scholarly journals Fluxionate Lewis acidity of the Zn2+ ion in carboxypeptidase A

1993 ◽  
Vol 289 (1) ◽  
pp. 185-193 ◽  
Author(s):  
W L Mock ◽  
D J Freeman ◽  
M Aksamawati
Keyword(s):  
Metal Ion ◽  
Competitive Inhibition ◽  
Dissociation Constants ◽  
Ph Dependence ◽  
Bound Water ◽  
Acid Dissociation ◽  
Lewis Acidity ◽  
Carboxypeptidase A ◽  
A Value ◽  
Straight Line

Competitive inhibition constants Ki for a series of phenol-ring-substituted derivatives of alpha-(2-hydroxyphenyl)benzenepropanoic acid have been ascertained by observing their influence on the catalytic hydrolysis of a peptide substrate by the zinc enzyme carboxypeptidase A. The pH-dependence of Ki shows that binding is maximal between two pKa values: one is that of the phenol group of the inhibitor, and the other uniformly has a value of 6, the pKa of a Zn(2+)-bound water molecule on the enzyme in the absence of substrate or inhibitor. This is the dependence expected if phenolate binds to the Zn2+ displacing its bound H2O/HO-. A log-log plot of the dissociation constants for the productive forms of inhibitor plus enzyme versus the acid dissociation constants of the phenolic residues in the inhibitors yields a straight line with a slope of +0.76. This number indicates that the active-site metal ion has special capacity for dispersing negative charge, such as builds up on the oxygen atom of a carboxamide group undergoing nucleophilic addition.

scholarly journals Binding to thermolysin of phenolate-containing inhibitors necessitates a revised mechanism of catalysis

1994 ◽  
Vol 302 (1) ◽  
pp. 57-68 ◽  
Author(s):  
W L Mock ◽  
M Aksamawati
Keyword(s):  
Metal Ion ◽  
Competitive Inhibition ◽  
Solvent Molecule ◽  
Bound Water ◽  
Amide Bond ◽  
Lewis Acidity ◽  
Ph Profile ◽  
Binding Characteristics ◽  
Ph Values ◽  
Mechanism Of Catalysis

Competitive inhibition as a function of pH for the metalloendoprotease thermolysin by derivatives of L-alpha-(2-hydroxyphenyl)benzenepropanoyl-L- tryptophanylglycylglycine exhibits a diagnostic bell shape. Binding is maximal between two pKa values: on the acidic limb the apparent Ki value is regulated by an unchanging enzymic ionization (pKa 5.3) which is also seen in the substrate-hydrolysis kinetics (kcat/Km), whereas the alkaline limb for inhibition varies and depends specifically on the pKa of the phenolic group in the inhibitor. Although it should be the phenolate form of the inhibitor that co-ordinates more efficiently to the active-site Zn2+, the apparent Ki shifts from pH-independent at pH values immediately below the inhibitor's pKa to progressively weaker binding at higher pH. This is explained by an anomalous acidity for the exchangeable solvent molecule that is attached to enzymic Zn2+ in the absence of substrate or inhibitor. Since OH- cannot be displaced from the enzyme as readily as H2O, a compensating pKa of 5.3 possessed by Zn(2+)-bound water rationalizes the binding characteristics, yielding the level pH profile exhibited at intermediate pH values. Recognition of the implicit heightened Lewis acidity of the metal ion in thermolysin leads to a revision of the mechanism of catalysis. The substrate amide bond becomes activated for hydrolysis by carbonyl-group co-ordination to the especially acidic Zn2+ ion (completely displacing the H2O/OH- species otherwise bound). The imidazole group of enzymic residue His-231, also discerned in the pH profile for kcat/Km from its pKa of 8, provides general-base assistance for hydration of the activated scissile linkage in the first committed step of catalysis. Additional evidence from inhibition patterns shows how substrate-binding energy may be employed in this scheme to promote hydrolysis of peptides by thermolysin.

