Michael W. Crowder

Hydrolysis of phosphate monoesters: a biological problem with multiple chemical solutions.

Formation of phosphate esters by kinases has long been recognized as an important process in biochemistry, but the reverse reaction, hydrolysis of phosphate esters by phosphatases, has attracted less attention. Recent work suggests that phosphatases are as important as kinases in regulatory processes, and that they constitute a diverse group of enzymes that utilize a variety of chemical means to accelerate phosphate ester hydrolysis.

Overexpression, Purification, and Characterization of the Cloned Metallo-β-Lactamase L1 from Stenotrophomonas maltophilia

ABSTRACT The metallo-β-lactamase L1 from Stenotrophomonas maltophilia was cloned, overexpressed, and characterized by spectrometric and biochemical techniques. Results of metal analyses were consistent with the cloned enzyme having 2 mol of tightly bound Zn(II) per monomer. Gel filtration chromatography demonstrated that the cloned enzyme exists as a tightly held tetramer with a molecular mass of ca. 115 kDa, and matrix-assisted laser desorption ionization and time-of-flight mass spectrometry indicated a monomeric molecular mass of 28.8 kDa. Steady-state kinetic studies with a number of diverse penicillin and cephalosporin antibiotics demonstrated that L1 effectively hydrolyzes all tested compounds, withkcat/Km values ranging between 0.002 and 5.5 μM−1 s−1. These characteristics of the recombinant enzyme are contrasted to those previously reported for metallo-β-lactamases isolated directly fromS. maltophilia.

Arabidopsis Glyoxalase II Contains a Zinc/Iron Binuclear Metal Center That Is Essential for Substrate Binding and Catalysis

Glyoxalase II participates in the cellular detoxification of cytotoxic and mutagenic 2-oxoaldehydes. Because of its role in chemical detoxification, glyoxalase II has been studied as a potential anti-cancer and/or anti-protozoal target; however, very little is known about the active site and reaction mechanism of this important enzyme. To characterize the active site and kinetic mechanism of the enzyme, a detailed mutational study of Arabidopsisglyoxalase II was conducted. Data presented here demonstrate for the first time that the cytoplasmic form of Arabidopsisglyoxalase II contains an iron-zinc binuclear metal center that is essential for activity. Both metals participate in substrate binding, transition state stabilization, and the hydrolysis reaction. Subtle alterations in the geometry and/or electrostatics of the binuclear center have profound effects on the activity of the enzyme. Additional residues important in substrate binding have also been identified. An overall reaction mechanism for glyoxalase II is proposed based on the mutational and kinetic data from this study and crystallographic data on human glyoxalase II. Information presented here provides new insights into the active site and reaction mechanism of glyoxalase II that can be used for the rational design of glyoxalase II inhibitors.

Mechanistic and Spectroscopic Studies of Metallo-β-lactamase NDM-1

In an effort to biochemically characterize metallo-β-lactamase NDM-1, we cloned, overexpressed, purified, and characterized several maltose binding protein (MBP)-NDM-1 fusion proteins with different N-termini (full-length, Δ6, Δ21, and Δ36). All MBP-NDM-1 fusion proteins were soluble; however, only one, MBP-NDM-1Δ36, exhibited high activity and bound 2 equiv of Zn(II). Thrombin cleavage of this fusion protein resulted in the truncated NDM-1Δ36 variant, which exhibited a k(cat) of 16 s(-1) and a K(m) of 1.1 μM when using nitrocefin as a substrate, bound 2 equiv of Zn(II), and was monomeric in solution. Extended X-ray absorption fine structure studies of the NDM-1Δ36 variant indicate the average metal binding site for Zn(II) in this variant consists of four N/O donors (two of which are histidines) and 0.5 sulfur donor per zinc, with a Zn-Zn distance of 3.38 Å. This metal binding site is very similar to those of other metallo-β-lactamases that belong to the B1 subclass. Pre-steady-state kinetic studies using nitrocefin and chromacef and the NDM-1Δ36 variant indicate that the enzyme utilizes a kinetic mechanism similar to that used by metallo-β-lactamases L1 and CcrA, in which a reactive nitrogen anion is stabilized and its protonation is rate-limiting. While they are very different in terms of amino acid sequence, these studies demonstrate that NDM-1 is structurally and mechanistically very similar to metallo-β-lactamase CcrA.

