Agonist dissociation equilibrium constant

Suppose that the two sites are identical (an oversimplification) and that the binding of the first molecule of agonist does not affect the affinity of the site that remains vacant. The dissociation equilibrium constant for each site is denoted by KA and the equilibrium constant for the isomerization between A2R and A2R by E, so that [A2R ] = [A2R],... 

We now consider what would happen if the binding of the first molecule of agonist altered the affinity of the second identical site. The dissociation equilibrium constants for the first and second bindings will be denoted by KA(U and KM2), respectively, and E is defined as before. 

These results show that if the relationship between the concentration of an agonist and the proportion of receptors that it occupies is measured directly (e.g., using a radioligand binding method), the outcome should be a simple hyperbolic curve. Although the curve is describable by the Hill-Langmuir equation, the dissociation equilibrium constant for the binding will be not KA but Ke, which is determined by both E and KA. 

Method 3. This method is more general than the other two in the sense that it is also applicable to full agonists, at least in principle. Suppose that we had some reliable means of determining the dissociation equilibrium constant for the combination of the agonist with its receptors. One procedure that has been used in the past is Furchgott s irreversible antag-... 

Here, KA and KB are the dissociation equilibrium constants for the binding of agonist and antagonist, respectively. This is the Gaddum equation, named after J. H. Gaddum, who was the first to derive it in the context of competitive antagonism. Note that if [B] is set to zero, we have the Hill-Langmuir equation (Section 1.2.1). 

The existence of a receptor reserve in many tissues has the implication that the value of the ECjq for a full agonist cannot give even an approximate estimate of the dissociation equilibrium constant for the combination of the agonist with its binding sites as already mentioned, when the response is half maximal, only a small fraction of the receptors may be occupied rather than the... 

Can an Irreversible Competitive Antagonist Be Used to Find the Dissociation Equilibrium Constant for an Agonist ... 

The use of the Schild method for estimation of the dissociation equilibrium constant of a competitive antagonist is described in detail in Chapter 1. The great advantage of the Schild method lies in the fact that it is a null method agonist occupancy in the absence or presence of antagonist is assumed to be equal when responses in the absence or presence of the antagonist are equal. Even when the relationship between occupancy and response is complex, the Schild method has been found to work well. 

RGS proteins (see Abramow-Newerly et al. 2006 De Vries et al. 2000 Druey 2001 Neitzel and Hepler 2006 Tinker 2006 Willars 2006 for reviews) comprise a wide and rather diverse family, with 30+ members, but with a common 120-residue RGS domain that enables binding to the G-protein a-subunit. The effect of this is to lower the energy requirement for GTP hydrolysis and hence to accelerate the GTPase activity of the a-subunit. This is reflected in the faster offset of Ga or Gpy-mediated effector responses noted above. Because the effective dissociation equilibrium constant for G-protein-effector interaction (Kdlss) is determined by l+ff/kon x G (where G is the concentration of activated G-protein), RGS-accelerated GTPase activity reduces effector response to agonist, effectively... 

Fig. 6. A multistate model of receptor function with three states. The receptor population consists of an inactive receptor conformation (R) in equilibrium with two (or more) active receptor conformations (R and R ). Each active conformation can differentially activate effector mechanisms, leading to response 1 or response2 in the absence of an agonist. Two isomerization constants (L and M) define the propensity of the receptor to adopt an active conformation in the absence of a ligand. Agonists can differentially stabilize R vs R depending on the value of the equilibrium dissociation constants KA and KA relative to KA. Inverse agonists can also have differential effects on response 1 vs. response2 depending upon the relative values of L and M and of the affinity constants. Additional active states with additional isomerization and affinity constants can be added. Adapted from Leff et al. (86) and Berg et al. (22).

A measure of the tendency of a ligand and its receptor to bind to each other is expressed as K in receptor occupancy studies. K is the equilibrium constant for the two processes of dmg-receptor combination and dissociation. K may be found for both agonists and antagonists, although sometimes the former poses more technical challenge due to alterations to the conformation of the binding site. In contrast, efficacy is a relative measure,... 

The equilibrium dissociation constant of the agonist-receptor complex (Ka) can be obtained by a regression of 1/[A] upon 1/[A7]. This leads to a linear regression from which... 

Equation 6.19 predicts an increasing IC50 with either increases in L or 1. In systems with low-efficacy inverse agonists or in systems with low levels of constitutive activity, the observed location parameter is still a close estimate of the KB (equilibrium dissociation constant of the ligand-receptor complex, a molecular quantity that transcends test system type). In general, the observed potency of inverse agonists only defines the lower limit of affinity. 

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