-glutathione adduct for subsequent hydrolysis. We show that GLXIs across kingdoms are more promiscuous than recognized previously and can act as aromatases without cofactors. In cotton, a specialized GLXI variant, SPG, has lost its GSH-binding sites and organelle-targeting signal, and evolved to aromatize cyclic sesquiterpenes bearing α-hydroxyketones to synthesize defense compounds in the cytosol. Notably, SPG is able to transform acetylated deoxynivalenol, the prevalent mycotoxin contaminating cereals and foods. We propose that detoxification enzymes are a valuable source of new catalytic functions and SPG, a standalone enzyme catalyzing complex reactions, has potential for toxin degradation, crop engineering and design of novel aromatics.

Alr2321, a multiple stress inducible glyoxalase I of Anabaena sp. PCC7120 detoxifies methylglyoxal and reactive species oxygen

Abiotic stresses enhance the cellular level of reactive oxygen species (ROS) which consequently leads to toxic methylglyoxal (MG) production. Glyoxalases (GlyI & GlyII) catalyze the conversion of toxic MG into non-toxic lactic acid but their properties and functions have been overlooked in cyanobacteria. This is the first attempt to conduct a genome-wide analysis of GlyI protein (PF00903) from Anabaena sp. PCC7120. Out of total nine GlyI domain possessing proteins, only three (Alr2321, Alr4469, All1022) harbour conserve His/Glu/His/Glu metal binding site at their homologous position and are deficient in conserved region specific for Zn²⁺ dependent members. Their biochemical, structural and functional characterization revealed that only Alr2321 is a homodimeric Ni²⁺ dependent active GlyI with catalytic efficiency 11.7 × 10⁶ M^-1 s^-1. It has also been found that Alr2321 is activated by various divalent metal ions and has maximum GlyI activity with Ni²⁺ followed by Co²⁺ > Mn²⁺ > Cu²⁺ and no activity with Zn^2+. Moreover, the expression of alr2321 was found to be maximally up-regulated under heat (19 fold) followed by cadmium, desiccation, arsenic, salinity and UV-B stresses. BL21/pGEX-5X2-alr2321 showed improved growth under various abiotic stresses as compared to BL21/pGEX-5X2 by increased scavenging of intracellular MG and ROS levels. Taken together, these results suggest noteworthy links between intracellular MG and ROS, its detoxification by Alr2321, a member of GlyI family of Anabaena sp. PCC7120, in relation to abiotic stress.

Recent findings on the physiological function of DJ-1: Beyond Parkinson's disease

Several mutations in the gene coding for DJ-1 have been associated with early onset forms of parkinsonism. In spite of the massive effort spent by the scientific community in understanding the physiological role of DJ-1, a consensus on what DJ-1 actually does within the cells has not been reached, with several diverse functions proposed. At present, the most accepted function for DJ-1 is a neuronal protective role against oxidative stress. However, how exactly this function is exerted by DJ-1 is not clear. In recent years, novel molecular mechanisms have been suggested that may account for the antioxidant properties of DJ-1. In this review, we critically analyse the experimental evidence, including some very recent findings, supporting the purported neuroprotective role of DJ-1 through different mechanisms linked to oxidative stress handling, as well as the relevance of these processes in the context of Parkinson's disease.

Characteristic Variations and Similarities in Biochemical, Molecular, and Functional Properties of Glyoxalases across Prokaryotes and Eukaryotes

The glyoxalase system is the ubiquitous pathway for the detoxification of methylglyoxal (MG) in the biological systems. It comprises two enzymes, glyoxalase I (GLYI) and glyoxalase II (GLYII), which act sequentially to convert MG into d-lactate, thereby helping living systems get rid of this otherwise cytotoxic byproduct of metabolism. In addition, a glutathione-independent GLYIII enzyme activity also exists in the biological systems that can directly convert MG to d-lactate. Humans and Escherichia coli possess a single copy of GLYI (encoding either the Ni- or Zn-dependent form) and GLYII genes, which through MG detoxification provide protection against various pathological and disease conditions. By contrast, the plant genome possesses multiple GLYI and GLYII genes with a role in abiotic stress tolerance. Plants possess both Ni²⁺- and Zn²⁺-dependent forms of GLYI, and studies on plant glyoxalases reveal the various unique features of these enzymes distinguishing them from prokaryotic and other eukaryotic glyoxalases. Through this review, we provide an overview of the plant glyoxalase family along with a comparative analysis of glyoxalases across various species, highlighting similarities as well as differences in the biochemical, molecular, and physiological properties of these enzymes. We believe that the evolution of multiple glyoxalases isoforms in plants is an important component of their robust defense strategies.

