Apparent Kinetic Properties Research Articles

Kinetic Insights into Boron-Based Materials Catalyzed Oxidative Dehydrogenation of Light Alkanes.

High selectivity toward alkenes in oxidative dehydrogenation (ODH) of light alkanes makes boron-based materials promising catalysts. However, many key mechanistic aspects are still debated due to the challenge of capturing fleeting reaction intermediates. Kinetic analysis, including determining reaction orders and activation energy, could be informative for reactions involving radical intermediates but has not been extensively exploited. This Review summarizes the current understanding of the apparent alkane reaction order and the apparent activation energy in the boron-catalyzed ODH. Despite varying compositions and structures, a majority of boron-based catalysts share many common features, including alkene selectivity, the evolution and the formation of active site, and the apparent kinetic properties. These common trends could be attributed to the shared gas-phase radical-mediated reaction pathways and the formation of active hydroxylated boron oxide species on boron-containing materials under ODH conditions. Values of apparent alkane reaction orders and apparent activation energies are sensitive and reliable experimental measures of the contributions of the gas-phase radical-mediated and surface-mediated pathways, suggesting the outline of a general mechanistic framework of the boron-catalyzed ODH.

Preparation of morph-genetic aluminum-doped calcium oxide templated from cotton and the calcium looping performance for energy storage in the presence of steam

Calcium looping (CaL), which can be combined with concentrated solar power (CSP) plants, is considered a promising technology for energy storage. To overcome the deactivation of calcium based materials with increasing cycles, a novel morph-genetic aluminum doped calcium oxide was prepared by a biomass template method using limestone, aluminum nitrate and cotton as raw materials. The effects of preparation parameters and heat storage conditions (especially in presence of steam) on the energy storage performance were studied. The results showed that the hollow tubular structure of the biomass template was preserved in the synthesized material and stabilized by the support of Ca12Al14O33. The presence of the hollow microtubular structure resulted in an increased pore volume of the synthesized materials and a change of the pore distribution, promoting the diffusion of CO2. This phenomenon also explained the superior performance of the synthesized material in terms of the apparent kinetic properties and resistance to sintering. The optimal synthesized material had a high energy storage density and carbonation fraction, which were more than three times those of limestone. The presence of steam during calcination served to reduce the calcination temperature and mitigate the sintering effect of CaO. An optimization route combining solar steam system and CaL-CSP system was proposed based on the positive effect of steam, which was promising for high-efficiency energy storage.

Apparent kinetic properties of soil phosphomonoesterase and β‐glucosidase are disparately influenced by pH

AbstractExtracellular enzymes catalyze organic matter transformations in soils, yet the study of soil enzymes does not typically evaluate how enzyme kinetic properties that govern activity in situ may shift with environmental parameters such as pH. To test this, we quantified changes in apparent kinetic properties (maximum catalysis rate [AppVmax] and substrate concentration that produces 50% of Vmax [AppKm]) of soil phosphomonoesterase (PME) and β‐glucosidase (BG) as a function of pH by using dual gradients of buffer pH (3–12) and substrate concentrations in a selected set of diverse soils. A third kinetic property, apparent specificity constant (AppKa = AppVmax/AppKm) was also evaluated as a function of pH. Results indicated widely differing pH optima of soil PME and BG AppVmax and AppKm, decoupling of which entailed unpredictable pH optima of AppKa. Additionally, changes in soil enzyme activity with pH may not be driven by shifts in the specificity constant because pH–AppVmax relationships are largely unrelated to co‐occurring changes in AppKa. These results invalidate the assumption that Km of a soil enzyme is constant across pH and emphasize the importance of assaying soil enzyme activities at sufficiently high (5 × Km) substrate concentrations at a given pH to avoid the mischaracterizing pH optima.

Kinetics study and performance comparison of CO2 separation using aqueous choline-amino acid solutions

The thermodynamic and kinetic properties of CO2 in aqueous choline-amino acids ([Cho][AA]s) are important information to demonstrate their performance. In this study, the apparent kinetic properties of CO2 in the five aqueous [Cho][AA]s, including the liquid-side mass-transfer coefficients, enhancement factor, and reaction rate constant, were systematically studied. Furthermore, a new ‘‘absorption ability’’ (AA) index was proposed, combining the apparent kinetic properties determined in this study and thermodynamic properties determined in our previous study. The CO2 separation performance using aqueous [Cho][AA]s was evaluated based on the AA and CO2 desorption enthalpy values. The results show that 30 wt% aqueous choline-serine is a promising absorbent for CO2 separation, and it is comparable to aqueous monoethanolamine.

Ldh activity in Euphausia mucronata and Calanus chilensis: implications for vertical migration behaviour

Specific lactic dehydrogenase (ldh) activity was measured in Euphausia mucronata and Calanus chilensis. Both key pelagic species inhabit areas of the Humboldt Current System that have permanent subsurface oxygen minimum layer. The specific ldh activity in E. mucronata was two orders of magnitude higher than that of C. chilensis, consistent with E. mucronata's ability to conduct daily vertical migrations through the oxygen minimum layer. In contrast, C. chilensis is restricted to inhabiting oxygenated waters over the oxygen minimum layer without carrying out daily vertical migrations across it. A positive correlation (P < 0.0001) was found between the specific ldh activity and body size in E. mucronata. This study also reports the apparent kinetic properties (V max and K m ) of ldh and the effect of temperature and pH on the specific activity for both species.

