# Avidity

In 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. A particular important aspect relates to the phenomenon of 'avidity entropy'.[1] Biomolecules often form heterogenous complexes or homogeneous oligomers and multimers or polymers. If clustered proteins form an organized matrix, such as the clathrin-coat, the interaction is a described as matricity.[citation needed]

## 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.[citation needed]

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.[2] 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.[citation needed]

### Affinity

Binding affinity is a measure of dynamic equilibrium of the ratio of on-rate (kon) and off-rate (koff) under specific concentrations of reactants. The affinity constant, Ka, is the inverse of the dissociation constant, 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.[citation needed]

There are several 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.[citation needed] 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.[3]

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

${\displaystyle {\ce {[Ab] + [Ag] <=> [AbAg]}}}$

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 (or equilibrium constant):

${\displaystyle K_{a}={\frac {k_{{\ce {on}}}}{k_{{\ce {off}}}}}={\frac {{\ce {[AbAg]}}}{{\ce {[Ab][Ag]}}}}}$

calculation of dissociation constant:

${\displaystyle K_{d}={\frac {k_{{\ce {off}}}}{k_{{\ce {on}}}}}={\frac {{\ce {[Ab][Ag]}}}{{\ce {[AbAg]}}}}}$

## Application

Avidity tests for rubella virus, Toxoplasma gondii, cytomegalovirus (CMV), varicella-zoster virus, human immunodeficiency virus (HIV), hepatitis viruses, Epstein-Barr virus, and others were developed a few years ago. These tests help to distinguish acute, recurrent or past infection by avidity of marker-specific IgG. Currently there are two avidity assays in use. These are the well known chaotropic (conventional) assay and the recently developed AVIcomp (avidity competition) assay.[4]

A number of technologies exist to characterise the avidity of molecular interactions including switchSENSE and surface plasmon resonance.[5][6]

## References

1. ^ Kitov PI, Bundle DR (2003). "On the nature of the multivalency effect: a thermodynamic model". Journal of the American Chemical Society. 125 (52): 16271–84. doi:10.1021/ja038223n. PMID 14692768.
2. ^ Rudnick SI, Adams GP (2009). "Affinity and avidity in antibody-based tumor targeting". Cancer Biotherapy & Radiopharmaceuticals. 24 (2): 155–61. doi:10.1089/cbr.2009.0627. PMC . PMID 19409036.
3. ^ Janeway, Charles A; Travers, Paul; Walport, Mark; Shlomchik, Mark J (2001). Immunobiology (5th ed.). New York: Garland Science. p. 104. ISBN 0-8153-3642-X.[page needed]
4. ^ Curdt I, Praast G, Sickinger E, Schultess J, Herold I, Braun HB, Bernhardt S, Maine GT, Smith DD, Hsu S, Christ HM, Pucci D, Hausmann M, Herzogenrath J (2009). "Development of fully automated determination of marker-specific immunoglobulin G (IgG) avidity based on the avidity competition assay format: application for Abbott Architect cytomegalovirus and Toxo IgG Avidity assays". Journal of Clinical Microbiology. 47 (3): 603–13. doi:10.1128/JCM.01076-08. PMC . PMID 19129411.
5. ^ Gjelstrup LC, Kaspersen JD, Behrens MA, Pedersen JS, Thiel S, Kingshott P, Oliveira CL, Thielens NM, Vorup-Jensen T (2012). "The role of nanometer-scaled ligand patterns in polyvalent binding by large mannan-binding lectin oligomers". Journal of Immunology. 188 (3): 1292–306. doi:10.4049/jimmunol.1103012. PMID 22219330.
6. ^ Vorup-Jensen T (2012). "On the roles of polyvalent binding in immune recognition: perspectives in the nanoscience of immunology and the immune response to nanomedicines". Advanced Drug Delivery Reviews. 64 (15): 1759–81. doi:10.1016/j.addr.2012.06.003. PMID 22705545.