Horseradish Peroxidase. XVII. Reactions of Compounds I and II with p-Aminobenzoic Acid in Deuterium Oxide

1975 ◽  
Vol 53 (11) ◽  
pp. 1563-1569 ◽  
Author(s):  
C. D. Hubbard ◽  
H. B. Dunford ◽  
W. D. Hewson
Keyword(s):  
Horseradish Peroxidase ◽  
Dissociation Constants ◽  
Deuterium Oxide ◽  
Isotope Effects ◽  
Ph Dependence ◽  
Order Rate Constant ◽  
Enzyme Concentration ◽  
Acid Dissociation ◽  
Aminobenzoic Acid ◽  
Kinetic Isotope

The kinetics of reactions of horseradish peroxidase compounds I and II (HRP-I and HRP-II, respectively) with p-aminobenzoic acid have been studied in ordinary water and in deuterium oxide solution over a pH (pD) range 3–10, at 25° and at an ionic strength of 0.11. Under the conditions of the experiments the rate of reaction is first order both in substrate concentration and in enzyme concentration in both solvents. An analysis of the pH dependence of the second order rate constant in H2O confirms the presence of two acid dissociation groups on the enzyme with pKa' of 8.6 and ∼0 for HRP-II, whereas for HRP-I the data suggest a pKa of 5.1 on the enzyme and reveal, as previously shown, the influence of the ionization of the substituted ammonium group of the substrate. In deuterium oxide the pD profiles are similar to those in water but significant shifts for both kinetic and acid dissociation constants are observed for both compounds.The numerical values of the isotope effects taken together with previous results in general confirm that with labile substrates the group of pKa 8.6 in H2O on HRP-II is involved in general acid catalysis. p-Aminobenzoic acid is intermediate between labile and somewhat unreactive substrates and behaves similarly to the ferrocyanide ion in that both acid dissociation groups (pKa's 8.6 and ∼0) are influential in the catalysis of substrate oxidation by HRP-II. The kinetic isotope effect for the HRP-I reaction with p-aminobenzoic acid at high pH (pD) is consistent with a rate determining proton transfer but the group of pKa 5.1 in H2O remains unidentified.

Tripodal trisamides based on nicotinic and picolinic acid derivatives

1998 ◽  
Vol 76 (4) ◽  
pp. 414-425 ◽  
Author(s):  
H R Hoveyda ◽  
Veranja Karunaratne ◽  
Christopher J Nichols ◽  
Steven J Rettig ◽  
Ashley KW Stephens ◽  
...  
Keyword(s):  
Hydrogen Bonding ◽  
Metal Ion ◽  
Dissociation Constants ◽  
Picolinic Acid ◽  
Direct Methods ◽  
Acid Dissociation ◽  
Coupling Reagent ◽  
Picolinic Acids ◽  
Metal Ion Chelation ◽  
Derivatives Of

A number of polydentate arylamide ligands have been prepared by coupling various acyclic tripodal or linear polyamines with derivatives of nicotinic and picolinic acids. Two synthetic procedures were utilized; tris{[(2-hydroxynicotinyl)carbonyl]-2-aminoethyl}amine (H3NICTREN) was prepared by Method A, the HOSu/DCC method, and the other arylamides in this study were prepared by Method B, the CDI method. Method A involved the reaction of N-hydroxysuccinimide with 2-hydroxynicotinic acid (in the presence of dicyclohexylcarbodiimide (DCC) as a dehydrative coupling reagent) to form the succinimide ester, followed by reaction with TREN to yield H3NICTREN. Method B involved reaction of a carboxylic acid (2-hydroxynicotinic, 3-hydroxypicolinic, nicotinic, or picolinic acids) with carbonyldiimidazole (CDI) to form the N-acylimidazolide, followed by reaction with the amine (TREN, TAME, spermidine, or TRPN) to yield the desired arylamide. The X-ray structure of 1,1,1-tris{[(3-hydroxypicolinyl)carbonyl]-2-aminomethyl}ethane (H3PICTAME) was determined; crystals of H3PICTAME are monoclinic, a = 10.257(2), b = 15.572(3), c = 15.208(2) Å, β = 96.124(15)°, Z = 4, space group P21/a. The structure was solved by direct methods and refined by full-matrix least-squares procedures to R = 0.041 and Rw = 0.038 for 2506 reflections with I >= 3 sigma (I). In the solid state, H3PICTAME contains an extensive hydrogen-bonding network, with eight intra- and one intermolecular H-bonds per molecule; the ligand is partially preorganized for metal ion chelation. The acid dissociation constants of H3NICTREN and those of 1,1,1-tris{[(2-hydroxynicotinyl)carbonyl]-2- aminomethyl}ethane (H3NICTAME) have been determined; pKa1 = 11.2 (10.68), pKa2 = 10.7 (10.58), pKa3 = 10.0 (9.71), and pKa4 = 6.25 for H3NICTREN (H3NICTAME); the high phenolic pKa's are consistent with the hydrogen bonding observed in the solid state.Key words: arylamide, hydrogen bonding, preorganization.