Identification and Characterization of a Glyoxalase I Gene in a Rapeseed Cultivar with Seed Thermotolerance

Glyoxalase I (GLYI) is a ubiquitous enzyme in all organisms that catalyzes the conversion of the potent cytotoxin methylglyoxal to S-D-lactoylglutathione. Although many reports suggest the importance of GLYI in the plant response to stress, its function in seeds requires further study. Here, we identified a heat-induced GLYI from Brassica napus seeds, BnGLYI, using a comparative proteomics approach. Two-dimensional gel analyses revealed that BnGLYI protein expression upon heat treatment was significantly elevated in thermotolerant seeds but was diminished in heat-sensitive seeds. The BnGLYI-2 and BnGLYI-3 genes from the heat-sensitive and thermotolerant cultivars, respectively, were characterized, and analyzed. Only two amino acid residue variations were found between the amino acid sequences of the two genes. Moreover, overexpressing BnGLYI-3 in yeast cells enhanced tolerance to heat and cold stress and significantly increased GLYI activity compared to overexpressing BnGLYI-2. In addition, BnGLYI-3 transformants showed enhanced superoxide dismutase activities under heat and cold treatment, whereas these activities were diminished for BnGLYI-2 transformants. Taken together, these results indicate that overexpression of the BnGLYI-3 gene imparts thermotolerance and cold tolerance in yeast and that the variations in BnGLYI-3 may play an important role in the responses to temperature stresses.

A unique Ni2+ -dependent and methylglyoxal-inducible rice glyoxalase I possesses a single active site and functions in abiotic stress response

The glyoxalase system constitutes the major pathway for the detoxification of metabolically produced cytotoxin methylglyoxal (MG) into a non-toxic metabolite D-lactate. Glyoxalase I (GLY I) is an evolutionarily conserved metalloenzyme requiring divalent metal ions for its activity: Zn(2+) in the case of eukaryotes or Ni(2+) for enzymes of prokaryotic origin. Plant GLY I proteins are part of a multimember family; however, not much is known about their physiological function, structure and metal dependency. In this study, we report a unique GLY I (OsGLYI-11.2) from Oryza sativa (rice) that requires Ni(2+) for its activity. Its biochemical, structural and functional characterization revealed it to be a monomeric enzyme, possessing a single Ni(2+) coordination site despite containing two GLY I domains. The requirement of Ni(2+) as a cofactor by an enzyme involved in cellular detoxification suggests an essential role for this otherwise toxic heavy metal in the stress response. Intriguingly, the expression of OsGLYI-11.2 was found to be highly substrate inducible, suggesting an important mode of regulation for its cellular levels. Heterologous expression of OsGLYI-11.2 in Escherichia coli and model plant Nicotiana tabacum (tobacco) resulted in improved adaptation to various abiotic stresses caused by increased scavenging of MG, lower Na(+) /K(+) ratio and maintenance of reduced glutathione levels. Together, our results suggest interesting links between MG cellular levels, its detoxification by GLY I, and Ni(2+) - the heavy metal cofactor of OsGLYI-11.2, in relation to stress response and adaptation in plants.

Tat-glyoxalase protein inhibits against ischemic neuronal cell damage and ameliorates ischemic injury

Methylglyoxal (MG), a metabolite of glucose, is the major precursor of protein glycation and induces apoptosis. MG is associated with neurodegeneration, including oxidative stress and impaired glucose metabolism, and is efficiently metabolized to S-D-lactoylglutathione by glyoxalase (GLO). Although GLO has been implicated as being crucial in various diseases including ischemia, its detailed functions remain unclear. Therefore, we investigated the protective effect of GLO (GLO1 and GLO2) in neuronal cells and an animal ischemia model using Tat-GLO proteins. Purified Tat-GLO protein efficiently transduced into HT-22 neuronal cells and protected cells against MG- and H2O2-induced cell death, DNA fragmentation, and activation of caspase-3 and mitogen-activated protein kinase. In addition, transduced Tat-GLO protein increased D-lactate in MG- and H2O2-treated cells whereas glycation end products (AGE) and MG levels were significantly reduced in the same cells. Gerbils treated with Tat-GLO proteins displayed delayed neuronal cell death in the CA1 region of the hippocampus compared with a control. Furthermore, the combined neuroprotective effects of Tat-GLO1 and Tat-GLO2 proteins against ischemic damage were significantly higher than those of each individual protein. Those results demonstrate that transduced Tat-GLO protein protects neuronal cells by inhibiting MG- and H2O2-mediated cytotoxicity in vitro and in vivo. Therefore, we suggest that Tat-GLO proteins could be useful as a therapeutic agent for various human diseases related to oxidative stress including brain diseases.

