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Deactivation, enzyme catalysis

The latter facilitates conformational movements during catalysis (such as the induced fit , see below) thereby underlining the pronounced dynamic character of enzyme catalysis. Besides the main polyamide backbone, the only covalent bonds are -S-S- disulfide bridges. Enzymes are intrinsically unstable in solution and can be deactivated by denaturation, caused by increased temperature, extreme pH, or an unfavorable dielectric environment such as high salt concentrations. [Pg.12]

Specific detail on Michaelis-Menten kinetics, quasi steady-state approximations, competitive and non-competitive inhibitions, substrate inhibition, rate expressions for enzyme catalysis and deactivations, Monod growth kinetics, etc. are not presented in an extensive manner although additional information is available in the work of Vasudevan for the interested leader. " Also note that the notation adopted by Vasudevan is employed throughout this chapter. [Pg.466]

Operation in biphasic mixtures using water-immiscible solvents introduces a linked equilibrium in the partition of educt and product and possible transport limitations at the interface, which have to be considered. Besides, enzyme deactivation at the interface and possible effects of the residual solvent solubility in aqueous buffers on enzyme stability have to be checked. Table 3 summarizes some data on stability of ADHs dissolved in aqueous buffers in a biphasic mixture with organic solvents [48]. Two different reactor concepts for continuous operation and enzyme catalysis in homogeneous phase have been studied—a bimembrane reactor [13,14] and an emulsion reactor [49]—which are discussed below with regard to reaction engineering. Using water-inuniscible solvents one can make use of the fact that NAD(P)/NAD(P)H are charged molecules and practically insoluble in apolar solvents. The coenzyme introduced in the reaction is therefore confined and physically immobilized with the enzymes in the aqueous phase. This facilitates efficient use of the coenzyme, especially if the volume fraction of the aqueous phase is kept low [13]. [Pg.848]

In a classic study on bovine pancreatic ribonuclease A at 90°C and pH conditions relevant for catalysis, irreversible deactivation behavior was found to be a function of pH (Zale, 1986) at pH 4, enzyme inactivation is caused mainly by hydrolysis of peptide bonds at aspartic acid residues as well as deamidation of asparagine and/or glutamine residues, whereas at pH 6-8, enzyme inactivation is caused mainly by thiol-disulfide interchange but also by fi-elimination of cystine residues, and deamidation of asparagine and/or glutamine residues. [Pg.502]

Enzymes are proteins that catalyze reactions. Thousands of enzymes have been classified and there is no clear limit as to the number that exists in nature or that can be created artificially. Enzymes have one or more catalytic sites that are similar in principle to the active sites on a solid catalyst that are discussed in Chapter 10, but there are major differences in the nature of the sites and in the nature of the reactions they catalyze. Mass transport to the active site of an enzyme is usually done in the liquid phase. Reaction rates in moles per volume per time are several orders of magnitude lower than rates typical of solid-catalyzed gas reactions. Optimal temperatures for enzymatic reactions span the range typical of living organisms, from about 4°C for cold-water fish, to about 40°C for birds and mammals, to over 100°C for thermophilic bacteria. Enzymatic reactions require very specific molecular orientations before they can proceed. As compensation for the lower reaction rates, enzymatic reactions are highly selective. They often require specific stereoisomers as the reactant (termed the substrate in the jargon of biochemistry) and can generate stereospecific products. Enzymes are subject to inhibition and deactivation like other forms of catalysis. [Pg.436]

SC-CO2 is also becoming increasingly important as reaction media [7] for a great variety of fundamental chemical reactions ranging from catalysis to polymerization, [8,9] to synthesis and growth of inorganic materials [1,2], to nanoparticle production and preparation processes [1,2,10,11], and to biotechnological applications such as activation and deactivation of enzymes [12], biomass conversion, and biocatalysis [1,2,13],... [Pg.434]

In pursuit of biomimetic catalysts, metaUoporphyrins have been extensively studied in attempts to mimic the active site of cytochrome P450, which is an enzyme that catalyzes oxidation reactions in organisms. In recent decades, catalysis of alkene epoxidation with metaUoporphyrins has received considerable attention. It has been found that iron [1-3], manganese [4,5], chromium [6], and cobalt porphyrins can be used as model compounds for the active site of cytochrome P450, and oxidants such as iodosylbenzene, sodium hypochlorite [7,8], hydrogen peroxide [9], and peracetic acid [10] have been shown to work for these systems at ambient temperature and pressure. While researchers have learned a great deal about these catalysts, several practical issues limit their applicability, especially deactivation. [Pg.472]

The majority of catalysts are subject to deactivation, e.g. to changes (deterioration) of activity with operation time. The time scale of deactivation depends on the type of process and can vary from a few seconds, as in fluid catalytic cracking (FCC), to several years, as in, for instance, ammonia synthesis. Due to the industrial importance, the modelling of deactivation was mainly developed for heterogeneous catalysis. Although the reasons for deactivation (inactivation) of homogeneous and enzymes could differ from solid catalysts, the mathematical approach can sometimes be very similar. [Pg.317]


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See also in sourсe #XX -- [ Pg.434 , Pg.438 ]




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