Affinity (biochemistry)

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The affinity (also binding affinity ) is in biochemistry a measure of the tendency of molecules to enter into a bond with other molecules, e.g. B. between the binding partners in protein - ligand interactions: the higher the affinity, the greater the association constant, K a (also known as the binding constant).


Today, however, the reciprocal measure, the dissociation constant , K d is commonly used in place of the association constant : the higher the affinity of a protein for its ligand, the lower the dissociation constant of the complex. Using the example of the formation / disintegration of an enzyme-substrate complex, [ES]

Enzyme-substrate interaction

The definitions according to the law of mass action or kinetic constants (rate constants k) are listed here:

Association constant
(binding constant)
Dissociation constant .

In enzyme kinetics, K m , the Michaelis constant , is occasionally given as a measure of the affinity of an enzyme for its substrate. This has a certain justification in practice, but theoretically the dissociation constant of the ES complex K d and the Michaelis constant must be strictly distinguished. According to Briggs and Haldane, the Michaelis constant also depends on the turnover number k 2 and is calculated as follows:


A conclusion from K m to K d is only possible if . This assumption is not always appropriate, but was originally made by Leonor Michaelis and Maud Menten to derive the Michaelis-Menten equation .

The affinity can be determined with ligand binding tests .

See also


  • Jeremy M. Berg, John L. Tymoczko, Lubert Stryer : Biochemistry. 6 edition, Spektrum Akademischer Verlag, Heidelberg 2007. ISBN 978-3-8274-1800-5 .
  • Donald Voet, Judith G. Voet: Biochemistry. 3rd edition, John Wiley & Sons, New York 2004. ISBN 0-471-19350-X .
  • Bruce Alberts , Alexander Johnson, Peter Walter, Julian Lewis, Martin Raff, Keith Roberts: Molecular Biology of the Cell , 5th Edition, Taylor & Francis 2007, ISBN 978-0815341062 .