Uranyl Sorption Onto Alumina

1996 ◽  
Vol 465 ◽  
Author(s):  
Anna-Maria M. Jacobsson ◽  
Robert S. Rundberg
Keyword(s):  
Metal Ion ◽  
Dissociation Constants ◽  
Surface Complexation ◽  
Uranyl Complex ◽  
Acid Dissociation ◽  
Oxide Surface ◽  
Acid Dissociation Constants ◽  
Surface Complexation Model ◽  
Surface Hydroxyl ◽  
Free Environment

ABSTRACTThe mechanism for the adsorption of uranyl onto alumina from aqueous solution was studied experimentally and the data were modeled using a triple layer surface complexation model. The experiments were carried out at low uranium concentrations (9×10-11 - 5×10-8M) in a CO2 free environment at varying electrolyte concentrations (0.01 – 1 M) and pH (4.5 – 12). The first and second acid dissociation constants, pKal and pKa2, of the alumina surface were determined from potentiometric titrations to be 7.2 ± 0.6 and 11.2 ± 0.4, respectively. The adsorption of uranium was found to be independent of the electrolyte concentration. We therefore conclude that the uranium binds as an inner sphere complex. The results were modeled using the code FITEQL. Two reactions of uranium with the surface were needed to fit the data, one forming a uranyl complex with a single surface hydroxyl and the other forming a bridged or bidentate complex reacting with two surface hydroxyls of the alumina. There was no evidence from these experiments of site heterogeneity. The constants used for the reactions were based in part on predictions made utilizing the Hard Soft Acid Base, HSAB, theory, relating the surface complexation constants to the hydrolysis of the sorbing metal ion and the acid dissociation constants of the mineral oxide surface.

scholarly journals The equilibrium constant of the phosphoglyceromutase reaction

1974 ◽  
Vol 139 (3) ◽  
pp. 491-497 ◽  
Author(s):  
John B. Clarke ◽  
Michael Birch ◽  
Hubert G. Britton
Keyword(s):  
Ionic Strength ◽  
Equilibrium Constant ◽  
Dissociation Constants ◽  
Ph Dependence ◽  
Binding Constants ◽  
Acid Dissociation ◽  
Effect Of Temperature ◽  
Acid Dissociation Constants ◽  
The Difference ◽  
Low Ionic Strength

The equilibrium constant of the phosphoglyceromutase reaction was determined over a range of pH (5.4–7.9), in solutions of different ionic strength (0.06–0.3) and in the presence of Mg2+, at 30°C and at 20°C. The values obtained (8.65–11.65) differ substantially from previously published values. The third acid dissociation constants were redetermined for 2- and 3-phosphoglycerate, and in contrast with previous reports the pK values (7.03 and 6.97 respectively at zero ionic strength) were closely similar. The Mg2+-binding constants were measured spectrophotometrically and the values, 286mm-1 and 255mm-1 for 2- and 3-phosphoglycerate at pH7 and ionic strength 0.02, were also very similar. From the relative lack of effect of temperature, pH and ionic strength it is concluded that the equilibrium constant differs from unity largely because of entropic factors. At low ionic strength, in the neutral region, the pH-dependence can be attributed to the small difference in the acid dissociation constants, but the difference in dissociation constants does not explain the pH-dependence in the acid region or at high ionic strength. Within physiological ranges of pH, Mg2+ concentration and ionic strength there will be little variation in equilibrium constant.

scholarly journals Pyrene Carboxylate Ligand Based Coordination Polymers for Microwave-Assisted Solvent-Free Cyanosilylation of Aldehydes

Molecules ◽  
2021 ◽  
Vol 26 (4) ◽  
pp. 1101
Author(s):  
Anirban Karmakar ◽  
Anup Paul ◽  
Elia Pantanetti Sabatini ◽  
M. Fátima C. Guedes da Silva ◽  
Armando J. L. Pombeiro
Keyword(s):  
Coordination Polymer ◽  
Single Crystal ◽  
Coordination Polymers ◽  
Metal Ion ◽  
Solvent Free ◽  
Lewis Acidity ◽  
Carboxylate Ligand ◽  
X Ray Diffraction ◽  
X Ray ◽  
Microwave Assisted

