Kinetics and mechanism of dissociation of zinc ion from carbonic anhydrase

Extremophilic behavior of catalytic amyloids sustained by backbone structuring

Enzyme function relies on the placement of chemistry defined by solvent and self-associative hydrogen bonding displayed by the protein backbone. Amyloids, long-range multi-peptide and -protein materials, can mimic enzyme functions while having a high proportion of stable self-associative backbone hydrogen bonds. Though catalytic amyloid structures have exhibited a degree of temperature and solvent stability, defining their full extremophilic properties and the molecular basis for such extreme activity has yet to be realized. Here we demonstrate that, like thermophilic enzymes, catalytic amyloid activity persists across high temperatures with an optimum activity at 81 °C where they are 30-fold more active than at room temperature. Unlike thermophilic enzymes, catalytic amyloids retain both activity and structure well above 100 °C as well as in the presence of co-solvents. Changes in backbone vibrational states are resolved in situ using non-linear 2D infrared spectroscopy (2DIR) to reveal that activity is sustained by reorganized backbone hydrogen bonds in extreme environments, evidenced by an emergent vibrational mode centered at 1612 cm^-1. Restructuring also occurs in organic solvents, and facilitates complete retention of hydrolysis activity in co-solvents of lesser polarity. We support these findings with molecular modeling, where the displacement of water by co-solvents leads to shorter, less competitive, bonding lifetimes that further stabilize self-associative backbone interactions. Our work defines amyloid properties that counter classical proteins, where extreme environments induce mechanisms of restructuring to support enzyme-like functions necessary for synthetic applications.

The Bioinorganic Chemistry of Mammalian Metallothioneins

The functions, purposes, and roles of metallothioneins have been the subject of speculations since the discovery of the protein over 60 years ago. This article guides through the history of investigations and resolves multiple contentions by providing new interpretations of the structure-stability-function relationship. It challenges the dogma that the biologically relevant structure of the mammalian proteins is only the one determined by X-ray diffraction and NMR spectroscopy. The terms metallothionein and thionein are ambiguous and insufficient to understand biological function. The proteins need to be seen in their biological context, which limits and defines the chemistry possible. They exist in multiple forms with different degrees of metalation and types of metal ions. The homoleptic thiolate coordination of mammalian metallothioneins is important for their molecular mechanism. It endows the proteins with redox activity and a specific pH dependence of their metal affinities. The proteins, therefore, also exist in different redox states of the sulfur donor ligands. Their coordination dynamics allows a vast conformational landscape for interactions with other proteins and ligands. Many fundamental signal transduction pathways regulate the expression of the dozen of human metallothionein genes. Recent advances in understanding the control of cellular zinc and copper homeostasis are the foundation for suggesting that mammalian metallothioneins provide a highly dynamic, regulated, and uniquely biological metal buffer to control the availability, fluctuations, and signaling transients of the most competitive Zn(II) and Cu(I) ions in cellular space and time.

The ovicidal, larvacidal and adulticidal properties of 5,5'-dimethyl-2,2'-bipyridyl against Drosophila melanogaster

Insecticide resistance has limited the number of available chemical options for insect pest control. Hence there is a need for new chemistries with novel modes of action. Here we investigate the mode of action for an insecticide that has not yet been released for commercial use. The ovicidal, larvacidal and adulticidal effects of 5,5′-dimethyl -2, 2′-dipyridyl (termed Ha44), which is being developed as a treatment for head lice, were evaluated in the Drosophila melanogaster model system. Ha44 demonstrated significant activity against embryos and was capable of arresting development at a number of stages of embryogenesis. The effects of Ha44 on embryos was shown to be reversible following the addition of the metal ions Fe(II) and Fe(III), Cu and Zn. When larvae were exposed to Ha44, lethality was recorded at similar concentrations to those observed for embryos. Using an eYFP reporter system it was shown that Ha44 was able to reduce the levels of both copper and zinc in the digestive tract, confirming the binding of Ha44 to these metals in vivo. Ha44 has further been shown to inhibit a zinc containing metalloproteinase in vitro. Exposure of adult flies to Ha44 resulted in lethality, but at higher concentrations than those observed for embryos and larvae. The median lethal dose in adult flies was shown to be associated with the type of exposure, with an LD-50 of 1.57 mM being recorded following the direct contact of flies with Ha44, while an LD-50 of 12.29 mM was recorded following the ingestion of the compound. The capacity of Ha44 to act on all stages of the life-cycle and potentially via a range of targets suggests that target site resistance is unlikely to evolve.