Investigation of metal binding and activation of Escherichia coli glyoxalase I: kinetic, thermodynamic and mutagenesis studies

GlxI (glyoxalase I) isomerizes the hemithioacetal formed between glutathione and methylglyoxal. Unlike other GlxI enzymes, Escherichia coli GlxI exhibits no activity with Zn(2+) but maximal activation with Ni(2+). To elucidate further the metal site in E. coli GlxI, several approaches were undertaken. Kinetic studies indicate that the catalytic metal ion affects the k (cat) without significantly affecting the K (m) for the substrate. Inductively coupled plasma analysis and isothermal titration calorimetry confirmed one metal ion bound to the enzyme, including Zn(2+), which produces an inactive enzyme. Isothermal titration calorimetry was utilized to determine the relative binding affinity of GlxI for various bivalent metals. Each metal ion examined bound very tightly to GlxI with an association constant ( K (a))>10(7) M(-1), with the exception of Mn(2+) ( K (a) of the order of 10(6) M(-1)). One of the ligands to the catalytic metal, His(5), was altered to glutamine, a side chain found in the Zn(2+)-active Homo sapiens GlxI. The affinity of the mutant protein for all bivalent metals was drastically decreased. However, low levels of activity were now observed for Zn(2+)-bound GlxI. Although this residue has a marked effect on metal binding and activation, it is not the sole factor determining the differential metal activation between the human and E. coli GlxI enzymes.

Flexible Metal Binding of the Metallo-β-lactamase Domain: Glyoxalase II Incorporates Iron, Manganese, and Zinc in Vivo†

Glyoxalase II belongs to the metallo-beta-lactamase superfamily of proteins, possessing the characteristic dinuclear active site. Within this protein family, glyoxalase II from Arabidopsis thaliana is the first member to be isolated with significant amounts of iron, manganese, and zinc when being recombinantly produced in Escherichia coli. Enzyme preparations with different ratios of these three metals all yield k(cat)/K(M) values in the range of 1.5-1.9 s(-1) microM(-1) with the substrate S-d-lactoylglutathione. X-ray absorption spectroscopy reveals binding of all three metals to the dinuclear active site with 5-6-fold coordination consisting of 2.5 +/- 0.5 histidine and 2.5 +/- 0.5 oxygen ligands. This model does not distinguish site-specific or distributed binding. The metal-metal distance is determined to be 3.18 +/- 0.06 A. Electron paramagnetic resonance spectroscopy gives evidence for several different types of dimetal sites, including spin-coupled Fe(III)Fe(II), Fe(III)Zn(II), and Mn(II)Mn(II) centers. The metal-ligand distances measured by X-ray absorption spectroscopy vary depending on the metal type and comply with their element-specific, characteristic values. This reflects a high degree of structural flexibility within the glyoxalase II dinuclear active site, which is considered as the structural basis for its broad metal selectivity.

Glutathione s-transferase omega 1-1 is a target of cytokine release inhibitory drugs and may be responsible for their effect on interleukin-1beta posttranslational processing

