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Affinity or dissociation constant?

Jim Hu jimhu at tamu.edu
Fri Jun 2 18:51:22 EST 2000

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.

Jim Hu
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.
> Frank
> Sent via Deja.com http://www.deja.com/
> Before you buy.

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