Curve binding

In order to elicit their effect, drug molecules must be bound to the cells of the effector organ. Binding commonly occurs at specific cell structures, namely, the receptors. The analysis of drug binding to receptors aims to determine the affinity of ligands, the kinetics of interaction, and the characteristics of the binding site itself. 

The law of mass action describes the hyperbolic relationship between binding (B) and ligand concentration (c). This relationship is characterized by the drug s affinity (1/I d) and the maximum binding (Bmax), i.e the total number of binding sites per unit of weight of membrane homogenate. 

Kd is the equilibrium dissociation constant and corresponds to that ligand concentration at which 50 % of binding sites are occupied. The values given in (A) and used for plotting the concentration-binding graph (B) result when Kd = 10. 

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In studying the af nity and number of such binding sites, use is made of membrane suspensions of different tissues. This approach is based on the expectation that binding sites will retain their characteristic properties during cell homogenization. 

Binding assays provide information about the af nity of ligands, but they do not give any clue as to whether a ligand is an agonist or antagonist (p. 60). 

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We can determine quantitatively the physiological significance of the sigmoid nature of the hemoglobin oxygen-binding curve, or, in other words, the biological importance of cooperativity. The equation... 

In addition to COg, Cl and BPG also bind better to deoxyhemoglobin than to oxyhemoglobin, causing a shift in equilibrium in favor of Og release. These various effects are demonstrated by the shift in the oxygen saturation curves for Hb in the presence of one or more of these substances (Figure 15.35). Note that the Og-binding curve for Hb + BPG + COg fits that of whole blood very well. 

The experimentally observed oxygen-binding curve for Hb does not fit the graph given in Figure A15.3 exactly. If we generalize Equation (A15.10) by replacing the exponent 4 by n, we can write the equation as... 

Archibald Hill demonstrated in 1913, well before any knowledge about the molecular organization of Hb existed, that the 02-binding behavior of Hb could be described by Equation (A15.12). If a value of 2.8 is taken for Equation (A15.12) fits the experimentally observed 02-binding curve for Hb very well (Figure A15.4). If the binding of O2 to Hb were an all-or-none phenomenon. 

FIGURE 4.3 Erroneous estimation of maximal binding with Scatchard plots. The saturation binding curve shown to the left has no data points available to estimate the true Bmax. The Scatchard transformation to the right linearizes the existing points, allowing an estimate of the maximum to be made from the x-axis intercept. However, this intercept in no way estimates the true Bmax since there are no data to define this parameter. 

COMPLEX BINDING PHENOMENA AGONIST AFFINITY FROM BINDING CURVES... 

FIGURE 4.15 Complex binding curves for agonists in G-protein unlimited receptor systems. 

Effect of excess receptor in binding experiments saturation binding curve (4.6.9)... 

Barlow method, 94f, 97, 260-261 from binding curves, 67-71 EC5o and, relationship between, 96-97 full, 91-95, 261... 

The 02-binding curve for myoglobin is hyperbolic, but for hemoglobin it is sigmoidal, a consequence of cooperative interactions in the tetramer. Cooperativ-ity maximizes the ability of hemoglobin both to load O2 at the PO2 of the lungs and to deliver O2 at the PO2 of the tissues. 

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