scholarly journals Purification and mechanistic properties of an extracellular α-l-arabinofuranosidase from Monilinia fructigena

1987 ◽  
Vol 245 (3) ◽  
pp. 843-849 ◽  
Author(s):  
M A Kelly ◽  
M L Sinnott ◽  
M Herrchen
Keyword(s):  
Ring Opening ◽  
Gel Filtration ◽  
Bond Breaking ◽  
Absolute Rate ◽  
Monilinia Fructigena ◽  
First Order ◽  
A Value ◽  
Leaving Group ◽  
Rate Limiting ◽  
Hydrolysis Of

1. The alpha-L-arabinofuranosidase isoenzyme designated AFIII [Laborda, Archer, Fielding & Byrde (1974) J. Gen. Microbiol. 81, 151-163] was purified by sequential isoelectric focusing, hydrophobic chromatography, gel filtration and chromatofocusing. 2. The enzyme is a monomer of Mr 40,000. 3. On inactivation of the enzyme with 3H-labelled 1-alpha-L-arabinofuranosylmethyl-3-p-nitrophenyltriazene, 0.64 mol of alpha-L-arabinofuranosylmethyl residues/mol of enzyme is estimated to become attached to protein. 5. Neither first-order nor second-order rate constants for hydrolyses of aryl alpha-L-arabinofuranosides are dependent upon leaving-group acidity [beta lg(V) = −0.16 +/− 0.11; Beta lg(V/K) = −0.11 +/- 0.07; n = 7; delta pKa = 4.5] 6. Bond-breaking is nonetheless rate-limiting, as is shown by a value of 18(V) of 1.030 +/− 0.007 for the hydrolysis of p-nitrophenyl arabinoside. 7. Proton-donation to the leaving group is thus far advanced at the rate-limiting transition state for this enzyme. 8. Four alpha-L-arabinofuranosyl pyridinium salts are substrates, and an approximate beta lg(V) value of −0.9 can be estimated. 9. The absolute rate enhancement with the 4-bromoisoquinolinium salt, 2.5 × 10(9), is comparable with that observed with pyranosidases. 10. Ring-opening mechanisms can therefore be dismissed, even though they are known in the acid-catalysed hydrolysis of arabinofuranosides.

Die γ-Radiolyse und Pulsradiolyse des Schwefelkohlenstoffs in wäßriger Lösung / The γ-Radiolysis and Pulse Radiolysis of Carbon Disulfide in Aqueous Solution

1973 ◽  
Vol 28 (1-2) ◽  
pp. 12-22 ◽  
Author(s):  
W. Roebke ◽  
M. Schöneshöfer ◽  
A. Henglein
Keyword(s):  
Rate Constant ◽  
Carbon Disulfide ◽  
Second Order ◽  
Γ Irradiation ◽  
Electrolytic Dissociation ◽  
First Order ◽  
A Value ◽  
Diffusion Controlled ◽  
Polymer Pyrolysis ◽  
Hydrolysis Of

A polymer (CHS2)n and sulfate are formed in the γ-irradiation of deaerated aqueous carbon disulfide solutions. The G-values are 3.6 and 0.41, respectively. In the presence of N,O, G (polymer) is decreased while G(SO4-) is increased. G(SO4-) can be decreased by isopropanol. G(polymer) is increased by H+ ions and reaches a value of 5 below pH = 2. Formic acid, hydrogen sulfide and carbonate are formed in the hydrolysis of the polymer. Pyrolysis at first leads to a red oil consisting of oligomer (HCS2)n and finally to H2S, CS2 plus a residue containing much carbon. The structure of the polymer is discussed.Pulse radiolytic experiments show that CS2 reacts with eaq (3.1 × 1010M-1s-1) and OH(7.4 × 109M-1s-1) in a diffusion controlled manner. The first product of the reaction with OH is SC(OH)S. The pK of the electrolytic dissociationSC(OH)Ṣ ⇄ SC(O-)S + H+is 4.4. The absorption spectra of SC(OH)S and SC(O-)S were measured. SC(OH)S disappears by second order with 2k = 1.6 × 109M-1s-1 at pH = 6. The product is a bivalent acid, the spectrum of which was measured. The second pK of this acid is 5.7, its first pK is lower than 4.Both eāq and H react with CS2 to form SCS-. The absorption spectrum of this radical anion was measured. The pK of the equilibriumSCSH ⇄SCS- + H+is about 1.6. In solutions of low H+-concentration, SCS- disappears by second order with 2k = 6.4 × 109M-1s-1. The structure of dithioformic acid is attributed to the resulting product. In solutions of high H+-concentration, SCS- (or SCSH) disappears by a fast first order process, the rate constant of which increases with H+-concentration. The carbeniat neutralizationis believed to be responsible for this process. The rate constant is 5.1 × 107M-1S-1. The spectrum of SC(H)S was measured. This radical disappears by second order with 2k = 7.4 × 109M-1s-1. The spectrum of the resulting product was also determined.It is concluded that the formation of the polymer and of SO4- occurs in processes in which the first products from the attack of eāq, H and OH on CS2 as well as molecules which were built up from these products are involved.

