Dissociation Constant

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Dissociation Constant
Dissociation Constant
In chemistry, biochemistry, and pharmacology, a dissociation constant is a specific type of
equilibrium constant that measures the propensity of a larger object to separate (dissociate)
reversibly into smaller components, as when a complex falls apart into its component
molecules, or when a salt splits up into its component ions.
The dissociation constant is usually denoted and is the inverse of the association constant. In
the special case of salts, the dissociation constant can also be cal ed an ionization constant.
One reason for the popularity of the dissociation constant in biochemistry and pharmacology
is that in the frequently encountered case where x=y=1, Kd has a simple physical
interpretation: when [A]=Kd, [B]=[AB] or equivalently [AB]/([B]+[AB])=1/2.
That is, Kd, which has the dimensions of concentration, equals the concentration of free A at
which half of the total molecules of B are associated with A.
This simple interpretation does not apply for higher values of x or y.
Affinity Constant
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This constant is the reverse of dissociation constant. This is a kind of mathematical constant
that describes the bonding affinity between two molecules at equilibrium.
For example, the affinity measure of an enzyme for its substrate.
This is an association constant which is used especially in relation to the binding of
macromolecules like in antigen-antibody, hormone-receptor, and enzyme-inhibitor reactions.
For example, the measure of the binding strength in hapten-antibody interaction.
Protein Ligand Binding
Ligands have ability to bind on the particular site of protein by the intermolecular forces such
as ionic bonds, hydrogen bonds and van der Walls forces.
When Ligand binds to a receptor protein, it changes the chemical conformation of receptor.
The tendency or strength of binding is called affinity.
The dissociation constant is usually brought into use to describe the accordance between a
ligand (L) and a protein (P) or it might also exhibit the extent of binding a ligand to a particular
site of protein.
This affinity between ligand and protein is affected by intermolecular interactions, non-
covalent in nature between ligand and protein. These are basically hydrogen bonding,
hydrophobic, electrostatic interactions, and Van der Wal s forces.
Where [P], [L] and [C] represent the protein's, ligand's and complex's molar concentration
respectively.
Now [C] shows the Ligand concentration at which the Protein and Ligand concentration are
bound, while [L] is for the concentration of ligand at which the concentration of protein with
ligand bound and lastly [P] is equivalent to the concentration of protein with no ligand bound.
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At equilibrium, [L] and [C] are equal to the [P]. The smal value of the dissociation constant
shows the good affinity between ligand and protein or that the ligands are tightly bound with
protein.
For example- A ligand with nano molar dissociation constant ((nM) ) exhibits a good affinity to
a particular protein than a ligand with micro molar ( M) dissociation constant.
Binding interactions of non-covalent nature between two molecules are rare because of Sub-
Pico molar dissociation constants but there are some exceptions.
For example- Biotin and avidin can have a binding with a range of 10-15M = 1fM =
0.000001nM dissociation constant. Similarly, ribonuclease inhibitor proteins may also bind to
ribonuclease with a similar 10-15M affinity.


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