Michaelis Menten saturation

Figure 5b shows the resulting steady-state flux. / s (obtained as the positive solution for c f from equation (23)) for a range of c"M values. At low c"M values (usually associated with low values), there is a linear dependence between and r M, as expected from the linearisation of the Langmuir isotherms (see equation (31), below). At large c M values, the usual Michaelis-Menten saturating effect of is also seen. 

Another interesting example of C-C bond formation can be found in reports from Li and co-workers who, in 2004, described the preparation of an MIP with peroxidase-like activity capable of dimerising the homovanillic acid (HVA) (73) [49]. In this case a polymer was prepared by using the HVA substrate, instead of a TSA, as a template and a haemin unit as catalytic centre (74). The polymerisation was carried out in the presence of acrylamide and vinyl-pyridine in order to add extra functionalities aiding substrate recognition. The imprinted polymer showed an enzyme-like activity, as confirmed by adherence to the Michaelis-Menten saturation model, and it was inhibited by ferulic acid (75), a structural analogue of the substrate, which is also capable of inhibiting the natural peroxidase. 

Several reports have shown that the kinetics of P-gp transport activity can be sufficiently described by one-site Michaelis-Menten saturable kinetics (199-206). Where JP.g ) is the flux mediated by P-gp transport activity,, /max is the maximal flux mediated by P-gp transport activity, Km is the Michaelis-Menten constant, and Ct is the concentration of substrate present at the target (binding) site of P-gp. When donor concentration is used in place of Ct, apparent Km and Jmax values are obtained. Binding affinity of the substrate to P-gp and the catalytic (ATPase) activity of P-gp combine to determine Km, and, /max is determined by the catalytic (ATPase) activity of P-gp and the expression of P-gp in the system (concentration of P-gp protein). It has recently been noted that since substrates must first partition or cross the membrane to access the binding site, accurate assessing of P-gp kinetics can be difficult (207). Furthermore, the requirement of first partitioning into the membrane has been shown to produce asymmetric apparent kinetics in polarized cells where AP and BL membrane compositions may be sufficiently different (206). 

In the kinetic considerations discussed above, a plot of 1 /V0 vs 1/[S0] yields a straight line, and the enzyme exhibits Michaelis-Menten (hyperbolic or saturation) kinetics. It is implicit in this result that all the enzyme-binding sites have the same affinity for the substrate and operate independently of each other. However, many enzymes exist as oligomers containing subunits or domains that function in the regulation of the catalytic site. Such enzymes do not exhibit classic Michaelis-Menten saturation kinetics. 

Can a phosphorylation-dephosphorylation switch be more sensitive to the level of kinase concentration than n = 1 as given in Equation 5.12 We note that the kinetic scheme in Equation (4.7) is obtained under the assumption of no Michaelis-Menten saturation. Since this assumption may not be realistic, let us move on to study the enzyme kinetics in Figure (5.2) in terms of saturable Michaelis-Menten kinetics. The mechanism by which saturating kinetics of the kinase and phosphatase leads to sensitive switch-like behavior is illustrated in Figure 5.4. The reaction fluxes as a function of / (the ratio [S ]/Sc) for two cases are plotted. The first case (switch off)... 

Figure 8.3. Rate as function of reactant concentration in reaction with Michaelis-Menten saturation kinetics (schematic).

Michaelis-Menten saturation kinetics should occur only at low substrate concentrations, near 10 M. Using a spectrophotometric assay it is possible to observe statistically significant deviations as shown by the Line-weaver-Burke and Eadie plots in Figure 14 using DOPA as the substrate in the catecholase reaction. 

The amount of verapamil presented to the liver, and its effective concentration in the region of the hepatic er zymes soon after oral dosing, are related to the rate at which verapamO is absorbed from the gastrointestinal tract into the portal vein and to the flow rate of blood in the portal vein to the liver. For instance, by hypothesizing a Michaelis-Menten metabolic process, when the absorption rate is slow and concentrations in the portal vein and liver are low, the hepatic metabolism of both enantiomers will be approximately first-order. Under these conditions, the K S ratio of the umnetabolized enantiomers leaving the liver will be closely related to the ratio of the Michaelis-Menten saturation constants (K ) for the enantiomers. The observed more rapid metabolism of S-verapamil than R-verapamil (i.e., S-verapamil has the lower systemic concentrations) is consistent with the lower reported for S-verapamil (16). 

Substrate dependence was measured at the pH with highest activity for each complex and followed Michaelis-Menten saturation behavior (Fig. 6.28 Table 6.9). Complex dependence was linear from 0 to 0.12 mM [complex]. The complexes are among the most active compounds known to date for the cleavage of BDNPP [32]. 

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