Stimulus-induced posttranslational processing of human monocyte interleukin-1beta (IL-1beta) is accompanied by major changes to the intracellular ionic environment, activation of caspase-1, and cell death. Certain diarylsulfonylureas inhibit this response, and are designated cytokine release inhibitory drugs (CRIDs). CRIDs arrest activated monocytes so that caspase-1 remains inactive and plasma membrane latency is preserved. Affinity labeling with [(14)C]CRIDs and affinity chromatography on immobilized CRID were used in seeking potential protein targets of their action. Following treatment of intact human monocytes with an epoxide-bearing [(14)C]CRID, glutathione S-transferase (GST) Omega 1-1 was identified as a preferred target. Moreover, labeling of this polypeptide correlated with irreversible inhibition of ATP-induced IL-1beta posttranslational processing. When extracts of human monocytic cells were chromatographed on a CRID affinity column, GST Omega 1-1 bound selectively to the affinity matrix and was eluted by soluble CRID. Recombinant GST Omega 1-1 readily incorporated [(14)C]CRID epoxides, but labeling was negated by co-incubation with S-substituted glutathiones or by mutagenesis of the catalytic center Cys(32) to alanine. Peptide mapping by high performance liquid chromatography-mass spectrometry also demonstrated that Cys(32) was the site of modification. Although S-alkylglutathiones did not arrest ATP-induced IL-1beta posttranslational processing or inhibit [(14)C]CRID incorporation into cell-associated GST Omega 1-1, a glutathione-CRID adduct effectively demonstrated these attributes. Therefore, the ability of CRIDs to arrest stimulus-induced IL-1beta posttranslational processing may be attributable to their interaction with GST Omega 1-1.

Glutathione conjugates and their synthetic derivatives as inhibitors of glutathione-dependent enzymes involved in cancer and drug resistance

Alterations in levels of glutathione (GSH) and glutathione-dependent enzymes have been implicated in cancer and multidrug resistance of tumor cells. The activity of a number of these, the multidrug resistance-associated protein 1, glutathione S-transferase, DNA-dependent protein kinase, glyoxalase I, and gamma-glutamyl transpeptidase, can be inhibited by GSH-conjugates and synthetic analogs thereof. In this review we focus on the function of these enzymes and carriers in cancer and anti-cancer drug resistance, in relation to their inhibition by GSH-conjugate analogs.

Catalytic mechanism of glyoxalase I: a theoretical study

Hybrid density functional theory is used to study the catalytic mechanism of human glyoxalase I (GlxI). This zinc enzyme catalyzes the conversion of the hemithioacetal of toxic methylglyoxal and glutathione to nontoxic (S)-D-lactoylglutathione. GlxI can process both diastereomeric forms of the substrate, yielding the same form of the product. As a starting point for the calculations, we use a recent crystal structure of the enzyme in complex with a transition-state analogue, where it was found that the inhibitor is bound directly to the zinc by its hydroxycarbamoyl functions. It is shown that the Zn ligand Glu172 can abstract the substrate C1 proton from the S enantiomer of the substrate, without being displaced from the Zn ion. The calculated activation barrier is in excellent agreement with experimental rates. Analogously, the Zn ligand Glu99 can abstract the proton from the R form of the substrate. To account for the stereochemical findings, it is argued that the S and R reactions cannot be fully symmetric. A detailed mechanistic scheme is proposed.

Brief History of Glyoxalase I and What We Have Learned about Metal Ion-Dependent, Enzyme-Catalyzed Isomerizations

Glyoxalase I, a member of the metalloglutathione (GSH) transferase superfamily, plays a critical detoxification role in cells by catalyzing the conversion of cytotoxic methylglyoxal (as the diastereomeric GSH-thiohemiacetals) to S-D-lactoylglutathione via a 1,2-hydrogen transfer. The mechanism-of-action of this Zn2+-metalloenzyme has been the subject of considerable controversy over the past 50 years. Key issues relate to the role of the active-site metal ion in catalysis and how the enzyme is able to use directly both diastereomeric thiohemiacetals as substrates. The results of recent X-ray crystallographic measurements on the enzyme in complex with a transition state analogue and site-directed mutagenesis studies now strongly support a base-mediated, proton-transfer mechanism in which the bound diastereomeric substrates undergo catalytic interconversion before the 1S-diastereomer goes to product via a Zn2+-coordinated, cis-enediolate intermediate. Comparisons with chemical model systems suggest that Zn2+-coordination of thiohemiacetal substrate will dramatically increase the thermodynamic and kinetic acidity of the C1-H bond of substrate. In the human enzyme, the carboxyl group of Glu (172) is well positioned to catalyze a suprafacial proton transfer between the adjacent carbons of substrate. The Zn2+-coordinated carboxyl group of Glu(99) is a reasonable candidate to catalyze proton transfer between the Zn2+-coordinated oxygen atoms of the enediolate intermediate. Other Zn2+ metalloenzymes appear to use similar reaction mechanisms to facilitate proton transfers.