Active site metal ion in UDP-3-O-((R)-3-hydroxymyristoyl)-N-acetylglucosamine deacetylase (LpxC) switches between Fe(II) and Zn(II) depending on cellular conditions

UDP-3-O-((R)-3-hydroxymyristoyl)-N-acetylglucosamine deacetylase (LpxC) catalyzes the deacetylation of UDP-3-O-((R)-3-hydroxymyristoyl)-N-acetylglucosamine to form UDP-3-O-myristoylglucosamine and acetate in Gram-negative bacteria. This second, and committed, step in lipid A biosynthesis is a target for antibiotic development. LpxC was previously identified as a mononuclear Zn(II) metalloenzyme; however, LpxC is 6-8-fold more active with the oxygen-sensitive Fe(II) cofactor (Hernick, M., Gattis, S. G., Penner-Hahn, J. E., and Fierke, C. A. (2010) Biochemistry 49, 2246-2255). To analyze the native metal cofactor bound to LpxC, we developed a pulldown method to rapidly purify tagged LpxC under anaerobic conditions. The metal bound to LpxC purified from Escherichia coli grown in minimal medium is mainly Fe(II). However, the ratio of iron/zinc bound to LpxC varies with the metal content of the medium. Furthermore, the iron/zinc ratio bound to native LpxC, determined by activity assays, has a similar dependence on the growth conditions. LpxC has significantly higher affinity for Zn(II) compared with Fe(II) with K(D) values of 60 ± 20 pM and 110 ± 40 nM, respectively. However, in vivo concentrations of readily exchangeable iron are significantly higher than zinc, suggesting that Fe(II) is the thermodynamically favored metal cofactor for LpxC under cellular conditions. These data indicate that LpxC expressed in E. coli grown in standard medium predominantly exists as the Fe(II)-enzyme. However, the metal cofactor in LpxC can switch between iron and zinc in response to perturbations in available metal ions. This alteration may be important for regulating the LpxC activity upon changes in environmental conditions and may be a general mechanism of regulating the activity of metalloenzymes.

Zinc modulation of a transient potassium current and histochemical localization of the metal in neurons of the suprachiasmatic nucleus

The effect of Zn2+ on a voltage-dependent, transient potassium current (IA) in acutely dissociated neurons from the suprachiasmatic nucleus was studied with the whole-cell patch-clamp technique. At micromolar concentrations, Zn2+ markedly potentiated IA activated from a holding potential of -60 mV, which is the resting potential of these neurons. This potentiation occurred at a Zn2+ concentration as low as 2 microM and increased with higher Zn2+ concentrations. The Zn2+ action appears to arise from a shift in the steady-state inactivation of IA to more positive voltages. At 30 microM, Zn2+ shifted the half-inactivation voltage by +20 mV (from -80 mV to -60 mV), and 200 microM Zn2+ shifted this voltage by +45 mV (from -80 mV to -35 mV). Histochemically, we have also observed Zn2+ staining throughout the suprachiasmatic nucleus; the staining is particularly intense in the ventrolateral region of the nucleus, which receives the major fiber inputs. Our findings suggest that Zn2+, presumably synaptically released, may modulate the electrical activity of suprachiasmatic nucleus neurons through IA. Because vesicular Zn2+ is fairly widespread in the central nervous system, it is conceivable that this kind of Zn2+ modulation on IA, and possibly on other voltage-activated currents, exists elsewhere in the brain.

