on- and off-rate of antibodies

ChenHA hzhen at freeuk.com
Tue Feb 1 22:04:51 EST 2000

Esbjorn Fiers wrote:

> Dear all,
> I am working with scFv phage display libraries. In some articles I come
> across the words on- and off-rate, being the rate at which an antibody
> binds its antigen and the rate at which an antibody dissociates from its
> antigen. Furthermore, they say that when antibody affinities become
> higher this is mostly due to a lower off-rate, with the on-rate being
> less important. I kind of have problems to completely understand this.
> Is there anybody who could explain this to me, or has anyone a reference
> in which this is clearly explained?

This is something that can be found in most textbooks on proteins, such as
the one by Creighton or the one by Fersht.  It would be best for you to look
into those books for detailed descriptions.  I would only give a very brief

The affinity or association constant (Ka) is determine by two rates - the
on-rate (or how fast something binds - k1 or kon or ka) and the off-rate (or
how fast something falls off - k-1 or koff or kd).  The affinity Ka (or its
reciprocal the dissociation constant, Kd) reflects the overall
protein-protein interaction and shows how favourable the interaction is.
High affinity interaction means that two proteins are strongly bound
together and low affinity interaction means two proteins only interact
loosely.  However, for some biological systems, it is often more useful to
know the on-rate and off-rate instead of just the affinity because you know
what it going on more clearly.  If an antibody has a low off-rate, then
obviously this would increase the affinity because Ka = kon/koff.  I don't
know much about the kinetics of antibody-antigen interaction, but I would
assume what the writer means is that the on-rate (kon) for most
antibody-antigen are fairly similar, therefore is not the main determinant
for the varying affinities observed for different antibody-antigen
interactions.   But don't trust me.  Read the books.


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