Binding constant

Short description: Chemical property

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The binding constant, or affinity constant/association constant, is a special case of the equilibrium constant K,[citation needed] and is the inverse of the dissociation constant.[1] It is associated with the binding and unbinding reaction of molecules. Consider a binding reaction between two molecules, X and Y, which is formalized as:

X + Y ⇌ XY

The reaction is characterized by the on-rate constant kon and the off-rate constant koff, which have units of M−1 s−1 and s−1, respectively. In equilibrium, the forward binding transition X + Y → XY should be balanced by the backward unbinding transition XY → X + Y. That is,

kon[X][Y]=koff[XY]

where [X] and [Y] represent the concentration of unbound molecules, while [XU] represents the concentration of bound complexes. The binding constant Ka is defined by

Ka=konkoff=[XY][X][Y].[2]

An often considered quantity is the dissociation constant Kd ≡ ⁠1/Ka⁠, which has the unit of concentration, despite the fact that strictly speaking, all association constants are unitless values. The inclusion of units arises from the simplification that such constants are calculated solely from concentrations, which is not the case. Once chemical activity is factored into the correct form of the equation, a dimensionless value is obtained. For the binding of receptor and ligand molecules in solution, the molar Gibbs free energy ΔG, or the binding affinity is related to the dissociation constant Kd via

ΔG=RTln⁡Kdc⊖,

in which R is the ideal gas constant, T temperature and the standard reference concentration c ~~o~~ = 1 mol/L.

## See also

* Binding coefficient

## References

1. ↑ Yartsev, Alex. "Affinity, association constant and dissociation constant | Deranged Physiology" (in en). https://derangedphysiology.com/main/cicm-primary-exam/required-reading/pharmacodynamics/Chapter 121/affinity-association-constant-and-dissociation-constant. [self-published source?]
2. ↑ "The Equilibrium Binding Constant and Binding Strength of Proteins (Video) | JoVE" (in en). https://www.jove.com/science-education/11513/the-equilibrium-binding-constant-and-binding-strength-of-proteins. [unreliable source?]

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Chemical equilibria

Concepts|

* Acid dissociation constant
* Binding constant
* Binding selectivity
* Buffer solution
* Chemical equilibrium
* Chemical stability
* Chelation
* Determination of equilibrium constants
* Dissociation constant
* Distribution coefficient
* Dynamic equilibrium
* Equilibrium chemistry
* Equilibrium constant
* Equilibrium unfolding
* Equilibrium stage
* Hammett acidity function
* Henry's law
* Liquid–liquid extraction
* Macrocycle effect
* Phase diagram
* Predominance diagram
* Phase rule
* Phase separation
* Reaction quotient
* Self-ionization of water
* Solubility equilibrium
* Stability constants of complexes
* Thermodynamic equilibrium
* Thermodynamic activity
* Vapor–liquid equilibrium

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