Two-substrate model

Korzekwa, K.R., Krishnamachary, N., Shou, M., Ogai, A., Parise, R.A., Rettie, A.E., Gonzalez, F.J. and Tracy, T.S. (1998) Evaluation of atypical cytochrome P450 kinetics with two-substrate models evidence that multiple substrates can simultaneously bind to cytochrome P450 active sites. Biochemistry, 37 (12), 4137-4147. 

The full kinetic scheme for the two-substrate model is given in Figure 3. If product release is fast relative to the oxidation rates, the velocity equation is simplified to Eq. (10) ... 

A second type of nonhyperbolic saturation kinetics became apparent during studies on the metabolism of naproxen to desmethylnaproxen (32). Studies with human liver microsomes showed that naproxen metabolism has biphasic kinetics and is activated by dapsone (T. Tracy, unpublished results). The unactivated data shows what appears to be a typical concentration profile for metabolism by at least two different enzymes. However, a similar biphasic profile was obtained with expressed enzyme (25). This biphasic kinetic profile is observed with the two-substrate model when V/rn2 > Eml and Kml Km2. The appropriate equation for the two-site model when [S] < Kml is... 

Korzekwa, K.R., et al. Evaluation of atypical cytochrome P450 kinetics with two-substrate models Evidence that multiple substrates can simultaneously bind to cytochrome P450. Biochemistry 1998, 37, 4137-4147. 

Gonzalez, Biochemistry, 37,4137 (1998). Evaluation of Atypical Cytochrome P450 Kinetics with Two-Substrate-Models Evidence that Multiple Substrates Can Simultaneously Bind to Cytochrome P450 Active Sites. 

Even the simplest detersive system is surprisingly complex and heterogeneous. It can nevertheless be conceptually resolved into simpler systems that are amenable to theoretical treatment and understanding. These simpler systems are represented by models for substrate-soHd soil and substrate-Hquid sod. In practice, many sod systems include soH—Hquid mixtures. However, removal of these systems can generally be analyzed in terms of the two simpler model systems. Although these two systems differ markedly in behavior and stmcture, and require separate treatment, there are certain overriding principles that apply to both. 

Fig. 7.6. Topographical model of the active site of pig liver esterase showing the catalytic OH group and two binding sites (1 and 2) capable of accommodating hydrophobic groups of the substrate. Binding to site 1 is stronger and, thus, dominates until the steric dimensions of the site are exceeded. The model shows two substrates in position, namely dimethyl 3-methylglu-tarate (top) and dimethyl 3-benzylglutarate (bottom), which are hydrolyzed preferentially to the (R)- and (5)-monoester, respectively [68].

Resistance to hydration was elucidated with tricyclic model compounds that lack the side chain and, hence, pharmacological activity. In this context, a useful comparison has been made between two meso compounds, namely 5W-dibenz.oja, dIcycloheplene 10,11-oxide (10.130, X = CH2) and d.v-slilbcnc oxide (10.7) [195]. The former compound proved to be a very poor substrate for rabbit liver microsomal EH, with a Km value comparable to that of cis-stilbene oxide, but Emax ca. 100-fold lower. This indicates that the two compounds have a comparable affinity for the enzyme, but that nucleophilic attack in the catalytic step is much less efficient for dibcnzo[ // cycloheplcnc 10,11-oxide than for d.v-slilbcnc oxide. This implies that the former compound acts better as an inhibitor than as a substrate of microsomal EH. Furthermore, there was also a fundamental steric difference in the reaction course of the two substrates, since the predominant stereoisomer formed from dibenzo //]cyclohep(ene 10,11-oxide had the (I OS, 11. -configuration,... 

A two-variable model taking into account the allosteric (i.e. cooperative) nature of the enzyme and the autocatalytic regulation exerted by the product shows the occurrence of sustained oscillations. Beyond a critical parameter value, the steady state admitted by the system becomes unstable and the system evolves toward a stable limit cycle corresponding to periodic behavior. The model accounts for most experimental data, particularly the existence of a domain of substrate injection rates producing sustained oscillations, bounded by two critical values of this control parameter, and the decrease in period observed when the substrate input rate increases [31, 45, 46]. 

The interaction of two substrates, the bond strength of adhesives are frequently measured by the peel test [76]. The results can often be related to the reversible work of adhesion. Due to its physical nature such a measurement is impossible to carry out for particulate filled polymers. Even interfacial shear strength widely applied for the characterization of matrix/fiber adhesion cannot be used in particulate filled polymers. Interfacial adhesion of the components is usually deduced indirectly from the mechanical properties of composites with the help of models describing composition dependence. Such models must also take into account interfacial interactions. 

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