330 Chapter 11
In the case of many growth factors, the binding of the ligand
to its receptor causes the receptors to form dimers that allow one
member to phosphorylate the other in a process of autophos-
phorylation. This activates the tyrosine kinase portion on the
cytoplasmic side of the receptor. However, the insulin receptor
is already a homodimer of two half-receptors (each composed
of two polypeptide chains), which is bound together by disulfide
bonds (chapter 2, see fig. 2.29) even before it binds to insulin.
The insulin receptor forms the shape of an inverted “V” that has
an insulin-binding site on its extracellular surface ( fig. 11.11 ).
Recent observations demonstrate that a single insulin molecule
binds to the sites and causes a portion of the polypeptide to
move. This conformational change results in autophosphoryla-
tion of the receptor, activating its tyrosine kinase activity.
The activated insulin receptor then phosphorylates insulin
receptor substrate proteins, which provide an enzymatic docking
station that activates a variety of other signaling molecules. These
signaling molecules cause the insertion of transport carrier proteins
for glucose into the plasma membrane (see fig. 11.30 ), and so pro-
mote the uptake of plasma glucose into tissue cells. In this way,
insulin promotes the lowering of the plasma glucose concentra-
tion. Some signaling molecules activate other second-messenger
systems within the target cells, allowing insulin and growth factors
to regulate different aspects of the metabolism of their target cells.
Figure 11.11 The receptor for insulin. The receptor is a homodimer of two half-receptors bound together to form
an inverted “V.” There are two insulin-binding sites on the extracellular side of the receptor, and the binding of insulin causes a
conformational change that results in autophosphorylation of the receptor. This activates its tyrosine kinase activity, leading to
phosphorylation of signaling molecules that result in the effects of insulin on its target cells.
Insulin
receptor
Insulin
Extracellular
fluid
Cytoplasm
P P
P P
P
ADP
ATP
ADP
ATP
ADP
ATP
ADP
ATP
P
Two half-receptors form
dimer prior to insulin
binding
- Insulin binding causes
autophosphorylation of
receptor
2.
Active tyrosine kinase
of receptor phosphorylates
signaling molecule
3.
Active signaling molecule
causes cascade of effects
4.
Glucose uptake and
anabolic reactions
(a) (b) (c)
PP
- The hormone binds to its receptor on the outer surface of the
target cell’s plasma membrane. - Hormone-receptor interaction stimulates the activity of a
membrane enzyme, phospholipase C. - Activated phospholipase C catalyzes the conversion
of particular phospholipids in the membrane to inositol
triphosphate (IP 3 ) and another derivative, diacylglycerol. - Inositol triphosphate enters the cytoplasm and diffuses to the
endoplasmic reticulum, where it binds to its receptor proteins
and causes the opening of Ca^21 channels. - Since the endoplasmic reticulum accumulates Ca^21 by active
transport, there exists a steep Ca^21 concentration gradient
favoring the diffusion of Ca^21 into the cytoplasm. - Ca^21 that enters the cytoplasm binds to and activates a protein
called calmodulin. - Activated calmodulin, in turn, activates protein kinase, which
phosphorylates other enzyme proteins. - Altered enzyme activity mediates the target cell’s response to
the hormone.
Table 11.5 | Sequence of Events Involving
the Ca^21 Second-Messenger System