Control of Cells by Chemical Messengers
121
Receptors
In Chapter 1, you learned that several classes of chemical mes-
sengers can communicate a signal from one cell to another.
These messengers include chemicals such as neurotransmit-
ters, whose signals are mediated rapidly and over a short dis-
tance. Other messengers, such as hormones, communicate
more slowly and over greater distances. Whatever the chemical
messenger, however, the cell receiving the signal must have a
way to detect the signal’s presence. Once a cell detects a signal,
a transduction mechanism is needed to convert that signal into
a biologically meaningful response, such as the cell-division
response to the delivery of growth-promoting signals.
The fi rst step in the action of any intercellular chemical
messenger is the binding of the messenger to specifi c target-
cell proteins known as
receptors
or
receptor proteins.
In
the general language of Chapter 3, a chemical messenger is
a ligand, and the receptor protein has a binding site for that
ligand. The binding of a messenger to a receptor protein initi-
ates a sequence of events in the cell leading to the cell’s response
to that messenger, a process called signal transduction.
The term
receptor
can be the source of confusion because
the same word is used to denote the “detectors” in a refl ex
arc, as Chapter 1 described. You should keep in mind that the
term
receptor
has two distinct meanings, but the context in
which the term is used makes the meaning clear.
CHO
CHO
Extracellular fluid
Intracellular fluid
NH
2
HOOC
Plasma
membrane
Hormone
binding site
Figure 5–1
Structure of a receptor that binds the
hormone epinephrine. The seven clusters of
amino acids embedded in the phospholipid
bilayer represent hydrophobic portions of the
protein’s alpha helix. Note that the binding
site for the hormone includes several of the
segments that extend into the extracellular
fl uid. The amino acids denoted by black
circles represent sites at which intracellular
substances can phosphorylate, and thereby
regulate, the receptor.
Adapted from Dohlman et al.
What is the nature of the receptors with which inter-
cellular chemical messengers combine? They are proteins or
glycoproteins located either in the cell’s plasma membrane or
inside the cell, mainly in the nucleus. The plasma membrane is
the much more common location, because a very large number
of messengers are water-soluble and thus cannot diffuse across
the lipid-rich plasma membrane. In contrast, the much smaller
number of lipid-soluble messengers pass through membranes
(mainly by diffusion but, in some cases, by mediated transport
as well) to bind to their receptors located inside the cell.
Plasma membrane receptors are transmembrane proteins;
that is, they span the entire membrane thickness. A typical
plasma membrane receptor is illustrated in
Figure 5–1
. Like
other transmembrane proteins, a plasma membrane recep-
tor has hydrophobic segments within the membrane, one or
more hydrophilic segments extending out from the membrane
into the extracellular fl uid, and other hydrophilic segments
extending into the intracellular fl
uid. It is to the extracellular
portions that the arriving chemical messenger binds. Also like
other transmembrane proteins, certain receptors may be com-
posed of two or more nonidentical subunits bound together.
The binding of a chemical messenger to its receptor protein
initiates the events leading to the cell’s response. The existence
of receptor proteins explains a very important characteristic of
intercellular communication—
specifi
city
(see
Table 5–1
for a
glossary of terms concerning receptors). Although a given chem-
ical messenger may come into contact with many different cells,
it infl uences only certain cells and not others. This is because
cells differ in the types of receptors they possess. Only certain
cell types, often just one, possess the specifi c receptor protein
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