Both. The association constant and the dissociation constant are
reciprocal. It just depends on how you write the binding equilibrium.
For a bimolecular reaction under pseudo-first order conditions (i.e.
protein is vast excess to DNA) you can think of the dissociation
constant as the concentration needed to get half-maximal occupancy of
the site. Thus, lower Kd means it takes less to get the same amount of
binding - this is what one thinks of for higher affinity or stronger
Note, however, that while the qualitative idea is still OK, the idea of
half-maximal binding at Kd breaks down when there are more than 2
components in the reaction, since the units become M^2, M^3 etc.
Dept. of Biochemistry and Biophysics
Texas A&M Univ.
frankb_1003 at my-deja.com wrote:
> Hello everybody!
>> My problem is that I havent fully understood what
> the difference is between affinity and
> dissociation constant. When there are two DNA-
> binding proteins and under certain conditions the
> first one binds stronger to its binding site than
> the second one: is the affinity of the first
> protein higher or is the dissociation constant
> lower or both?
>> I would be thankful for some answers.
>> Sent via Deja.com http://www.deja.com/> Before you buy.