scholarly journals Catalysis by acetylcholinesterase in two-hydronic-reactive states. Integrity of deuterium oxide effects and hydron inventories

1992 ◽  
Vol 285 (2) ◽  
pp. 451-460 ◽  
Author(s):  
E Salih
Keyword(s):  
Kinetic Parameters ◽  
Conformational Change ◽  
Rate Constants ◽  
Literature Abstract ◽  
Deuterium Oxide ◽  
Ph Dependence ◽  
Reversible Inhibitor ◽  
Induced Fit ◽  
Rate Limiting ◽  
Hydrolysis Of

Low 2H2O effects (1.0-1.5) for the parameter k(cat.)/Km in the hydrolysis of various substrates by acetylcholinesterase (AcChE) is due to normal 2H2O effects (1.8-2.8) for the parameter k(cat.) and 2H2O effects of 1.0-2.5 for the parameter Km. The analyses and interpretations of 2H2O effects in the literature utilizing the parameter k(cat.)/Km, which led to the proposal of ‘isotope insensitivity’ of the catalytic steps and the hypothesis of a rate-limiting substrate-induced-fit conformational change, are incorrect. Since k(cat.) is the only parameter that can represent the hydron-transfer step solely, the 2H2O effect can most appropriately be evaluated by using this parameter. Calculations and comparison of acylation (k+2) and deacylation (k+3) rate constants show that acylation is rate-determining for most substrates and the improved binding -0.84 to -2.09 kJ/mol (-0.2 to -0.5 kcal/mol) in 2H2O obscures the normal 2H2O effect on k(cat.) when the ratio k(cat.)/Km is utilized. Consistent with this, measurements of the inhibition constant (KI(com.)) for a reversible inhibitor, phenyltrimethylammonium, lead to KI(com.)H2O = 39 +/- 3 microM and KI(com.)2H2O = 24.5 +/- 3.5 microM, an 2H2O effect of 1.59 +/- 0.26. pH-dependence of k(cat.) in 2H2O is subject to variability of the pK(app.) values, as evaluated in terms of the two-hydronic-reactive states (EH and EH2) of AcChE, and is due to an uneven decrease in 2H2O of the kinetic parameters k'cat. for the EH2 state relative to k(cat.) for the EH state, thus leading to variable shifts in pK(app.) values of between 0.5 and 1.2 pH units for this parameter. The observed pH-independent limiting rate constants for k(cat.)/Km(app.) are made to vary between 0.5 and 1.0 in 2H2O by effects on kinetic parameters for the EH2 state, k'cat./K'm varying between 0.2 and 0.7 relative to the EH state, with k(cat.)/Km varying between 0.4 and 1.0. The effects observed on k(cat.)/Km(app.) are ultimately the result of variable effects of 2H2O on k'cat. and K'm for the EH2 state relative to k(cat.) and Km for the EH state of AcChE. These effects are responsible for the variable shifts and more than 0.5 pH unit of the pK(app.) values in 2H2O for pH-k(cat.)/Km profiles. The upward-bowing hydron inventories for k(cat.)/Km are the result of linear hydron inventories for k(cat.) and downward-bowing on Km and are not due to the rate-limiting substrate-induced fit process as claimed in the literature.(ABSTRACT TRUNCATED AT 400 WORDS)

Kinetics and mechanism of gold(III) incorporation into tetrakis(1-methylpyridium-4-yl)porphyrin in aqueous solution

2004 ◽  
Vol 08 (11) ◽  
pp. 1269-1275 ◽  
Author(s):  
Ahsan Habib ◽  
Masaaki Tabata ◽  
Ying Guang Wu
Keyword(s):  
Aqueous Solution ◽  
Rate Constants ◽  
Kinetic Data ◽  
Ph Dependence ◽  
Equilibrium Constants ◽  
Hydroxyl Group ◽  
Net Charge ◽  
First Order ◽  
Kinetics Of ◽  
Hydrolysis Of

