# Avidity

In the context of biochemistry, avidity refers to the accumulated strength of multiple affinities of individual non-covalent binding interactions, such as between a protein receptor and its ligand, and is commonly referred to as functional affinity. As such, avidity is distinct from affinity, which describes the strength of a single interaction. However, because individual binding events increase the likelihood of other interactions to occur (i.e. increase the local concentration of each binding partner in proximity to the binding site), avidity should not be thought of as the mere sum of its constituent affinities but as the combined effect of all affinities participating in the biomolecular interaction. Biomolecules often form heterogenous complexes or homogenous oligomers and multimers or polymers. If clustered proteins form an organized matrix, such as the clathrin-coat, the interaction is a described as matricity.

## Antibody-antigen interaction

Avidity is commonly applied to antibody interactions in which multiple antigen-binding sites simultaneously interact with the target antigenic epitopes, often in multimerized structures. Individually, each binding interaction may be readily broken, however, when many binding interactions are present at the same time, transient unbinding of a single site does not allow the molecule to diffuse away, and binding of that weak interaction is likely to be restored.

Each antibody has at least two antigen-binding sites, therefore antibodies are bivalent to multivalent. Avidity (functional affinity) is the accumulated strength of multiple affinities.[1] For example IgM is said to have low affinity but high avidity because it has 10 weak binding sites for antigen as opposed to the 2 stronger binding sites of IgG, IgE and IgD with higher single binding affinities.

### Affinity

Binding affinity is a measure of dynamic equilibrium between the reciprocal on-rate (kon, calculated from Ka) and off-rate (koff, calculated from Kd) under specific concentrations of reactants in solution. There is always an inverse relationship between affinity and Kd. The strength of complex formation in solution is related to the stability constants of complexes, however in case of large biomolecules, such as receptor-ligand pairs, their interaction is also dependent on other structural and thermodynamic properties of reactants plus their orientation and immobilization.

There are multiple methods to investigate protein–protein interactions existing with differences in immobilization of each reactant in 2D or 3D orientation. The measured affinities are stored in public databases, such as the Ki Database and BindingDB. As an example, affinity is the binding strength between the complex structures of the epitope of antigenic determinant and paratope of antigen-binding site of an antibody. Participating non-covalent interactions may include hydrogen bonds, electrostatic bonds, van der Waals forces and hydrophobic forces.[2]

Calculation of binding affinity for bimolecular reaction (1 antibody binding site per 1 antigen):

$[Ab] + [Ag] \leftrightharpoons [AbAg]$

where [Ab] is the antibody concentration and [Ag] is the antigen concentration, either in free ([Ab],[Ag]) or bound ([AbAg]) state.

calculation of association constant: $K_a = \frac{k_{on}}{k_{off}} = \frac{[AbAg]}{[Ab][Ag]}$

calculation of dissociation constant: $K_d = \frac{k_{off}}{k_{on}} = \frac{[Ab][Ag]}{[AbAg]}$

where ratio of ka and kd describes the binding affinity:

$K = \frac{K_a}{K_d} = \frac{[AbAg]}{[Ab][Ag]}$

where K is the equilibrium constant.

## Application

Avidity test for rubella virus, Toxoplasma gondii, cytomegalovirus (CMV), varicella-zoster virus, human immunodeficiency virus (HIV), hepatitis viruses, Epstein-Barr virus, and others was developed few years ago. This test helps us to distinguish acute, recurrent or past infection by avidity of marker-specific IgG. Currently there are two avidity assay in use. These are well known chaotropic (conventional) assay and recently developed AVIcomp (avidity competition) assay.[3]