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In Chemistry and Biochemistry , 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 Affinity Constant . In the special case of Salt s, the dissociation constant can also be called an Ionization Constant . For a general reaction : in which a complex breaks down into ''x'' A subunits and ''y'' B subunits, the dissociation constant is defined : where [B , and [AxBy] are the concentrations of A, B, and the Complex AxBy, respectively. PROTEIN-LIGAND BINDING The dissociation constant is commonly used to describe the Affinity between a Ligand () (such as a Drug ) and a Protein ('''''') i.e. how tightly a Ligand binds to a particular Protein . Ligand-protein affinities are influenced by Non-covalent Intermolecular Interactions between the two molecules such as Hydrogen Bonding , Electrostatic Interactions , Hydrophobic and Van Der Waals Force s. The formation of a ligand-protein complex () can be described by a two-state process : the corresponding dissociation constant is defined : where ]," class="copylinks" target="_blank">['''''' and [''''''] represent the concentrations of the protein, ligand and complex, respectively. The dissociation constant has Molar units (M), which correspond to the concentration of ligand ]" class="copylinks" target="_blank">at which the binding site on a particular protein is half occupied, i.e. the concentration of ligand, at which the concentration of protein with ligand bound ['''''' , equals the concentration of protein with no ligand bound ['''''']. The smaller the dissociation constant, the more tightly bound the ligand is, or the higher the affinity between ligand and protein. For example, a ligand with a nanomolar (nM) dissociation constant binds more tightly to a particular protein than a ligand with a micromolar (M) dissociation constant. Sub-nanomolar dissociation constants as a result of non-covalent binding interactions between two molecules are rare. Nevertheless, there are some important exceptions. Biotin and Avidin bind with a dissociation constant of roughly M = 1 fM = 0.000001 nM.1 While Ribonuclease Inhibitor proteins may also bind to Ribonuclease with a similar M affinity.2 The dissociation constant for a particular ligand-protein interaction can change significantly with solution conditions (e.g. Temperature , PH and salt concentration). The effect of different solution conditions is to effectively modify the strength of any Intermolecular Interactions holding a particular ligand-protein complex together. Drugs can produce harmful side effects through interactions with proteins for which they were not meant to or designed to interact. Therefore much pharmaceutical research is aimed at designing drugs that bind to only their target proteins with high affinity (typically 0.1-10 nM) or at improving the affinity between a particular drug and its '' In-vivo '' protein target. ANOTHER NOTATION A dissociation constant is sometimes expressed by its p, which is defined as: : These p's are mainly used for Covalent dissociations (i.e., reactions in which chemical bonds are made or broken) since such dissociation constants can vary greatly. DISSOCIATION CONSTANT OF WATER As a frequently used special case, the dissociation constant of Water is often expressed as Kw: (The concentration of water is not included in the definition of , for reasons described in the article Equilibrium Constant . The value of Kw varies with temperature, as shown in the table below. This variation must be taken into account when making precise measurements of quantities such as PH . ACID BASE REACTIONS For the (-COOH) group, p''K''2 refers to its Amino (-NH3) group and the p''K''3 is the p''K'' value of its Side Chain . REFERENCES SEE ALSO |
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