Kinetic binding allosteric modulators

In general, the kinetics of most allosteric modulators have been shown to be faster than the kinetics of binding of the tracer ligand. This is an initial assumption for this experimental approach. Under these circumstances, the rate of dissociation of the tracer ligand (pA t) n the presence of the allosteric ligand is given by [11, 12]... 

The regulation of enzymes by metabolites leads to the concept of allostenc regulation. Allosteric means other structure. Allosteric modulators can bind at a site other than the active site in question and cause activation or inhibition. These modulators can include the substrate itself, which binds at another active site in a multi-subunit enzyme. In fact, allosterically modulated enzymes almost always have a complex quaternary structure (multiple subunits) and exhibit non-Michaelis-Menten kinetics. 

The fact that the aliosterically preferred conformation may be relatively rare in the library of conformations available to the receptor may have kinetic implications. Specifically, if the binding site for the modulator appears only when the preferred conformation is formed spontaneously, then complete conversion to alios terically modified receptor may require a relatively long period of equilibration. For example, the allosteric p38 MAP kinase inhibitor BIRB 796 binds to a conformation of MAP kinase requiring movement of a Phe residue by 10 angstroms (so-called out conformation). The association rate for this modulator is 8.5 x 105 M-1 s-1, 50 times slower than that required for other inhibitors (4.3 x 107 M 1 s-1). The result is that while other inhibitors reach equilibrium within 30 minutes, BIRB 376 requires 2 full hours of equilibration time [8],... 

The activity of allosteric enzymes is adjusted by reversible binding of a specific modulator to a regulatory site. Modulators may be the substrate itself or some other metabolite, and the effect of the modulator may be inhibitory or stimulatory. The kinetic behavior of allosteric enzymes reflects cooperative interactions among enzyme subunits. 

Some allosteric enzymes are also classified by the way in which they are affected by the binding of a modulator some affect the value of Km without affecting that of Vmtx- They are classed as K-series enzymes while others, which affect Fmax without affecting Km, are called M-series enzymes. Figure 5.35 shows the characteristic kinetic patterns observed for K-series and M-series enzymes. There are, of course, exceptions to these two extremes of kinetic behaviour. 

A change in the kinetics of an enzyme, resulting from the revernble alteration of its protein conformation by the binding of a modulating or allosteric effector (see Allostery, COvalent modification of enzymes). 

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