scholarly journals Human Carboxylesterase 2 in Cocaine Metabolism

2021 ◽  
Author(s):  
Pedro R. Figueiredo ◽  
Ricardo D. González ◽  
Alexandra T.P. Carvalho
Keyword(s):  
Serum Proteins ◽  
Therapeutic Potential ◽  
The Body ◽  
Molecular Mechanics Calculations ◽  
Accelerated Molecular Dynamics ◽  
Rate Limiting Step ◽  
A Value ◽  
First Time ◽  
Rate Limiting ◽  
Hydrolysis Of

Increased hydrolysis of cocaine to non-toxic compounds is a promising way to prevent cocaine-induced toxicity. However, the short half-life of cocaine in the blood and the rapid conversion in the body to the hydrolysis-resistant metabolite benzoylecgonine, limits the therapeutic potential of serum proteins. Therefore, hydrolysis by tissue-specific hydrolases that do not generate benzoylecgonine deserves further investigation. Here, we report for the first time, the mechanism of cocaine hydrolysis by the human Carboxylesterase 2. We have combined conventional and accelerated Molecular Dynamics, which allowed us to identify the structural motions of the α1 and α10’ helices that act as a putative lid. Quantum Mechanics/Molecular Mechanics calculations on the full cycle showed that the rate-limiting step is the formation of benzoic acid (deacylation step) with an ΔG of 18.3 kcal.mol-1 (a value in close conformity with the experimental value of 19.7 kcal.mol-1).

scholarly journals Insights into the mechanisms controlling the residual corrosion rate of borosilicate glasses

2020 ◽  
Vol 4 (1) ◽  
Author(s):  
Stephane Gin ◽  
Xiaolei Guo ◽  
Jean-Marc Delaye ◽  
Frédéric Angeli ◽  
Kamalesh Damodaran ◽  
...  
Keyword(s):  
Deep Understanding ◽  
Borosilicate Glasses ◽  
New Paradigm ◽  
Residual Rate ◽  
A Value ◽  
Gel Layer ◽  
Multiple Processes ◽  
High Level ◽  
Rate Limiting ◽  
Hydrolysis Of

AbstractBorosilicate glasses are widely used to confine high-level radioactive wastes. The lifetime of these materials could reach hundreds of thousands of years if leaching of the glass into groundwater enables the formation of a passivating gel layer. Even in this regime, the glass will never stop corroding as thermodynamic equilibrium between glass and solution cannot be achieved. Therefore, accurate predictions of glass durability including passivation, require a deep understanding of the mechanisms controlling the so-called residual rate. However, despite tremendous efforts, these mechanisms remain poorly understood. Here, focusing on the behavior of the soluble elements of the International Simple Glass (B, Na, and Ca), we show that the residual rate is controlled by the behavior of B, a glass former supposed to dissolve instantaneously when in contact with water and thus widely considered as an ideal tracer. We then demonstrate that B release is controlled by multiple processes highly dependent on the pH. At the beginning of the passivating layer formation, the hydrolysis of B-O-Si linkages is rate-limiting and has an activation energy of ∼60 kJ mol−1, a value slightly lower than that for breaking Si-O-Si linkages. Once the fraction of closed pores resulting from gel restructuring is high enough, then diffusion of both reactants (water molecules) and some products (mainly Baq, Caaq) through the growing gel layer becomes rate-limiting. Consequently, B and Ca accumulate in an inner layer referred to as the active zone, with potential feedback on the B-O-Si hydrolysis. A new paradigm, including B as a key element of the system, is proposed to develop a comprehensive model for the corrosion of borosilicate glass.

