Affinity and avidity are an example of those pesky terms that seem interchangeable but in reality, are quite different. In this blog post we will be exploring the differences between them and hopefully dispelling that vagueness you feel next time you come across them in a paper.
What is binding affinity?
Let’s start off with defining affinity; the binding affinity of a protein-protein interaction can be described as the strength of the interaction between a receptor and its respective ligand. An example of this is an antibody’s epitope and an antigen. In this interaction the antigen binds to the epitope through a variety of different types of bonds such as Hydrogen bonds, ionic bonds, Van der Waals interactions and electrostatic forces.
More accurately, binding affinity is a measure of the dynamic equilibrium of the ratio of Kon (rate at which a complex forms) and Koff (rate at which a complex dissociates).
If we take the example of an antibody and an antigen the dynamic equilibrium would look like this:
[Ab] + [Ag] ⇌ [AbAg]
[Ab] = Concentration of Antibody
[Ag] = Concentration of Antigen
*Where 1 antibody binding site can bind 1 antigen
kon, therefore is the rate at which the antigen is binding to the antibody, whereas koff is the rate at which the antibody-antigen complex is dissociating. You can calculate how much of the antibody-antigen complex is present at the point of equilibrium. This is the association constant, Ka, and it can be calculated by dividing the kon by the koff.
Ka = kon / koff
Inversely, you can calculate the dissociation constant, KD, of this interaction by dividing the koff by the kon.
KD = koff / kon = [Ab] [Ag] / [AbAg]
So now that we know binding affinity is the strength of attraction between a receptor and its ligand, let’s look at how avidity is different from it.
What is avidity?
Avidity describes the measure of overall or accumulated strength of a protein-protein complex, i.e. The total strength of all the non-covalent interactions between an antibody binding to its ligand/s. It is determined by three parameters:
1. The binding affinity of the complex
2. The valency of the proteins
3. The structural arrangement of the proteins in the complex.
Let’s break it down and use the antibody-antigen complex as an example again. The avidity of an antibody-antigen complex would depend upon the affinity of the antibody’s epitope and the antigen as previously explained. The valency of both the antigen and antibody refers to how many binding sites an antibody or antigen may have. One example of this is how IgM antibodies have ten binding sites (decavalent) whereas IgE antibodies only have two binding sites (bivalent). Therefore, IgM antibodies have a greater valency to IgE antibodies.
And finally, the structural arrangement of the antibodies and antigens refers to any conformational changes in the tertiary structure of either protein that could help or limit interactions between the two proteins. So, if a non-competitive inhibitor binds to an antibody and changes its structure, it can no longer bind to an antigen and therefore the avidity of that interaction has decreased.
Overall this means that a protein that is multi-valent, with high binding affinity at each active site, that are also accessible to the ligand, will have high avidity. For example, an aggregate of IgM antibodies binding to antigens that are immobilized on a surface will have a high avidity, whereas freely floating IgE and antigens will have a lower avidity.
In conclusion – the binding affinity is the strength of an interaction between two molecules, whereas avidity is the total strength of all non-covalent interactions between the two proteins. When assessing a drug candidate, binding affinity is most relevant (how strongly does the drug interact with the target?) – this can be tested by techniques such as MDS, SPR or MST.
Avidity is mostly used to semi-quantitively characterize a potential antibody for an antigen epitope. A common method for the estimation of antibody avidity consists of the assessment of the stability of antigen–antibody complexes in the presence of chaotropic agents.