In Vivo Properties of Phosphoenolpyruvate Carboxylase in Crassulacean Acid Metabolism Plants. Is Pineapple CAM Not Regulated by PEPC Phosphorylation?

The in vivo sensitivity to inhibition by L-malate of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) in rapidly prepared crude extracts (within 2 min) was investigated for two CAM plants Kalanchoe pinnata and pineapple (Ananas comosus (L.) Merr. cv. Smooth-cayenne N67-10) . The phosphorylation process may not occur in pineapple PEPC. Variations were also observed in apparent kinetic properties and molecular mass as well as subunit composition of PEPC from these plants. Two different subunits (major polypeptide 112 kDa and minor polypeptide 119 kDa) were separated from day and night forms of PEPC from K. pinnata but only one identical subunit (107 kDa) from pineapple. The integrity of both forms of in vivo PEPC from these plants was demonstrated by Western blot analyses using anti-N-terminus antibody. K. pinnata lost malate sensitivity about 87% whereas pineapple PEPC lost only 21% sensitivity at night in the presence of 2 mM L-malate. The apparent Ki (L-malate) was 6.4-fold higher in phase I (5.8 mM) than that in phase III (0.9 mM) from K. pinnata but it was only 1.0 and 0.5 mM for day and night PEPC respectively from pineapple. These findings suggest phosphorylation process may not be the major regulatory mechanism in pineapple PEPC.

Multicompartment, numerical model of cellular events in the pharmacokinetics of gene therapies.

DNA expression vectors may be administered to patients like conventional medicines to have a finite and controlled duration of action. The clinical application of these medicines will require a precise understanding of the kinetics of the administered gene, the mRNA transcript, and the gene product. The apparent kinetic properties of the therapeutic gene product, including the level and duration of action, will be determined by various intrinsic kinetic processes including: (i) distribution and biological fate of the DNA expression vector; (ii) rates of DNA uptake into cells and dynamics of intracellular trafficking; (iii) half-life of the DNA vector in the cell; (iv) transcription rate; (v) half-life of mRNA; (vi) translation rate; and (vii) post-translational processing, distribution, and fate of the gene product. To consider in a theoretical manner how the intrinsic kinetics of cellular processes may affect the apparent level of a therapeutic gene product over time, we have constructed a multicompartment, numerical model. The model has six compartments, designated MILIEU, ENDOSOME, CELL, RNA, PROTEIN, and PRODUCT. The apparent level and kinetics of the gene product over time are calculated with different values for the intrinsic t1/2 of DNA in the MILIEU, ENDOSOME, and CELL; the intrinsic t1/2 of mRNA; the intrinsic t1/2 of the gene product; endosomal stability; and transcription rate. The model demonstrates how first-order kinetics can result from the summation of complex kinetic processes and provides a theoretical basis for future pharmacokinetic studies. This theoretical model illustrates how the half-lives of DNA, RNA, and gene product each affect the level of the product and highlights strategies for enhancing the therapeutic profile of gene therapies.

In situ behavior of the pyrimidine pathway enzymes in Saccharomyces cerevisiae: 2. Reaction mechanism of aspartate transcarbamylase dissociated from carbamylphosphate synthetase by genetic alteration

The reaction mechanism of Saccharomyces cerevisiae aspartate transcarbamylase was studied in permeabilized cells of a mutant in which this enzyme is not associated to carbamylphosphate synthetase. The results obtained indicate an ordered mechanism in which carbamylphosphate binds first, followed by aspartate, with dissociation of the products in the order phosphate then carbamylaspartate. Interestingly, this clear-cut mechanism differs from the more complex behavior shown by aspartate transcarbamylase when this enzyme is associated to carbamylphosphate synthetase in wild-type S. cerevisiae (B. Penverne and G. Hervé, Arch. Biochem. Biophys. (1983) 225, 562–575). This difference indicates that the association of the two enzymes within the multienzymatic complex alters the apparent kinetic properties of aspartate transcarbamylase. Such an enzyme-enzyme interaction might be related to the channeling of carbamylphosphate from one catalytic site to the other one.

Effect of creatine kinase association with heart mitochondria on its apparentkinetic properties

Effect of creatine kinase association with heart mitochondria on its apparentkinetic properties

Fructose 2,6-bisphosphate as a contaminant of commercially obtained fructose 6-phosphate: Effect on PP i:fructose 6-phosphate phosphotransferase

Fructose 6-phosphate from several commercial sources was shown to be contaminated with fructose 2,6-bisphosphate. This contaminant was identified by its activation of PP i:fructose 6-phosphate phosphotransferase, extreme acid lability and behaviour on ion-exchange chromatography. The apparent kinetic properties of PP i:fructose 6-phosphate phosphotransferase from castor bean endosperm were considerably altered when contaminated fructose 6-phosphate was used as a substrate. Varying levels of fructose 2,6-bisphosphate in the substrate may account for differences that have been observed in the properties of the above enzyme from several plant sources.