Modelling of Anaerobic Digestion Processes (A Discussion of Concepts)

1991 ◽  
Vol 24 (8) ◽  
pp. 17-33 ◽  
Author(s):  
P. L. McCarty ◽  
F. E. Mosey
Keyword(s):  
Anaerobic Digestion ◽  
Mathematical Models ◽  
Volatile Fatty Acids ◽  
Rate Equations ◽  
Monod Kinetics ◽  
Anaerobic Digesters ◽  
First Order ◽  
Intermediate Metabolism ◽  
Rate Limiting ◽  
Hydrolysis Of

The concepts behind some of the more widely used mathematical models of the anaerobic digestion process are described and discussed together with some recent microbiological and biochemical studies that might provide a basis for the next generation of mathematical models. First order rate equations are commonly used for the initial hydrolysis of complex organics and Monod kinetics are widely used to describe methanogenesis from acetate. Depending on circumstances, either of these reactions may be considered as the rate-limiting reaction of the overall fermentation. Thermodynamic calculations provide a valuable tool for modelling intermediate metabolism but the metabolic repertoire of these bacteria still provides a few surprises for the fermentation chemist. A possible new concept for modelling patterns of volatile fatty acids during overload and recovery of anaerobic digesters is described and discussed.

Model Reactions for the Degradation of DNA-4′ Radicals in Aqueous Solution. Fast Hydrolysis of α-Alkoxyalkyl Radicals with a Leaving Group in β-Position Followed by Radical Rearrangement and Elimination Reactions

1982 ◽  
Vol 37 (11-12) ◽  
pp. 1205-1227 ◽  
Author(s):  
Günter Behrens ◽  
Günther Koltzenburg ◽  
Dietrich Schulte-Frohlinde
Keyword(s):  
Aqueous Solution ◽  
Radical Cations ◽  
Cyclic Ethers ◽  
Γ Irradiation ◽  
A Value ◽  
Leaving Group ◽  
Catalyzed Reactions ◽  
Hydrolysis Of ◽  
Model Reactions ◽  
Hydrolysis Reactions

Abstract α-Alkoxyalkyl radicals with a leaving group L = Cl or OCOCH3 in β-position are produced by H-abstraction from the corresponding saturated substrates by ·OH, SO·4- or (CH3)3CO· radicals. From ESR spectroscopic observations it is concluded that in aqueous solution at pH 5 -9 the following fast hydrolysis reactions take place: The rate constants of these reactions and for the hydrolysis of CH3O-ĊH-CH2Cl are k ≥ 106 s-1, whereas the rate constant for CH3O-ĊH-CH2OCOCH3 was determined to be ≈ 2 × 103 s-1 at room temperature. The radicals with L = Cl cannot be scavenged by O2 which fact leads to a value of k ≥ 2 × 10-7 s-1. α-Alkoxyalkene radical cations are assumed as intermediates in the hydrolysis reactions. The radicals with L = OCOCH3 and the radical CH3O--ĊH-CH2Cl are observable in acetone solution ESR spectroscopically. In aqueous solution at pH below 3 proton catalyzed reactions are observed by ESR spectroscopy: Radicals resulting from H-abstraction at the CH3O-groups of the substrates or at the 5-positions of the cyclic ethers are also observed. The ESR parameters and the pH-ranges of existence of the above radicals are given. Support of the reported reactions comes from quantitative analysis of stable products such as H+, Cl- or CH3OH after 60Co-γ-irradiation of N2O saturated aqueous solutions of the substrates. The behaviour of the radicals is used as a model to describe a modified version of the degradation of DNA-4′ radicals in aqueous solution in the absence of oxygen.

scholarly journals The molecular weight and thiol residues of acetyl-coenzyme A synthetase from ox heart mitochondria

1973 ◽  
Vol 133 (1) ◽  
pp. 23-36 ◽  
Author(s):  
John C. Londesborough ◽  
Sung Ling Yuan ◽  
Leslie T. Webster
Keyword(s):  
Molecular Weight ◽  
Binding Sites ◽  
Gel Filtration ◽  
Enzymic Activity ◽  
Slow Phase ◽  
Conformation Change ◽  
Acetyl Coa ◽  
Direct Role ◽  
First Order ◽  
Rate Limiting

