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|  | The Equilibrium Constant Is a Measure of the Strength of an Interaction Between Two Molecules5
The precise strength of the bonding between two molecules is a useful index of the specificity of their interaction. To illustrate how the binding strength is measured, let us consider a reaction in which molecule A binds to molecule B. The reaction will proceed until it reaches an equilibrium point, at which the rates of formation and dissociation are equal (Figure 3-9). The concentrations of A, B, and the complex AB at this point can be used to determine an equilibrium constant (K) for the reaction, as explained in Figure 3-9. This constant is sometimes termed the affinity constant and is commonly employed as a measure of the strength of binding between two molecules: the stronger the binding, the larger is the value of the affinity constant.
The equilibrium constant of a reaction in which two molecules bind to each other is related directly to the standard free-energy change for the binding (DG°) by the equation described in Table 3-3. The table also lists the delta-G° values corresponding to a range of K values. Affinity constants for simple binding interactions in biological systems often range between 103 and 1012 liters/mole; this corresponds to binding energies in the range 4-17 kcal/mole, which could arise from 4 to 17 average hydrogen bonds.
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