Conformational change

After binding of the analyte to the ligand, the whole complex is rearranging itself following first order kinetics (1). The rate of changing of the conformation is assumed equal in both directions and before the analyte can dissociate, it must change to the first binding state (2).

It is not immediately apparent that a conformational change can have an effect on the response measured because it is mass-based and with a conformational change, no change in mass occurs. The effect is indirect because the second conformation alters the equilibrium between bound and free forms of the analyte. The second apparent rate constant increases with increasing analyte concentration (3). Post binding conformational changes are believed to be a feature of some receptor-hormone and antibody-antigen interaction (4),(5). All conformational change interactions should be confirmed with additional experiments such as ultracentrifugation, circular dichroisme or NMR (6).

In the article of Rich and Myszka (7) there is a myth busting section with several myths about biosensors. One of the myths concerns the conformational change model. The authors conclude that there are no plausible biological interactions that justify the use of this model. In addition, since the relevant time scale for conformational change is normally in seconds or less, the Biacore (probably most SPR machines) can not be useful, since these can only monitor conformational changes in minute time scale (6).

Reaction equation

Differential equation

Below an example of a sensorgram generated with the differential equations from above.