Catalytic parameters

It is important to compare the catalytic properties of Prussian blue with known hydrogen peroxide transducers. Table 13.2 presents the catalytic parameters, which are of major importance for analytical chemistry selectivity and catalytic activity. It is seen that platinum, which is still considered as the universal transducer, possesses rather low catalytic activity in both H202 oxidation and reduction. Moreover, it is nearly impossible to measure hydrogen peroxide by its reduction on platinum, because the rate of oxygen reduction is ten times higher. The situation is drastically improved in case of enzyme peroxidase electrodes. However, the absolute records of both catalytic activity... 

A priori, the pH effect on Ucat may derive from the pH dependence of both the ATm(pII) and the kcat(pH). Figure 6 displays the variation of the catalytic parameters Km and kcat as a function of pH. As seen from Figure 6, at pH > 9 the fccat is independent of pH, whereas... 

The simple theory outlined above has to be modified to account for the pH dependence of the catalytic parameters in mechanisms more complicated than the basic Michaelis-Menten. 

Enzyme-catalyzed reactions involve specific, rapid combination of substrate and enzyme to form a complex that is rapidly converted to products through transition states that are controlled by the enzyme s environment. Since enzymes are homogeneous chemical catalysts, we expect them to operate by routes that parallel some of the same processes in reactions that do not involve enzymes. The relative magnitude of enzymic and nonenzymic catalytic parameters has been called catalytic proficiency by Wolfenden6,17 24 and this has been a subject of intense current interest.7,25 32 Wolfenden noted that while nonenzymic reactions have diverse rates, enzyme-catalyzed processes are highly evolved to be comparable in rate, no matter how slow their nonenzymic counterparts. 

One may agree with this definition, if the main task of the simulation is that it approaches as closely as possible the simulated object in physicochemical characteristics. In this case, of course, if corresponding models of every catalytic parameter are obtained, it is possible not only to estimate their relative importance, but by combining them, also to update the model structure in order to approach the efficiency of the simulated enzyme. 

Table 9.3 The Ka values for monovalent cations and the effect of each cation on the catalytic parameters of tryptophanase37 ...

Note The kinetic constant for activation with each cation and the catalytic parameters for S-o-nitrophenyl-L-cysteine (SOPC) (0.6 mM) were determined from double reciprocal plots at 25° C and pH 8.0. 

For the purpose of this chapter, we define an immobilized enzyme as a composite consisting of two essential components the noncatalytic structural component (carrier) and the catalytic functional component, the enzyme. Therefore, an immobilized enzyme has to be characterized by two sets of variables, the noncatalytic and the catalytic parameters [87]. 

Considering the catalytic parameters, which were acquired simultaneously with the various spectra, the data show that propane conversion became measurable at 523 K and increased with increasing temperature until it remained almost constant at temperatures > 623 K (Fig. 18B). During this treatment, virtually all the monomeric and polymeric O = surface species and the major part of the V2O5... 

A question of particular interest concerns the catalytic advantage that a selenol in the active site of an enzyme has in comparison to a thiol group. It has been approached by replacing the selenocysteine in the formate dehydrogenase Ft by a cysteine residue and by determining the catalytic parameters of the Se-wild-type species with the mutant S-species. Table 1 presents the data obtained. They show that koit of the Se enzyme is more than 300-fold higher than that of the S variant and that the affinity to the substrate is only marginally affected. Similar replacements of selenocysteine by cysteine in other enzymes like the deiodinase type 1 or methionine sulfoxide reductase B corroborated these results. 

Table 1 Comparison of catalytic parameters of formate dehydrogenases...

PTE was also entrapped in additives for latex coating of biodefensive surfaces. Such PTE-based additives for paints and coatings were shown to retain catalytic parameters and stability of the enzyme (McDaniel et al, 2006). For decontamination of OPs in the environment and remediation, an alternative approach, phytodegradation by transgenic plants (e.g. tobacco) expressing a bacterial PTE, has been considered as a potentially low-cost, effective, and environmentally friendly method (Wang et al, 2008). 

