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Alberts B, Bray D, Lewis J, et al. Molecular Biology of the Cell. 3rd edition. New York: Garland Science; 1994.

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Cover of Molecular Biology of the Cell

Molecular Biology of the Cell. 3rd edition.

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Figure 3-9. The principle of equilibrium.

Figure 3-9The principle of equilibrium

The equilibrium between molecules A and B and the complex AB is maintained by a balance between the two opposing reactions shown in (1) and (2). As shown in (3), the ratio of the rate constants for the association and the dissociation reactions is equal to the equilibrium constant (K) for the reaction. Molecules A and B must collide in order to react, and the rate in reaction (2) is therefore proportional to the product of their individual concentrations. As a result, the product [A] x [B] appears in the final expression for K, where [ ] indicates concentration.

As traditionally defined, the concentrations of products appear in the numerator and the concentrations of reactants appear in the denominator of the equation for an equilibrium constant. Thus the equilibrium constant in (3) is that for the association reaction A + B → AB. For simple binding interactions this constant is called the affinity constant or association constant (in units of liters per mole); the larger the value of the association constant ( K a), the stronger is the binding between A and B. The reciprocal of K ais the dissociation constant (in units of moles per liter); the smaller the value of the dissociation constant ( K d), the stronger is the binding between A and B.

From: Molecular Recognition Processes

Copyright © 1994, Bruce Alberts, Dennis Bray, Julian Lewis, Martin Raff, Keith Roberts, and James D Watson.


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