Role of the Zn1 and Zn2 sites in Metallo-β-lactamase L1

In an effort to probe the role of the Zn(II) sites in metallo-beta-lactamase L1, mononuclear metal ion containing and heterobimetallic analogues of the enzyme were generated and characterized using kinetic and spectroscopic studies. Mononuclear Zn(II)-containing L1, which binds Zn(II) in the consensus Zn1 site, was shown to be slightly active; however, this enzyme did not stabilize a nitrocefin-derived reaction intermediate that had been previously detected. Mononuclear Co(II)- and Fe(III)-containing L1 were essentially inactive, and NMR and EPR studies suggest that these metal ions bind to the consensus Zn2 site in L1. Heterobimetallic analogues (ZnCo and ZnFe) analogues of L1 were generated, and stopped-flow kinetic studies revealed that these enzymes rapidly hydrolyze nitrocefin and that there are large amounts of the reaction intermediate formed during the reaction. The heterobimetallic analogues were reacted with nitrocefin, and the reactions were rapidly freeze quenched. EPR studies on these samples demonstrate that Co(II) is 5-coordinate in the resting state, proceeds through a 4-coordinate species during the reaction, and is 5-coordinate in the enzyme-product complex. These studies demonstrate that the metal ion in the Zn1 site is essential for catalysis in L1 and that the metal ion in the Zn2 site is crucial for stabilization of the nitrocefin-derived reaction intermediate.

Structural studies on a mitochondrial glyoxalase II.

Glyoxalase 2 is a β-lactamase fold-containing enzyme that appears to be involved with cellular chemical detoxification. Although the cytoplasmic isozyme has been characterized from several organisms, essentially nothing is known about the mitochondrial proteins. As a first step in understanding the structure and function of mitochondrial glyoxalase 2 enzymes, a mitochondrial isozyme (GLX2-5) from Arabidopsis thaliana was cloned, overexpressed, purified, and characterized using metal analyses, EPR and 1H NMR spectroscopies, and x-ray crystallography. The recombinant enzyme was shown to bind 1.04 ± 0.15 eq of iron and 1.31 ± 0.05 eq of Zn(II) and to exhibit kcat and Km values of 129 ± 10 s-1 and 391 ± 48 μm, respectively, when using S-d-lactoylglutathione as the substrate. EPR spectra revealed that recombinant GLX2-5 contains multiple metal centers, including a predominant Fe(III)Z-n(II) center and an anti-ferromagnetically coupled Fe(III)Fe(II) center. Unlike cytosolic glyoxalase 2 from A. thaliana, GLX2-5 does not appear to specifically bind manganese. 1H NMR spectra revealed the presence of at least eight paramagnetically shifted resonances that arise from protons in close proximity to a Fe(III)Fe(II) center. Five of these resonances arose from solvent-exchangeable protons, and four of these have been assigned to NH protons on metal-bound histidines. A 1.74-Å resolution crystal structure of the enzyme revealed that although GLX2-5 shares a number of structural features with human GLX2, several important differences exist. These data demonstrate that mitochondrial glyoxalase 2 can accommodate a number of different metal centers and that the predominant metal center is Fe(III)Zn(II).

Glyoxalase II from A. thaliana requires Zn(II) for catalytic activity

Cytosolic glyoxalase II from Arabidopsis thaliana, GLX2‐2, was overexpressed and purified to homogeneity using Q‐sepharose chromatography. MALDI‐TOF mass spectrometry studies indicated a molecular weight of 28 767 Da. Using steady‐state kinetics studies, the purified enzyme exhibited a K m of 660±100 μM and a k cat of 484±92 s−1 at 37°C. Metal analyses demonstrated that the enzyme binds 2.1±0.5 moles of Zn(II) per monomer; the binding of Zn(II) is essential for enzyme viability and activity. Sequence comparison of glyoxalase II enzymes from human, A. thaliana, and yeast and the metallo‐β‐lactamases reveal that all metal binding ligands of the metallo‐β‐lactamases are conserved in glyoxalase II enzymes, suggesting that all glyoxalase II enzymes are Zn(II) metalloenzymes. These results and their implications are discussed in light of previous studies on glyoxalase II, and an active site for the glyoxalase II enzymes is proposed.