Characterization of meprin, a membrane-bound metalloendopeptidase from mouse kidney

Meprin is an intrinsic protein of the brush border, a specialized plasma membrane, of the mouse kidney. It is a metalloendopeptidase that contains 1 mol of zinc and 3 mol of calcium per mol of the 85,000-Mr subunit. The enzyme is isolated, and active, as a tetramer. The behaviour of the enzyme on SDS/polyacrylamide gels in the presence and absence of beta-mercaptoethanol indicates that the subunits are of the same Mr (approx. 85,000) and held together by intersubunit S--S bridges. Eight S-carboxymethyl-L-cysteine residues were detected after reduction of the enzyme with beta-mercaptoethanol and carboxymethylation with iodoacetate. The enzyme is a glycoprotein and contains approx. 18% carbohydrate. Most of the carbohydrate is removed by endoglycosidase F, indicating that the sugar residues are N-linked. The isoelectric point of the enzyme is between pH 4 and 5, and the purified protein yields a pattern of evenly spaced bands in this range on isoelectric focusing. The peptide-bond specificity of the enzyme has been determined by using the oxidized B-chain of insulin as substrate. In all, 15 peptide degradation products were separated by h.p.l.c. and analysed for their amino acid content and N-terminal amino acid residue. The prevalent peptide-bond cleavages were between Gly20 and Glu21, Phe24 and Phe25 and between Phe25 and Tyr26. Other sites of cleavage were Leu6-Cysteic acid7, Ala14-Leu15, His10-Leu11, Leu17-Val18, Gly8-Ser9, Leu15-Tyr16, His5-Leu6. These results indicate that meprin has a preference for peptide bonds that are flanked by hydrophobic or neutral amino acid residues, but hydrolysis is not limited to these bonds. The ability of meprin to hydrolyse peptide bonds between small neutral and negatively charged amino acid residues distinguishes it from several other metalloendopeptidases.

Metal binding to angiotensin converting enzyme: implications for the metal binding site

Angiotensin converting enzyme interacts with the chelator, 1,10-phenanthroline (OP) to form an OP-Zn-ACE ternary complex, which subsequently dissociates to OP-Zn and apoenzyme. The association and dissociation rate constants for the reaction OP + Zn-ACE in equilibrium OP-Zn-ACE have been determined and compared with those of known OP-metal complexes. Such constants were also used to calculate the rate constant for formation of the OP-Zn complex from OP-Zn-ACE. The rate of dissociation of zinc from ACE has been measured in the presence of EDTA (which acts only as a metal scavenger) as a function of chelator concentration, at different pH values, and with different buffers. The stability constant for the binding of zinc to apoACE log Kc = 8.2, determined by equilibrium dialysis using atomic absorption spectroscopy to assess metal concentration, is much smaller than that for Zn-carboxypeptidase A. Zn-thermolysin, or Zn-carbonic anhydrase. This weak binding is attributable to the zinc dissociation rate constant of ACE, 7.5 X 10(-3) sec-1 at pH 7.0, which is much greater than that of the other zinc metalloenzymes. These results lead to inferences regarding the metal binding site of ACE.

Catalytic properties and inhibition of Cd2+-carbonic anhydrases

Cd2+ derivatives of human carbonic anhydrases I and II and bovine red cell carbonic anhydrase (carbonate hydro-lyase, EC 4.2.1.1) have been prepared. The metal ion in these derivatives is readily displaced by Zn2+. The Cd2+-carbonic anhydrases have appreciable 4-nitrophenyl acetate hydrolase activities. These activities increase with pH as if dependent on the basic form of a group with pKa near 10. The Cd2+-carbonic anhydrases also have significant CO2 hydration activities. The Cd2+ derivatives are strongly inhibited by monovalent anions. In particular, I- is a much more potent inhibitor of the Cd2+ enzymes than of the native enzymes. Acetazolamide (5-acetylamido-1,3,4-thiadiazole 2-sulfonamide) is also a strong inhibitor although its affinity for the Cd2+ enzyme is less than its affinity for the native enzyme.