1. A constant molecular weight of 57000 was obtained by gel filtration of highly purified acetyl-CoA synthetase over a 1000-fold range of enzyme concentrations. The amino acid analysis is reported. 2. With native enzyme at 20°C the relatively rapid reaction of four thiol residues with p-hydroxymercuribenzoate caused an immediate inhibition reversible by either CoA or mercaptoethanol. Other substrates did not protect against this rapid inhibition. 3. The much slower reaction of the remaining four thiol residues was independent of the concentration of the mercurial, first-order with respect to enzyme, and had a large energy of activation (+136kJ/mol), suggesting that a conformation change in the protein was rate-limiting. This slow phase of the reaction was accompanied by an irreversible inactivation of the enzyme. 4. The effects of substrates on this irreversible inactivation at pH7.0 in 5 mm-MgCl2 indicated strong binding of ATP and pyrophosphate by the enzyme (concentrations for half-maximal effects, K½, were <30μm and <10μm respectively) and weaker binding of acetyl-CoA (K½ about 1 mm), AMP (K½ about 2mm) and acetate. In the presence of acetate, MgCl2 and p-hydroxymercuribenzoate, titration of the enzyme with ATP revealed at least two ATP binding sites/mol. 5. The experiments suggest that reaction of the thiol residues with mercurial causes loss of enzymic activity by altering the structure of the enzyme, rather than that the thiol residues play a direct role in the catalysis.

Metal Ion Promoted Hydrolysis of Polyphosphates

1993 ◽  
Vol 46 (7) ◽  
pp. 1021 ◽  
Author(s):  
RJ Geue ◽  
AM Sargeson ◽  
R Wijesekera
Keyword(s):  
Time Scale ◽  
Metal Ion ◽  
The Other ◽  
Phosphate Esters ◽  
Bond Rupture ◽  
Hydroxo Complexes ◽  
Leaving Group ◽  
Rapid Hydrolysis ◽  
Rate Limiting ◽  
Hydrolysis Of

Various CoIII amine aqua hydroxo complexes have been evaluated for their ability to hydrolyse pyrophosphate to phosphate rapidly. Two very effective reagents have emerged, [Co( trpn )(OH)(OH2)]2+ ( trpn = 3,3′,3″-nitrilotris(propan-1-amine)) and [Co( tamen )(OH)-OH2)]2+ ( tamen = 6-(4-amino-2-azabutyl)-6-methyl-1,4-diazacycloheptane), and the latter has been examined closely to assess details of the mechanistic path for hydrolysis. The study shows substitution of the reagent on the substrate is fast and not rate determining. Two moles of the reagent are required to bind to [(en)2Co(P2O7)]- (en = ethane-1,2-diamine) to effect rapid hydrolysis. One supplies the coordinated OH- nucleophile to cleave the phosphorus anhydride, and the other binds as a chelate to the leaving group to assist bond rupture. An unproductive side path, to form a symmetrical dichelate of the form [(en)2Co(P2O7)Co( tamen )]2+, becomes rate limiting. However, if this path can be reduced in significance or eliminated, the study implies that polyphosphates and phosphate esters should hydrolyse on the subsecond time scale.

The effect of polymeric and model imidazolium halides on the rate of hydrolysis of 4-acetoxy-3-nitrobenzoic acid

1985 ◽  
Vol 50 (4) ◽  
pp. 845-853 ◽  
Author(s):  
Miloslav Šorm ◽  
Miloslav Procházka ◽  
Jaroslav Kálal
Keyword(s):  
Catalyst Concentration ◽  
Low Molecular Weight ◽  
Imidazolium Chloride ◽  
First Order ◽  
Nitrobenzoic Acid ◽  
Rate Of Hydrolysis ◽  
Acid Catalyzed ◽  
Hydrolysis Of ◽  
Ethanol In Water ◽  
Direct Attack

The course of hydrolysis of an ester, 4-acetoxy-3-nitrobenzoic acid catalyzed with poly(1-methyl-3-allylimidazolium bromide) (IIa), poly[l-methyl-3-(2-propinyl)imidazolium chloride] (IIb) and poly[l-methyl-3-(2-methacryloyloxyethyl)imidazolium bromide] (IIc) in a 28.5% aqueous ethanol was investigated as a function of pH and compared with low-molecular weight models, viz., l-methyl-3-alkylimidazolium bromides (the alkyl group being methyl, propyl, and hexyl, resp). Polymers IIb, IIc possessed a higher activity at pH above 9, while the models were more active at a lower pH with a maximum at pH 7.67. The catalytic activity at the higher pH is attributed to an attack by the OH- group, while at the lower pH it is assigned to a direct attack of water on the substrate. The rate of hydrolysis of 4-acetoxy-3-nitrobenzoic acid is proportional to the catalyst concentration [IIc] and proceeds as a first-order reaction. The hydrolysis depends on the composition of the solvent and was highest at 28.5% (vol.) of ethanol in water. The hydrolysis of a neutral ester, 4-nitrophenyl acetate, was not accelerated by IIc.

Sign in / Sign up

Export Citation Format

Share Document