The new coordination polymers (CPs) [Zn(μ-1κO1:1κO2-L)(H2O)2]n·n(H2O) (1) and [Cd(μ4-1κO1O2:2κN:3,4κO3-L)(H2O)]n·n(H2O) (2) are reported, being prepared by the solvothermal reactions of 5-{(pyren-4-ylmethyl)amino}isophthalic acid (H2L) with Zn(NO3)2.6H2O or Cd(NO3)2.4H2O, respectively. They were synthesized in a basic ethanolic medium or a DMF:H2O mixture, respectively. These compounds were characterized by single-crystal X-ray diffraction, FTIR spectroscopy, thermogravimetric and elemental analysis. The single-crystal X-ray diffraction analysis revealed that compound 1 is a one dimensional linear coordination polymer, whereas 2 presents a two dimensional network. In both compounds, the coordinating ligand (L2−) is twisted due to the rotation of the pyrene ring around the CH2-NH bond. In compound 1, the Zn(II) metal ion has a tetrahedral geometry, whereas, in 2, the dinuclear [Cd2(COO)2] moiety acts as a secondary building unit and the Cd(II) ion possesses a distorted octahedral geometry. Recently, several CPs have been explored for the cyanosilylation reaction under conventional conditions, but microwave-assisted cyanosilylation of aldehydes catalyzed by CPs has not yet been well studied. Thus, we have tested the solvent-free microwave-assisted cyanosilylation reactions of different aldehydes, with trimethylsilyl cyanide, using our synthesized compounds, which behave as highly active heterogeneous catalysts. The coordination polymer 1 is more effective than 2, conceivably due to the higher Lewis acidity of the Zn(II) than the Cd(II) center and to a higher accessibility of the metal centers in the former framework. We have also checked the heterogeneity and recyclability of these coordination polymers, showing that they remain active at least after four recyclings.

scholarly journals THE pH DEPENDENCE AND DISSOCIATION CONSTANTS OF ESTERASES AND PROTEASES TREATED WITH DIISOPROPYL FLUOROPHOSPHATE

1957 ◽  
Vol 226 (2) ◽  
pp. 867-872 ◽  
Author(s):  
L.A. Mounter ◽  
H.C. Alexander ◽  
Kenneth D. Tuck ◽  
Lien Tien H. Dien
Keyword(s):  
Dissociation Constants ◽  
Ph Dependence ◽  
Diisopropyl Fluorophosphate

Aortic tortuosity index as a predictor of type A aortic dissection

2020 ◽  
Vol 41 (Supplement_2) ◽  
Author(s):  
F Scalbert ◽  
O Milleron ◽  
M Para ◽  
R Raffoul ◽  
Q Pellenc ◽  
...  
Keyword(s):  
Aortic Dissection ◽  
Type A ◽  
Ct Scans ◽  
Control Group ◽  
Type A Aortic Dissection ◽  
Total Length ◽  
A Value ◽  
Straight Line ◽  
Patients At Risk ◽  
Geometric Length

Abstract Introduction The risk ok type A aortic dissection (AAD) depends on the degree of aortic wall's alteration, which can result in dilatation or tortuosity. The estimate of this risk relies solely on the evaluation of the diameter of the ascending aorta. Purpose The purpose of this study is to evaluate the presence and importance of aortic tortuosity in patients with type A aortic dissection. Method Postoperative CT scans of patients with type A aortic dissection were compared with CT scans from controls matched for gender and age. After 3D reconstruction, total length (actual distance along aortic center line = Ltot) and geometric length (length of a straight line between start and end of the aortic segment = Lgeo) were measured to calculate the tortuosity index (TI = Ltot / Lgeo). Results Ltot, Lgeo and TI from different aortic segments of the AAD group were higher than in the control group. Ltot and TI of the whole aorta (from aortic valve to bifurcation) were greater in patients with type A aortic dissection (527.7±46.1 mm vs. 475.8±39.7, p<0.0001; and 2.05±0.24 vs. 1.98±0.21, p=0.002 respectively). Total length and TI were greater after exclusion of the ascending part, and a value of this TI >1.3 identifies AAD patients with an accuracy of 74.8% (AUC = 0.792, p<0.0001). TI is altered by risk factors for aortic dissection: it increases with hypertension and age but not by tobacco use, and TI decreases in diabetes. Conclusions Type A aortic dissection is associated with longer aorta and increased aortic tortuosity. This index may help recognize patients at risk for type A aortic dissection. Calculation of tortuosity indexes Funding Acknowledgement Type of funding source: None

Sign in / Sign up

Export Citation Format

Share Document