Enzyme transfer model

This system displays a two-enzyme kinetic model in which bioconversion is controlled by the interaction between the two reactions and the mass transfer. This situation offers a more realistic model for the conditions occurring in vivo, in which some pathways of intermediary metabolism consist of linear sequences of reactions. These pathways take place in highly organized compartments. 

Figure 9.3 Schematic representation of possible reaction sites for PoHNL-catalyzed cleavage/synthesis of mandelonitrile (MN) (a) adsorbed enzyme model [24] and (b) mass transfer model [27]. BA benzaldehyde white dots enzyme.

In sulfite oxidase and related enzymes, transfer of the oxygen atom from Mo to substrate and its replacement from H2O both appear to occur in the Mo(VI) to Mo(IV) transformation regeneration of the Mo(VI) state is achieved via two one-electron oxidations which are mediated by the internal electron transfer chain and which generate transient Mo(V) states (Scheme 3) (7, 233, 234). A number of model catalytic systems have been reported in which O2 and/or H2O are key participants, but the role of water has not been clarified (196,198,201, 202, 235, 236). 

Arad et al. (1990) simulated the reaction sequence of papain by constructing several enzyme-substrate models with molecular mechanics and following reaction paths with semiempirical quantum mechanics. AMBER force field (Weiner et al., 1986a) was employed for the construction. AMI (Dewar et al, 1985) results for proton affinities of the modeled molecules were compared to 4-31G and to experiments. AMI underestimates the proton affinities of methanethiol and of imidazole but overestimates the proton affinity of methanol. However, the proton transfer reactions from methanol to imidazole and from methanethiol to imidazole are overestimated by only 6 and 11 kcal/mol, respectively, and PT from imidazolium to formamide is underestimated by 6 kcal/mol. 

Kinetic Aspects of Oxygen Transfer Reactions -Enzymes vs. Models... 

Mechanisms of Asymmetric Epoxidation Reactions 558 Nature s Hydride Reducing Agent 566 The Captodative Effect 573 Stereoelectronics in an Acyl Transfer Model 579 The Swern Oxidation 580 Gas Phase Eliminations 588 Using the Curtin-Hammett Principle 593 Aconitase—An Enzyme that Catalyzes Dehydration and Rehydration 595... 

Denys, S., van Loey, A.M., and Hendrickx, M.E. (2000) A modelling approach for evaluation process uniformity during batch high hydrostatic pressure processing combination of a numerical heat transfer model and enzyme inactivation kinetics. Innovative... 

With these assumptions in hand, interpretation of real assay data involves plotting a model-derived value for concentration of NAD at the enzyme surface (NAD ). The value for the Mnad+ can be fitted to allow the Lineweaver-Burk plot to intercept the x-axis at a value that yields the value of Km as determined in solution. The value for Vmax is then read as the intercept at the y-axis (Figure 12.2). This approach permits derivation of a Vmax for the electrode that is independent of the effects of mass transfer. If one further assumes that the immobilization process does not affect the turnover rate of the immobilized enzyme (relative to its activity in solution), then this value of Vmax (which represents the total activity of all bound enzyme) can also be used to estimate the amount of immobilized enzyme. This model can be particularly useful when fabricating electrodes using immobilization techniques that entrap a fraction of enzyme from bulk solutions, such as direct physical absorption or co-immobilization within gels. 

Calculation of Conformational Free Energies for a Model of a Bilobal Enzyme Protein kinases catalyze the transfer of phosphate from adenosine triphosphate (ATP) to protein substrates and are regulatory elements of most known pathways of signal transduction. 

This study is particularly noteworthy in the evolution of QM-MM studies of enzyme reactions in that a number of technical features have enhanced the accuracy of the technique. First, the authors explicitly optimized the semiempirical parameters for this specific reaction based on extensive studies of model reactions. This approach had also been used with considerable success in QM-MM simultation of the proton transfer between methanol and imidazole in solution. 

Enzymatic reactions frequently undergo a phenomenon referred to as substrate inhibition. Here, the reaction rate reaches a maximum and subsequently falls as shown in Eigure 11-lb. Enzymatic reactions can also exhibit substrate activation as depicted by the sigmoidal type rate dependence in Eigure 11-lc. Biochemical reactions are limited by mass transfer where a substrate has to cross cell walls. Enzymatic reactions that depend on temperature are modeled with the Arrhenius equation. Most enzymes deactivate rapidly at temperatures of 50°C-100°C, and deactivation is an irreversible process. 

Keller, H. J., and Soos,-Z. G. Solid Charge-Transfer Complexes of Phenazines. 127, 169-216 (1985). Kellogg, R. M. Bioorganic Modelling — Stereoselective Reactions with Chiral Neutral Ligand Complexes as Model Systems for Enzyme Catalysis. 101, 111-145 (1982). 

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