The strength of ligand-protein binding is a property of the binding site known as affinity. The affinity of a binding site for a ligand determines how likely it is that a bound ligand will leave the protein surface and return to its unbound state. Binding sites that tightly bind a ligand are called high-affinity binding sites;

those to which the ligand is weakly bound are low-affinity binding sites.

Affinity and chemical specificity are two distinct, although closely related, properties of binding sites. Chemical specificity, as we have seen, depends only on the shape of the binding site, whereas affinity depends on the strength of the attraction between the protein and the ligand. Thus, different proteins may be able to bind the same ligand—that is, may have the same chemical specificity—but may have different affinities for that ligand. For example, a ligand may have a negatively charged ionized group that would bind strongly to a site containing a positively charged amino acid side chain but would bind less strongly to a binding site having the same shape but no positive charge (Figure 4-4). In addition, the closer the surfaces of the ligand and binding site are to each other, the stronger the attractions. Hence, the more closely the ligand

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

PART ONE Basic Cell Functions

I. Basic Cell Functions

4. Protein Activity and Cellular Metabolism

© The McGraw-Hill Companies, 2001


Protein 1

High-affinity binding site

Protein 2

Intermediate-affinity binding site

Protein 3

Low-affinity binding site

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