The kinetics of the reaction of the tetrakis(1-methylpyridium-4-yl)porphyrin tetracation, [ H 2( TMPyP )]4+, with gold(III) ions were studied along with equilibria of gold(III) species in aqueous medium at 25°C, I = 0.10 M ( NaNO 3). The equilibrium constants for the formation of [ AuCl 4-n( OH ) n ]- ( n = 0,…,4), defined as β n = [ AuCl 4- n ( OH ) n ]- [ Cl -] n / [ AuCl 4-][ OH -] n were found to be that log β1 = 7.94 ± 0.03, log β2 = 15.14 ± 0.03, log β3 = 21.30 ± 0.05 and log β4 = 26.88 ± 0.05. The overall reaction was first order with respect to each of the total [ Au (III)] and [ H 2 TMPyP 4+]. On the basis of pH dependence on rate constants and the hydrolysis of gold(III), the rate expression can be written as d [ Au ( TMPyP )5+]/ dt = ( k 1[ AuCl 4-] + k2[ AuCl 3( OH )-] + k3[ AuCl 2( OH )2-] + k4[ AuCl ( OH )3-])[ H 2 TMPyP 4+], where k1, k2, k3 and k4 were found to be (2.16 ± 0.31) × 10-1, (6.56 ± 0.19) × 10-1, (1.07 ± 0.24) × 10-1, and (0.29 ± 0.21) × 10-1 M -1. s -1, respectively. The kinetic data revealed that the trichloromonohydroxogold(III) species, [ AuCl 3( OH )]-, is the most reactive. The higher reactivity of [ AuCl 3( OH )]- is explained by hydrogen bonding formation between the hydroxyl group of [ AuCl 3( OH )]- and the pyrrole hydrogen atom of [ H 2( TMPyP )]4+. Furthermore, applying the Fuoss equation to the observed rate constants at different ionic strengths, the apparent net charge of [ H 2( TMPyP )]4+ was calculated to be +3.5.

Kinetic Analysis of the Catalytic Properties of Peptides in Ester Hydrolysis

1976 ◽  
Vol 31 (11-12) ◽  
pp. 675-678 ◽  
Author(s):  
Dorothee Petz ◽  
Friedhelm Schneider
Keyword(s):  
Kinetic Analysis ◽  
Aspartic Acid ◽  
Rate Constants ◽  
Catalytic Properties ◽  
Ph Dependence ◽  
Ester Hydrolysis ◽  
Saturation Kinetics ◽  
Catalytic Behaviour ◽  
Hydrolysis Of

Abstract The catalytic properties of peptides containing histidine, cysteine and aspartic acid in ester hydrolysis were studied. Saturation kinetics were found for the reaction of p-nitrophenyl acetate (NPA) with Z-His-Ala-Asp-Gly-Cys-NH2 and Z-His-Ala-Gly-Gly-Cys-NH2 . The Brönsted equation for the hydrolysis of tert-butyloxycarbonyl-L-alanine-p-nitrophenylester (Boc-Ala-ONp) catalyzed by simple imidazole and SH-compounds was determined. The catalytic behaviour of the peptides in ester hydrolysis could not be described by the Brönsted equations for imidazole or thiole catalyzed hydrolysis of NPA and Boc-Ala-ONp. The pH dependence of the rate constants of the catalyzed ester hydrolysis gave no linear plots in 1/k versus H+ diagrams.

scholarly journals The effect of methanol and dioxan on the rates of the β-galactosidase-catalysed hydrolyses of some β-d-galactopyranosides: rate-limiting degalactosylation. The pH-dependence of galactosylation and degalactosylation

1973 ◽  
Vol 133 (1) ◽  
pp. 81-87 ◽  
Author(s):  
Michael L. Sinnott ◽  
Odile M. Viratelle
Keyword(s):  
Steady State ◽  
Ph Dependence ◽  
Methanol Concentration ◽  
The Other ◽  
Fractional Rate ◽  
Rate Of Increase ◽  
Rate Limiting ◽  
Hydrolysis Of

1. The effect of methanol on the β-galactosidase-catalysed hydrolysis of some nitrophenyl β-d-galactopyranosides has been studied under steady-state conditions. 2. The initial fractional rate of increase of kcat. as a function of methanol concentration with 2,4- and 3,5-dinitrophenyl β-d-galactopyranosides, but not with the other substrates studied, indicated that degalactosylation of the enzyme was rate-limiting. 3. The decrease in kcat. at high methanol concentrations for these substrates is considered to arise from causes other than galactosylation becoming rate-limiting. 4. Both galactosylation and degalactosylation of the enzyme require protonation of a group of pKa approx. 9.