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.

Computer simulation of primary kinetic isotope effects in the proposed rate-limiting step of the glyoxalase I catalyzed reaction

The proposed rate-limiting step of the glyoxalase I catalyzed reaction is the proton abstraction from the C1 carbon of the substrate by Glu(172). Here we examine primary kinetic isotope effects and the influence of quantum dynamics on this process by computer simulations. The calculations utilize the empirical valence bond method in combination with the molecular dynamics free energy perturbation technique and path integral simulations. For the enzyme-catalyzed reaction a H/D kinetic isotope effect of 5.0 +/- 1. 3 is predicted in reasonable agreement with the experimental result of about 3. Furthermore, the magnitude of quantum mechanical effects is found to be very similar for the enzyme reaction and the corresponding uncatalyzed process in solution, in agreement with other studies. The problems associated with attaining the required accuracy in order for the present approach to be useful as a diagnostic tool for the study of enzyme reactions are also discussed.

The human glutathione transferase P1-1 specific inhibitor TER 117 designed for overcoming cytostatic-drug resistance is also a strong inhibitor of glyoxalase I

gamma-L-Glutamyl-S-(benzyl)-L-cysteinyl-R-(-)-phenylglycine (TER 117) has previously been developed for selective inhibition of human glutathione S-transferase P1-1 (GST P1-1) based on the postulated contribution of this isoenzyme to the development of drug resistance in cancer cells. In the present investigation, the inhibitory effect of TER 117 on the human glyoxalase system was studied. Although designed as an inhibitor specific for GST P1-1, TER 117 also competitively inhibits glyoxalase I (K(I) = 0.56 microM). In contrast, no inhibition of glyoxalase II was detected. Reduced glyoxalase activity is expected to raise intracellular levels of toxic 2-oxoaldehydes otherwise eliminated by glyoxalase I. The resulting toxicity would accompany the potentiation of cytostatic drugs, caused by inhibition of the detoxication effected by GST P1-1. TER 117 was designed for efficient inhibition of the most abundant form GST P1-1/Ile105. Therefore, the inhibitory effect of TER 117 on a second allelic variant GST P1-1/Val105 was also studied. TER 117 was shown to competitively inhibit both GST P1-1 variants. The apparent K(I) values at glutathione concentrations relevant to the intracellular milieu were in the micromolar range for both enzyme forms. Extrapolation to free enzyme produced K(I) values of approximately 0.1 microM for both isoenzymes, reflecting the high affinity of GST P1-1 for the inhibitor. Thus, the allelic variation in position 105 of GST P1-1 does not affect the inhibitory potency of TER 117. The inhibitory effects of TER 117 on GST P1-1 and glyoxalase I activities may act in synergy in the cell and improve the effectiveness of chemotherapy.

Reaction mechanism of glyoxalase I explored by an X-ray crystallographic analysis of the human enzyme in complex with a transition state analogue

The structures of human glyoxalase I in complexes with S-(N-hydroxy-N-p-iodophenylcarbamoyl)glutathione (HIPC-GSH) and S-p-nitrobenzyloxycarbonylglutathione (NBC-GSH) have been determined at 2.0 and 1.72 A resolution, respectively. HIPC-GSH is a transition state analogue mimicking the enediolate intermediate that forms along the reaction pathway of glyoxalase I. In the structure, the hydroxycarbamoyl function is directly coordinated to the active site zinc ion. In contrast, the equivalent group in the NBC-GSH complex is approximately 6 A from the metal in a conformation that may resemble the product complex with S-D-lactoylglutathione. In this complex, two water molecules occupy the liganding positions at the zinc ion occupied by the hydroxycarbamoyl function in the enediolate analogue complex. Coordination of the transition state analogue to the metal enables a loop to close down over the active site, relative to its position in the product-like structure, allowing the glycine residue of the glutathione moiety to hydrogen bond with the protein. The structure of the complex with the enediolate analogue supports an "inner sphere mechanism" in which the GSH-methylglyoxal thiohemiacetal substrate is converted to product via a cis-enediolate intermediate. The zinc ion is envisioned to play an electrophilic role in catalysis by directly coordinating this intermediate. In addition, the carboxyl of Glu 172 is proposed to be displaced from the inner coordination sphere of the metal ion during substrate binding, thus allowing this group to facilitate proton transfer between the adjacent carbon atoms of the substrate. This proposal is supported by the observation that in the complex with the enediolate analogue the carboxyl group of Glu 172 is 3.3 A from the metal and is in an ideal position for reprotonation of the transition state intermediate. In contrast, Glu 172 is directly coordinated to the zinc ion in the complexes with S-benzylglutathione and with NBC-GSH.

