(Science: chemistry) The ratio of the reverse and forward rate constants for a reaction of the type:
a _ B = ab
at equilibrium the equilibrium constant (K) equals the product of the concentrations of reactants divided by the concentration of product and has dimensions of concentration.
kd = (concentration a.concentration B) / (concentration ab).
The affinity constant (Ka) is the reciprocal of the equilibrium constant.
Dimension: moles per litre. In general the concept of kd is more readily understood than that of Ka, for example: in considering the conversion of a to ab by the binding of ligand B, the kd = B when a = ab. Thus kd is equal to the ligand concentration which produces half maximal conversion (response).