Kinetics of alkaline hydrolysis of mono- and dimethyl esters of 2,3- and 3,4-thiophenedicarboxylic acids, cyanothiophenecarboxylic acids and their dihydroanalogues

1987 ◽  
Vol 52 (8) ◽  
pp. 2005-2018
Author(s):  
Milan Struhárik ◽  
Pavel Hrnčiar ◽  
Dušan Loos
Keyword(s):  
Kinetic Parameters ◽  
Alkaline Hydrolysis ◽  
Rate Constants ◽  
Activation Parameters ◽  
Hydrolysis Rate ◽  
Electronic Effects ◽  
Cyclic System ◽  
Kinetics Of ◽  
Hydrolysis Of ◽  
Reactivity Order

Kinetics of alkaline hydrolysis of mono- and dimethyl esters of 2,3- and 3,4-thiophenedicarboxylic acids and their dihydroanalogues have been studied in 70% dioxane. The rate measurements have been carried out by the titrimetric method and the k1, k2 rate constants evaluated by the Frost-Schwemer method. The activation parameters of these reactions have been calculated. The results are compared with kinetic parameters of alkaline hydrolysis of methyl and dimethyl phthalates. The hydrolysis rate is significantly affected by the cyclic system to which the methoxycarbonyl groups are bound, the reactivity order being: Dihydrothiophene > thiophene > benzene. Also measured were the kinetics of alkaline hydrolyses of methyl 4-cyano-3-thiophenecarboxylate and 3-cyano-2-thiophenecarboxylate and of their dihydroanalogues. No meaningful preference of the hydrolysis of the group at the position 2 has been observed in case of 2,3-isomers. However, it has been confirmed that the hyper- and hypo-ortho transfer of electronic effects operates in the thiophene nucleus.

Influence of medium on alkaline hydrolysis of succinic acid monomethyl and monopropyl esters

1980 ◽  
Vol 45 (11) ◽  
pp. 2873-2882
Author(s):  
Vladislav Holba ◽  
Ján Benko
Keyword(s):  
Succinic Acid ◽  
Kinetic Parameters ◽  
Alkaline Hydrolysis ◽  
Specific Interactions ◽  
Nonaqueous Media ◽  
The Media ◽  
Kinetics Of ◽  
Hydrolysis Of

The kinetics of alkaline hydrolysis of succinic acid monomethyl and monopropyl esters were studied in mixed aqueous-nonaqueous media at various temperatures and ionic strengths. The results of measurements are discussed in terms of electrostatic and specific interactions between the reactants and other components of the reaction mixture. The kinetic parameters in the media under study are related to the influence of the cosolvent on the solvation sphere of the reactants.

Kinetics of hydrolysis of peroxodisulphate ions in acidic medium to peroxomonosulphuric acid

1981 ◽  
Vol 46 (5) ◽  
pp. 1229-1236 ◽  
Author(s):  
Jan Balej ◽  
Milada Thumová
Keyword(s):  
Ionic Strength ◽  
Rate Constants ◽  
Acidic Medium ◽  
Measured Data ◽  
Molar Ratios ◽  
Starting Solution ◽  
Kinetics Of Hydrolysis ◽  
Rate Of Hydrolysis ◽  
Kinetics Of ◽  
Hydrolysis Of

The rate of hydrolysis of S2O82- ions in acidic medium to peroxomonosulphuric acid was measured at 20 and 30 °C. The composition of the starting solution corresponded to the anolyte flowing out from an electrolyser for production of this acid or its ammonium salt at various degrees of conversion and starting molar ratios of sulphuric acid to ammonium sulphate. The measured data served to calculate the rate constants at both temperatures on the basis of the earlier proposed mechanism of the hydrolysis, and their dependence on the ionic strength was studied.

Salt effect in hydrolysis of 3-acyl-1,3-diphenyltriazenes

1981 ◽  
Vol 46 (12) ◽  
pp. 3104-3109 ◽  
Author(s):  
Miroslav Ludwig ◽  
Oldřich Pytela ◽  
Miroslav Večeřa
Keyword(s):  
Sodium Chloride ◽  
Temperature Dependence ◽  
Potassium Chloride ◽  
Rate Constants ◽  
Zinc Sulphate ◽  
Salt Effect ◽  
Sodium Sulphate ◽  
Potassium Sulphate ◽  
Lithium Sulphate ◽  
Hydrolysis Of

Rate constants of non-catalyzed hydrolysis of 3-acetyl-1,3-diphenyltriazene (I) and 3-(N-methylcarbamoyl)-1,3-diphenyltriazene (II) have been measured in the presence of salts (ammonium chloride, potassium chloride, lithium chloride, sodium chloride and bromide, ammonium sulphate, potassium sulphate, lithium sulphate, sodium sulphate and zinc sulphate) within broad concentration ranges. Temperature dependence of the hydrolysis of the substrates studied has been measured in the presence of lithium sulphate within temperature range 20° to 55 °C. The results obtained have been interpreted by mechanisms of hydrolysis of the studied substances.

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