(a) Allosteric modulation and (b) covalent modulation of a protein's functional binding site.

since its binding to the regulatory site allosterically modulates the shape, and thus the activity, of the functional site.

The regulatory site to which modulator molecules bind is the equivalent of a molecular switch that controls the functional site. In some allosteric proteins, the binding of the modulator molecule to the regulatory site turns on the functional site by changing its shape so that it can bind the functional ligand. In other cases, the binding of a modulator molecule turns off the functional site by preventing the functional site from binding its ligand. In still other cases, binding of the modulator molecule may decrease or increase the affinity of the functional site. For example, if the functional site is 50 percent saturated at a particular ligand concentration, the binding of a modulator molecule that increases the affinity of the functional site may increase its saturation to 75 percent.

To summarize, the activity of a protein can be increased without changing the concentration of either the protein or the functional ligand. By controlling the concentration of the modulator molecule, and thus the percent saturation of the regulatory site, the functional activity of an allosterically regulated protein can be increased or decreased.

We have spoken thus far only of interactions between regulatory and functional binding sites. There is, however, a way that functional sites can influence each other in certain proteins. These proteins are composed of more than one polypeptide chain held together by electrical attractions between the chains. There may be only one binding site, a functional binding site, on each chain. The binding of a functional ligand to one of the chains, however, can result in an alteration of the functional binding sites in the other chains. This happens because the change in shape of the chain with the bound ligand induces a change in the shape of the other chains. The interaction between the functional binding sites of a multimeric (more than one polypeptide chain) protein is known as coopera-tivity. It can result in a progressive increase in the affinity for ligand binding as more and more of the sites become occupied. Such an increase occurs, for example, in the binding of oxygen to hemoglobin, a protein composed of four polypeptide chains, each containing one binding site for oxygen (Chapter 15).

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Protein Activity and Cellular Metabolism CHAPTER FOUR

Protein Activity and Cellular Metabolism CHAPTER FOUR

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