Page 233 - Color Atlas of Biochemistry

224 Organelles
Membrane receptors
To receive and pass on chemical or physical
signals, cells are equipped with receptor pro-
teins. Many of these are integral membrane
proteins in the plasma membrane, where
they receive signals from their surroundings.
Other receptor proteins are located in inter-
cellular membranes. The receptors for lipo-
philic hormones are among the few that func-
tion in a soluble form. They regulate gene
transcription in the nucleus (see p. 378).
A. Principle of receptor action
Membrane-located receptors can be divided
into three parts, which have different tasks.
The receptor domain reacts specifically to a
given signal. Signals of this type can be of a
purely physical nature. For example, many
organisms react to light. In this way, plants
adapt growth and photosynthesis to light
conditions, while animals need light recep-
tors for visual processing (C; see p. 358). Me- known as 7-helix receptors. Via their effector
chanoreceptors are involved in hearing and in
pressure regulation, among other things.
Channels that react to action potentials (see
p. 350) can be regarded as receptors for elec-
trical impulses.
However, most receptors do not react to
physical stimuli, but rather to signal mole-
cules. Receptors for these chemical signals
containbinding sites inthe receptor domain
that are complementary to each ligand. In this
respect, they resemble enzymes (see p. 94).
As the effector domain of the receptor is usu-
ally separated by a membrane, a mechanism
for signal transfer between the domains is
needed. Little is yet known regarding this. It
is thought that conformation changes in the
receptor protein play a decisive part. Some
receptors dimerize after binding of the ligand, The cells of theimmunesystem communicate
thereby bringing the effector domains of two
molecules into contact (see p. 392).
The way in which the effector works differs
from case to case. By binding or interconver-
sion, many receptors activate special media-
tor proteins, which then trigger a signal cas-
cade (signal transduction; see p. 384). Other
receptors function as ion channels.This is
particularly widespread in receptors for neu-
rotransmitters (see p. 354).
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
All rights reserved. Usage subject to terms and conditions of license.
B. Insulin receptor
The receptor for the hormone insulin (see
p. 76) belongs to the family of 1-helix recep-
tors.
These molecules span the membrane with
only one α-helix. The subunits of the dimeric
receptor (red and blue) each consist of two
polypeptides (α and β)bound by disulfide
bonds. The α-chains together bind the insulin,
while the β-chains contain the transmem-
brane helix and, at the C-terminus, domains
with tyrosine kinase activity. In the activated
state, the kinase domains phosphorylate
themselves and also mediator proteins (re-
ceptor substrates) that set in motion cascades
of further phosphorylations (see pp. 120 and
388).
C. 7-helix receptors
A large group of receptors span the mem-
brane with α-helices seven times. These are
domains, they bind and activate trimeric pro-
teins, which in turn bind and hydrolyze GTP
and are therefore called G proteins. Most G
proteins, in turn, activate or inhibit enzymes
that create secondary signaling molecules
(second messengers; see p. 386). Other G pro-
teins regulate ion channels. The illustration
shows the complex of the light receptor rho-
dopsin, with the associated G protein trans-
ducin (see p. 358). The GTP-binding α-subunit
(green) and the γ-subunit (violet) of transdu-
cin are anchored in the membrane via lipids
(see p. 214). The β-subunit is shown in detail
on p. 72.
D. T-cell receptor
with each other particularly intensively. The
T-cell receptor plays a central role in the acti-
vation of T lymphocytes (see p. 296). The cell
at the top has been infected with a virus, and
it indicates this by presenting a viral peptide
(violet) with the help of a class I MHC protein
(yellow and green). The combination of the
two molecules is recognized by the dimeric
T-cell receptor (blue) and converted into a
signal that activates the T cell (bottom) and
thereby enhances the immune response to
the virus.