Histochemical localization and quantification of glucose-6-phosphate dehydrogenase in bovine leydig cells.

Some of the critical steps in the qualitative histochemical localization of glucose-6-phosphate dehydrogenase (freezing procedures, incubation techniques and the influence of intermediate electron carriers, respiratory chain inhibitors and different tetrazolium salts) were evaluated in sections of bovine testis as a prerequisite for the microdensitometric estimation of the activity of the enzyme in bovine Leydig cells in situ. A modification of the gel incubation method of Rieder et al. (1978) gave the best results and was used for the quantitative investigations. Quantitative data for the dehydrogenase activity gained from microdensitometry of the formazan final reaction products in Leydig cells in situ were compared with the results of assays of the activity in homogenates of testis. The following apparent kinetic properties of glucose-6-phosphate dehydrogenase were obtained for the enzyme in Leydig cells in situ: Vmax = 0.11 absorbance units/min, Km = 0.37 mM. The quantitative characterization of glucose-6-phosphate activity in Leydig cells in situ appears to be suitable for combined morphological and functional diagnoses of small tissue samples such as testicular biopsies. This would give valuable information of the functional status of Leydig cells in normal and diseased testicular tissue.

N-Acetylglutamate synthetase in human liver: Regulation of activity by L-Arginine and N-Acetylglutamate

A sensitive assay for the determination of N-Acetylglutamate synthetase activity in human liver has been developed. The activity was markedly stimulated by L-Arginine and inhibited by its product N-Acetylglutamate. Apparent kinetic properties were similar to those of the rat liver enzyme. This work provides the first evidence for the presence of N-Acetylglutamate synthetase activity in human.

Polyadenylate metabolism in the nuclei and cytoplasm of Saccharomyces cerevisiae.

A procedure has been designed for the simultaneous isolation, in a single step, of the nuclei and cytoplasm of Saccharomyces cerevisiae alphas288c spheroplasts. We have examined the polyadenylate poly(A)-containing RNA in these fractions and their kinetics of synthesis. Nuclear RNA saturates with [3H] adenine within 10 min. Labeled RNA appears very quickly in the cytoplasm, exceeding the amount of labeled nuclear RNA within 2 min after the addition of [3H] adenine. Nuclear poly(A)-containing RNA is approximately 10% of the total cellular poly(A)-containing RNA obtained from spheroplasts labeled for 15 min. Nuclear poly(A)-containing RNA is not as large as the giant heterogeneous nuclear RNA of animal cells. The distribution of molecular size in nuclear and cytoplasmic populations of poly(A)-containing RNA is very broad with the average size of the nuclear species being moderately larger than the cytoplasmic species. Three distinct size classes of poly(A), with different apparent kinetic properties, are obtained from yeast poly(A)-containing RNA. Their electrophoretic mobility suggests molecular lengths of approximately 20, 40, and 60 nucleotides (Groner, G., Hynes, N., and Phillips, S. (1975) Biochemistry, 13, 5378-5383). Each of these poly(A) classes are present in the mRNA from large and small polyribosomes.

Chemical fixation of chymotrypsin to water-insoluble crosslinked dextran (sephadex) and solubilization of the enzyme derivatives by means of dextranase.

AbstractCatalytically active chymotrypsin derivatives can be synthesized from cyanogen bromide‐activated Sephadex G200. In most cases the apparent catalytic activity of the covalently fixed enzyme appears to be considerably decreased in comparison to the activity of the free enzyme. However, by proper choice of the reaction conditions for the activation, enzyme conjugates with high activity, even toward a high molecular substrate, can be synthesized. These latter derivatives may be of practical value for the digestion of proteins. Crosslinked dextran as carrier was chosen because of the possbility, of digesting it enzymatically by dextranase. Sephadex G200, if activated at or below pH 10.3, will combine with chymotrypsin to yield digestable products. Changes of apparent kinetic properties of the fixed enzyme can accordingly be studied during the degradation process. On the solubilization of the insoluble conjugate, a total recovery of activity of the fixed enzyme can be obtained in cases the carrier has been activated by a sufficiently mild procedure. The high apparent Michaelis constant Km of insoluble chymotrypsin–Sephadex toward N‐acetyl‐L‐tyrosine ethyl ester shifts back on solubilization to the value of free chymotrypsin. We therefore propose that the decreased activity of an insoluble chymotrypsin–Sephadex is due to diffusional effects shown by the gel matrix toward the substrate. Similarly observed shifts in optimum pH are explained by accumulation of hydrogen ions in the gel. The organic chemical reaction used for coupling the enzyme to the polymer can therefore be performed without decreasing the inherent catalytic activity of the enzyme. The route described for fixing chymotrypsin to Sephadex followed by solubilization of the products may be useful as a synthetic method for binding proteins, peptides, and other amino group‐containing substances to soluble carriers, e.g., for the modification of pharmaceuticals.