68
Chapter 3
of binding sites is 100 percent saturated. When half the avail-
able 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 affi nity 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 satura-
tion of binding sites increases with increasing ligand concen-
tration until all the sites become occupied (
Figure 3–30
).
Assuming that the ligand is a molecule that exerts a biological
effect when it binds to a protein, the magnitude of the effect
would also increase with increasing numbers of bound ligands
until all the binding sites were occupied. Further increases in
ligand concentration would produce no further effect because
there would be no additional sites to be occupied. To gener-
alize, a continuous increase in the magnitude of a chemical
stimulus (ligand concentration) that exerts its effects by bind-
ing to proteins will produce an increased biological response
until the point at which the protein-binding sites are 100 per-
cent saturated.
The second factor determining the percent of binding
site saturation is the affi nity 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 affi nity for a ligand, even a low ligand con-
centration will result in a high degree of saturation because,
once bound to the site, the ligand is not easily dislodged. A
low-affi nity site, on the other hand, requires a higher con-
centration of ligand to achieve the same degree of saturation
(
Figure 3–31
). One measure of binding site affi nity is the
ligand concentration necessary to produce 50 percent satura-
tion; the lower the ligand concentration required to bind to
half the binding sites, the greater the affi nity of the binding
site (see Figure 3–31).
Competition
As we have seen, more than one type of ligand can bind to
certain binding sites (see Figure 3–28). In such cases,
compe-
tition
occurs between the ligands for the same binding site.
In other words, the presence of multiple ligands able to bind
+
Ligand
High-affinity
binding site
Intermediate-affinity
binding site
Low-affinity
binding site
Protein 1
Protein 2
Protein 3
Figure 3–29
Three binding sites with the same chemical specifi city but different
affi nities for a ligand.
Ligand
Protein
Percent saturation
100
E
D
C
B
A
75
50
25
0
Ligand concentration
100%
saturation
ABCDE
Figure 3–30
Increasing ligand concentration increases the number of binding sites occupied—that is, it increases the percent saturation. At 100 percent
saturation, all the binding sites are occupied, and further increases in ligand concentration do not increase the number bound.
previous page 96 Vander's Human Physiology The Mechanisms of Body Function read online next page 98 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off