Saturation

An equilibrium is rapidly reached between unbound ligands in solution and their corresponding protein binding sites such that at any instant some of the free ligands become bound to unoccupied binding sites and some of the bound ligands are released into solution. A single binding site is either occupied or unoccupied. The term saturation refers to the fraction of to tal binding sites that are occupied at any given time. When all the binding sites are occupied, the population of binding sites is 100 percent saturated. When half the available sites are occupied, the system is 50 percent saturated, and so on. A single binding site would also be 50 percent saturated if it were occupied by a ligand 50 percent of the time.

The percent saturation of a binding site depends upon two factors: (1) the concentration of unbound ligand in the solution, and (2) the affinity of the binding site for the ligand.

The greater the ligand concentration, the greater the probability of a ligand molecule encountering an unoccupied binding site and becoming bound. Thus, the percent saturation of binding sites increases with increasing ligand concentration until all the sites become occupied (Figure 4-5). Assuming that the ligand is a molecule that exerts a biological effect when it is bound to a protein, the magnitude of the effect would also increase with increasing numbers of bound li-gands until all the binding sites were occupied. Further increases in ligand concentration would produce no further effect since there would be no additional sites to be occupied. To generalize, a continuous increase in the magnitude of a chemical stimulus (ligand concentration) that exerts its effects by binding to proteins will produce an increased biological response up to the point at which the protein binding sites are 100 percent saturated.

Ligand Protein

• • • • • ••

••• •••• ..y.-*:

• • • •

• * • • •

v

C

D

OL 25

Ligand concentration

FIGURE 4-5

Increasing ligand concentration increases the number of binding sites occupied—that is, increases the percent saturation. At 100 percent saturation, all the binding sites are occupied, and further increases in ligand concentration do not increase the amount bound.

Ligand concentration

FIGURE 4-5

Increasing ligand concentration increases the number of binding sites occupied—that is, increases the percent saturation. At 100 percent saturation, all the binding sites are occupied, and further increases in ligand concentration do not increase the amount bound.

OL 25

Vander et al.: Human I I. Basic Cell Functions I 4. Protein Activity and I I © The McGraw-Hill

Physiology: The Cellular Metabolism Companies, 2001

Mechanism of Body Function, Eighth Edition

Protein Activity and Cellular Metabolism CHAPTER FOUR

Protein Y —

Ligand

50% bound 25% bound

Protein X

ing sites. By occupying the binding sites, the drug decreases the amount of natural ligand that can be bound.

50% bound 25% bound

FIGURE 4-6

When two different proteins, X and Y, are able to bind the same ligand, the protein with the higher-affinity binding site (protein Y) has more bound sites at any given ligand concentration until 100 percent saturation.

FIGURE 4-6

When two different proteins, X and Y, are able to bind the same ligand, the protein with the higher-affinity binding site (protein Y) has more bound sites at any given ligand concentration until 100 percent saturation.

The second factor determining the percent of binding site saturation is the affinity of the binding site. Collisions between molecules in a solution and a protein containing a bound ligand can dislodge a loosely bound ligand, just as tackling a football player may cause a fumble. If a binding site has a high affinity for a ligand, even a low ligand concentration will result in a high degree of saturation since, once bound to the site, the ligand is not easily dislodged. A low-affinity site, on the other hand, requires a much higher concentration of ligand to achieve the same degree of saturation (Figure 4-6). One measure of binding-site affinity is the ligand concentration necessary to produce 50 percent saturation; the lower the ligand concentration required to bind to half the binding sites, the greater the affinity of the binding site (Figure 4-6).

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Responses

  • leon
    What is the 50 percent saturation point of a solution of ligands and proteins?
    6 years ago

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