Allosteric Modulation
Whenever a ligand binds to a protein, the attracting forces between the ligand and the protein alter the protein's shape. For example, as a ligand approaches a binding site, these attracting forces can cause the surface of the binding site to bend into a shape that more closely approximates the shape of the ligand's surface.
Moreover, as the shape of a binding site changes, it produces changes in the shape of other regions of the protein, just as pulling on one end of a rope (the polypeptide chain) causes the other end of the rope to move. Therefore, when a protein contains two binding sites, the noncovalent binding of a ligand to one site can alter the shape of the second binding site and, hence, the binding characteristics of that site. This is termed allosteric (other shape) modulation (Figure 4-7a), and such proteins are known as allosteric proteins.
One binding site on an allosteric protein, known as the functional site (also termed the active site), carries out the protein's physiological function. The other binding site is the regulatory site, and the ligand that binds to this site is known as a modulator molecule
Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition
PART ONE Basic Cell Functions
Ligand-
Functional site
Activation of functional site
Protein
Regulatory site
Modulator molecule
Functional site
(b) Covalent modulation
Protein
I Phosphoprotein phosphatase l 2_
OH PO4
Protein
I Phosphoprotein phosphatase l 2_
OH PO4
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