Kinetics and mechanisms of the recombination of Zn2+, Co2+, and Ni2+ with the metal-depleted catalytic site of horse liver alcohol dehydrogenase

The kinetics of the recombination of the metal-depleted active site of horse liver alcohol dehydrogenase (LADH) with metal ions have been studied over a range of pH and temperature. The formation rates were determined optically, by activity measurements, or by using the pH change during metal incorporation with a pH-indicator as monitor. The binding of Zn2+, Co2+, and Ni2+ ions occurs in a two-step process. The first step is a fast equilibrium reaction, characterized by an equilibrium constant K1. The spectroscopic and catalytic properties of the native or metal-substituted protein are recovered in a slow, monomolecular process with the rate constant k2. The rate constants k2 5.2 X 10(-2) sec-1 (Zn2+), 1.1 X 10(-3) sec-1 (Co2+), and 2 X 10(-4) sec-1 (Ni2+). The rate constants increase with increasing pH. Using temperature dependence, the activation parameters for the reaction with Co2+ and Ni2+ were determined. Activation energies of 51 +/- 2.5 kJ/mol (0.033 M N-Tris-(hydroxymethyl)methyl-2-aminomethane sulfonic acid (TES), pH 6, 9) for Co2+ and 48.5 +/- 4 kJ/mol (0.033 M TES, pH 7, 2) for Ni2+ at 23 degrees C were found. The correspondent activation entropies are - 146 +/- 10 kJ/mol K for Co2+ and - 163 +/- 9 kJ/mol K for Ni2+. Two protons are released during the binding of Zn2+ to H4Zn(n)2 LADH in the pH range 6.8-8.1. The binding of coenzyme, either reduced or oxidized, prevents completely the incorporation of metal ions, suggesting that the metal ions enter the catalytic site via the coenzyme binding domain and not through the hydrophobic substrate channel.

Coordination chemical studies on metalloenzymes. IX. Properties of the ternary complex between cobalt(II)-bovine carbonic anhydrase and bidentate ligands

The spectrum, thermodynamic parameters, and proton longitudinal relaxation time of the ternary complex between various bidentate ligands (2-pyridinecarboxylate, 2-quinolincarboxylate, 8-quinolinecarboxylate, and 2-pyridylacetate) and cobalt(II)-bovine carbonic anhydrase were measured to clarify the nature of the ternary complex. The formation constants of the ternary complexes of bidentate ligands are in the order of (2-pyridinecarboxylate approximately greater than 8-quinolinecarboxylate much greater than 2-quinolinecarboxylate approximately equal to 2-pyridylacetate). The degree of the shift of the band characteristic of five-coordinate species at 13-15 kcm-1 (cm-1 X 10(-3)) and that of the higher energy band at 21-22 kcm-1 decrease almost in the same order. These results are explained on the basis of the contribution of the bond formation between the nitrogen atom of the heterocyclic ring of ligands and cobalt ion. The formation constants of the ternary complex of bidentate ligands were compared to the stability constants of various ligands with a cobalt ion but there is no correlation in these values. The rate constant of break-up of the ternary complex was discussed on the coordination geometry of the ternary complex on the basis of the degree of the distortion.

Ligand substitution reactions of metallothioneins with EDTA and apo-carbonic anhydrase

The reactions of Zn-, Zn,Cd-, and Cd-thioneins with EDTA and apo-carbonic anhydrase have been studied. The ligand substitution reaction of zinc with EDTA is multiphasic, having both associative and dissociative components in the rate expression. The cadmium sites are about 2 orders of magnitude less reactive. In contrast, apo-carbonic anhydrase abstracts zinc from Zn-thionein and Zn,Cd-thionein in second-order processes that are 2-3 orders of magnitude more rapid than those involving EDTA and approach the rate for unligated Zn2+ with the apo-protein. In comparison with other zinc proteins, Zn-thionein contains unusually reactive metal sites, suggesting that this protein may be a physiological zinc transfer protein, able to donate zinc to zinc-requiring apo macromolecules.