The kinetic and catalytic parameters of the reaction involved as well as the design and operating variables of the membrane reactor all play important roles in the reactor performance. They will be treated in some detail in this chapter. 

Pyrococcus furiosus. It was subjected to standard biochemical analysis to determine basic catalytic parameters. The processes under study were of the form ... 

Significant alterations in the chemisorptive and catalytic parameters of supported metal nanoparticles are observed upon doping semiconductive carriers with cations of valence different than that of the host cations. These alterations may be... 

An alternative interpretation of the phenomenon of metal-support interactions induced by doping of semiconductive carriers with aliovalent cations is based on the theory of electrochemical promotion or the NEMCA effect. According to this interpretation, the charge carriers transported from the carrier to the metal particles are oxygen ions, which diffuse to the surface of the metal particles, thus altering the surface work function and, subsequently, chemisorptive and catalytic parameters. Work is currently in progress to elucidate the mechanism of induction of metal-support interactions by carrier doping. 

While this initial report clearly indicates the phase transfer catalytic properties of the silacrowns in both soluble and immobilized forms, much information is required to define their catalytic parameters and synthetic limitations. Current information regarding optimum concentrations, temperatures and times for conversions, determination of stability constants and structural conformation is incomplete. Long term hydrolytic stability in particular, under basic conditions may be a serious limiting factor. 

Magnetic particles coated with copolymer of acrylamide and acrylonitriles were successfully obtained. The peroxidase immobilized on these particles preserves its high relative activity. It was established that the catalytic parameters of the immobilized peroxidase change as follows t, - 30°C, pH - 7 n Km - 2.3 x 10 . ... 

Electronic properties are of particular interest in SMRs because they control the cleavage and formation of covalent bonds characteristic of a biotransformation reaction (i.e., the catalytic step). Correlations between electronic parameters and catalytic parameters obtained from in vitro studies (e.g., or cat) allow a rationalization of substrate selectivity and some insight into reaction mechanism. 

An example of a quantitative SMR study correlating electronic properties and catalytic parameters is provided by the glutathione conjugation of para-substituted l-chloro-2-nitro-benzene derivatives (183). The values of log/j2 (second order rate constant of the nonenzy-matic reaction) and log (enzymatic reaction catalyzed by various glutathione transferase preparations) were correlated with the Hammett resonance cr value of the substrates, a measure of their electrophilicity. Regression equations with positive slopes and values in the range 0.88-0.98 were obtained. These results quantitate the influence of substrate electrophilicity on nucleophilic substitutions mediated by glutathione, be they enzymatic or nonenzymatic. 

Weisfield, L.B. "The Estimation of Catalytic Parameters of Metal Acetylacetonates in Isocyanate Polymerization Reaction." Journal of Applied Polymer Science 1961, vol. 5, pp. 424-427. 

Catalytic. Typically the effects of the cryosolvent on the catalytic parameters may be determined by examining the effect of increasing concentration of the cosolvent on kinetic parameters, such as kcat, Km, and rate constants corresponding to more elementary steps in the overall reaction such as kacyiation, kdegaiactosyintion, as well as inhibition constants (e.g., Ki) and pH-rate profiles. In general, only the expected effects on... 

Substitution of the active site Zn(II) of LADH by Co(II), Ni(II), Cu(II), or Fe(II) introduces spectroscopic probes, which are sensitive to the coordination geometry, the electrostatic environment, and the protein conformation. UV-visible absorbance signatures consisting of d- d and ligand-to-metal charge transfer (LMCT) transitions have proven to be useful probes of the site environment (3, 24-26, 29, 34, 35). The Co(II) and Ni(II) derivatives exhibit catalytic parameters (i.e., hydride transfer rates, Km and lq values) that are similar to those of the native Zn(II)-enzyme (25). The Cu(II) and Fe(II) enzymes show reduced reactivities and catalytic parameters, which indicate these derivatives are of limited use as analogues of the native enzyme (24). 

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