LOW-MOLECULAR-WEIGHT CHROMIUM-BINDING SUBSTANCE AND BIOMIMETIC CR3O(O2CCH2CH3)6(H2O)3+ DO NOT CLEAVE DNA UNDER PHYSIOLOGICALLY-RELEVANT CONDITIONS

Abstract Chromium(III) tris(picolinate), Cr(pic)3, is currently a very popular nutritional supplement; however, at physiologically-relevant concentrations, it has recently been demonstrated to cleave DNA [J.K. Speetjens, R.A. Collins, J.B. Vincent, S.A. Woski, Chem. Res. Toxicol. 12 (1999) 483]. A number of other chromium-containing compounds have been proposed as substitutes for Cr(pic)3. Of particular interest are low-molecular-weight chromium-binding substance (LMWCr) and [Cr3O(O2CCH2CH3)6(H2O)3]+ 1. The former compound has recently been identified as the biologically active form of chromium in mammals, activating the kinase activity of insulin receptor in the presence of insulin. Complex 1 is a functional biomimetic for LMWCr. Both compounds have been proposed as possible nutritional supplements and therapeutics for adult-onset diabetes. This work demonstrates that these complexes, unlike Cr(pic)3, are poor DNA-cleaving agents and may represent safer materials for human consumption.

Differential Binding of Co(II) and Zn(II) to Metallo-β-Lactamase Bla2 from Bacillus anthracis

In an effort to probe the structure, mechanism, and biochemical properties of metallo-beta-lactamase Bla2 from Bacillus anthracis, the enzyme was overexpressed, purified, and characterized. Metal analyses demonstrated that recombinant Bla2 tightly binds 1 equiv of Zn(II). Steady-state kinetic studies showed that mono-Zn(II) Bla2 (1Zn-Bla2) is active, while di-Zn(II) Bla2 (ZnZn-Bla2) was unstable. Catalytically, 1Zn-Bla2 behaves like the related enzymes CcrA and L1. In contrast, di-Co(II) Bla2 (CoCo-Bla2) is substantially more active than the mono-Co(II) analogue. Rapid kinetics and UV-vis, (1)H NMR, EPR, and EXAFS spectroscopic studies show that Co(II) binding to Bla2 is distributed, while EXAFS shows that Zn(II) binding is sequential. To our knowledge, this is the first documented example of a Zn enzyme that binds Co(II) and Zn(II) via distinct mechanisms, underscoring the need to demonstrate transferability when extrapolating results on Co(II)-substituted proteins to the native Zn(II)-containing forms.

N-Heterocyclic dicarboxylic acids: Broad-spectrum inhibitors of metallo-β-lactamases with co-antibacterial effect against antibiotic-resistant bacteria

In an effort to identify novel, broad-spectrum inhibitors against the metallo-β-lactamases (MβLs), several N-heterocyclic derivatives were tested as inhibitors of MβLs CcrA, ImiS, and L1, which are representative enzymes from the distinct MβL subclasses. Three N-heterocyclic dicarboxylic acid derivatives were competitive inhibitors of CcrA and L1, exhibiting K(i) values ≤2 μM, while only 2,4-thiazolidinedicarboxylic acid (1b) was a competitive inhibitor of ImiS. Two 2-mercapto-1,3,4-thiadiazole derivatives were noncompetitive inhibitors of CcrA and ImiS, exhibiting K(i) values <7 μM; however, these same compounds did not inhibit L1. Two 2-mercapto-1,3,4-triazole derivatives were shown not to inhibit any of the tested MβLs. The N-heterocyclic derivatives were tested for antibacterial activity by examining the MIC values for existing antibiotics in the presence/absence of these derivatives. Consistent with the steady-state inhibition data, the inclusion of three N-heterocyclic dicarboxylic acid derivatives resulted in lower MIC values when using Escherichia coli BL21(DE3) cells containing the CcrA or L1 plasmids or Klebsiella pneumoniae (ATCC 700603), while 1b was the only dicarboxylic acid derivative to lower the MIC value of E. coli cells containing the ImiS plasmid. Inclusion of the 2-mercapto-1,3,4-thiadiazole derivatives resulted in lower MIC values for E. coli cells containing ImiS or L1 plasmids; however, these derivatives did not alter the MIC values for K. pneumoniae or E. coli cells containing the L1 plasmid. None of the N-heterocyclic derivatives affected the MIC of two methicillin resistant Staphylococcus aureus (MRSA) strains. Taken together, these studies demonstrate that N-heterocyclic dicarboxylic acids 1a-c and pyridylmercaptothiadiazoles 2a,b are good scaffolds for future broad-spectrum inhibitors of the MβLs.