Energetics of the proposed rate-determining step of the glyoxalase I reaction

The proposed rate-limiting step of the reaction catalyzed by glyoxalase I is the proton abstraction from the C1 carbon atom of the substrate by a glutamate residue, resulting in a high-energy enolate intermediate. This proton transfer reaction was modelled using molecular dynamics and free energy perturbation simulations, with the empirical valence bond method describing the potential energy surface of the system. The calculated rate constant for the reaction is approximately 300-1500 s(-1) with Zn2+, Mg2+ or Ca2+ bound to the active site, which agrees well with observed kinetics of the enzyme. Furthermore, the results imply that the origin of the catalytic rate enhancement is mainly associated with enolate stabilization by the metal ion.

Involvement of an active-site Zn2+ ligand in the catalytic mechanism of human glyoxalase I

The Zn2+ ligands glutamate 99 and glutamate 172 in the active site of human glyoxalase I were replaced, each in turn, by glutamines by site-directed mutagenesis to elucidate their potential significance for the catalytic properties of the enzyme. To compensate for the loss of the charged amino acid residue, another of the metal ligands, glutamine 33, was simultaneously mutated into glutamate. The double mutants and the single mutants Q33E, E99Q, and E172Q were expressed in Escherichia coli, purified on an S-hexylglutathione matrix, and characterized. Metal analysis demonstrated that mutant Q33E/E172Q contained 1.0 mol of zinc/mol of enzyme subunit, whereas mutant Q33E/E99Q contained only 0.3 mol of zinc/mol of subunit. No catalytic activity could be detected with the double mutant Q33E/E172Q (<10(-8) of the wild-type activity). The second double mutant Q33E/E99Q had 1.5% of the specific activity of the wild-type enzyme, whereas the values for mutants Q33E and E99Q were 1.3 and 0. 1%, respectively; the E172Q mutant had less than 10(-5) times the specific activity of the wild-type. The crystal structure of the catalytically inactive double mutant Q33E/E172Q demonstrated that Zn2+ was bound without any gross changes or perturbations. The results suggest that the metal ligand glutamate 172 is directly involved in the catalytic mechanism of the enzyme, presumably serving as the base that abstracts a proton from the hemithioacetal substrate.

Overproduction and characterization of a dimeric non-zinc glyoxalase I from Escherichia coli: evidence for optimal activation by nickel ions

The ubiquitous glyoxalase system converts toxic alpha-keto aldehydes into their corresponding nontoxic 2-hydroxycarboxylic acids, utilizing glutathione (GSH) as a cofactor. The first enzyme in this system, glyoxalase I (GlxI), catalyzes the isomerization of the hemithioacetal formed nonenzymatically between GSH and cytotoxic alpha-keto aldehydes. To study the Escherichia coli GlxI enzyme, the DNA encoding this protein, gloA, was isolated and incorporated into the plasmid pTTQ18. Nucleotide sequencing of the gloA gene predicted a polypeptide of 135 amino acids and Mr of 14 919. The gloA gene has been overexpressed in E. coli and shown to encode for GlxI. An effective two-step purification protocol was developed, yielding 150-200 mg of homogeneous protein per liter of culture. Electrospray mass spectrometry confirmed the monomeric weight of the purified protein, while gel filtration analysis indicated GlxI to be a homodimer of 30 kDa. Zinc, the natural metal ion found in the Homo sapiens and Saccharomyces cerevisiae GlxI, had no effect on the activity of E. coli GlxI. In contrast, the addition of NiCl2 to the growth medium or to purified E. coli apo-GlxI greatly enhanced the enzymatic activity. Inductively coupled plasma and atomic absorption analyses indicated binding of only one nickel ion per dimeric enzyme, suggesting only one functional active site in this homodimeric enzyme. In addition, the apoprotein regained maximal activity with one molar equivalence of nickel chloride, indicative of tight metal binding. The effects of pH on the kinetics of the nickel-activated enzyme were also studied. This is the first example of a non-zinc activated GlxI whose maximal activation is seen with Ni2+.