Download AP Bio: Chp.11 Cell Communication G-protein

AP Bio: Chp.11
Cell Communication
G-protein-linked receptors:
• vary in their binding sites and for
recognizing different G-proteins.
• Most have seven alpha helices penetrating
the plasma membrane.
• When a G-protein-linked receptor binds with
a signal or ligand, it becomes activated and
interacts with a G-protein.
Examples: Yeast mating factors, epinephrine,
neurotransmitters and other hormones.
• G-protein-linked receptors activate G proteins.
• G proteins are found in the plasma membrane.
• They are activated when the G-protein-linked
receptors causes GTP to replace GDP on the G-protein.
• The activated G-protein moves laterally to
interact with a given enzyme also located in the
plasma membrane
• This causes a certain biochemical pathway to be
activated.
• Once this occurs, the G-protein cleaves a
phosphate group from GTP and makes it GDP
and thus the G-protein becomes inactivated.
• These biochemical pathways include
developmental pathways, sensory perception in
vision and smell.
Note-Bacteria that often cause disease
by secreting toxins. These toxins may
interfere with G-protein functions. 60%
of all medicines exert their effect by
influencing G-protein pathways.
Which molecule would be considered a ligand if it were
small?
A) A B) B C) C D) D
E) E
A
The molecule that attaches to the G-linked receptor
protein is the ligand. A ligand is a small molecule that
attaches to the large molecule. In this case A is
considered to be the ligand if it were small.
b. Tyrosine-Kinase receptors• when activated can activate more than one signaltransduction pathway at one time.
• Example: cell reproduction requires a number of
biochemical pathways to be activated at once.
• The tyrosine-kinase receptor in the inactive form are two
separate polypeptide chains forming alpha helices that
span the membrane.
• Inside the cell, the two polypeptides have tyrosines on tails
that extend into the cytoplasm.
• Both have a receptor site for a signal or ligand.
• Once both receptors are activated, the polypeptides move
toward one another and bond with one another to form a
dimer.
• The dimer is phosphorylated at every
tyrosine by ATP.
• The receptor is activated & can trigger a
number of transduction-signal pathways.
• This may include the transfer of a phosphate
to another enzyme.
• Triggering a number of these pathway is the
main difference between this receptor and
the G-protein-linked receptor.
The activation of receptor tyrosine kinases is always
characterized by
A) dimerization and phosphorylation.
B) IP3 binding.
C) a phosphorylation cascade.
D) GTP hydrolysis.
E) channel protein conformational change.
A
The molecule has two separated halves that joint together
once the ligands attach to the receptor site. Once joined,
this molecule with two identical halves is called a dimer.
On the molecule there are places where a phosphate group
will attach. This place of attachment has tyrosine located
there.
c. Ligand-gated ion channels.
• are protein pores on plasma
membrane that open or close in
response to a chemical signal
• allows or prevents the flow of ions
into or out of the cell.
• Examples include Na+ or Ca++.
• Once the signal attaches to the
gated channel, the protein
changes shape & a pore opens;
allowing for an immediate change
in the concentration of ions.
d. Intracellular receptors• NOT found in plasma membrane,
they are dissolved in cytoplasm.
• Signal must be lipid soluble to
make into the cytosol.
• Signals are usually steroid
hormones, thyroxin, & nitric
oxide.
• Example: Testosterone enters the
cell & binds with a receptor
protein found in the cytoplasm.
This complex moves to the nucleus
where is acts a transcription
factor & helps transcribe certain
genes.
• Only targeted cells have
intracellular receptors.
Binding of a signal molecule to which type of receptor leads
directly to a change in the distribution of anions and/or
cations on opposite sides of the membrane?
A) receptor tyrosine kinase
B) G-protein-linked receptor
C) phosphorylated receptor tyrosine kinase dimer
D) ligand-gated ion channel
E) intracellular receptor
D
The ligand-gated channel as illustrated above leads directly
to a change in the distribution of anions and cations.
Lipid-soluble signal molecules, such as testosterone, cross
the membranes of all cells but affect only target cells
because
A) only target cells retain the appropriate DNA segments.
B) intracellular receptors are present only in target cells.
C) most cells lack the Y chromosome required.
D) only target cells possess the cytosolic enzymes that
transduce the testosterone.
E) only in target cells is testosterone able to initiate the
phosphorylation cascade leading to activated transcription
factor.
B
Lipid-soluble signal molecules,
such as testosterone, cross
the membranes of all cells but
affect only target cells
because intracellular receptors
are present only in target
cells.
Transduction:
• After reception, the cell needs to process the signal,
this is a multi-step process.
• Usually the molecules that process the signal are
proteins that are activated by phosphorylating them.
• Protein kinases are proteins that transfer phosphate
groups from ATP to another protein.
• Serine and threonine are two amino acids that receive
the phosphate group on the receiving protein kinase.
• This cascading affect increases the number of proteins
activated, amplifying the effect very quickly.
• A biochemical pathway usually has a cascading
effect where one protein kinase will activate
another protein kinase by transferring a
phosphate group on to it.
• These protein kinases are extremely important.
• Approximately 1% of all our genes code for
various protein kinases.
• Protein phosphatase is a protein that removes a
phosphate group from proteins and can
deactivate a protein kinase.
Phosphorylation cascades involving a series of protein
kinases are useful for cellular signal transduction because
A) they are species specific.
B) they always lead to the same cellular response.
C) they amplify the original signal manifold.
D) they counter the harmful effects of phosphatases.
E) the number of molecules used is small and fixed.
C
Phosphorylation cascades involving a series of protein
kinases are useful for cellular signal transduction because
they amplify the original signal manifold.
Between the receptor molecule
that is activated and signaltransduction pathway that is
to be activated, there may be a
second messenger.
Note-The hormone or signal
that attaches the to the
receptor site is called the first
messenger.
The most common secondary
messengers include:
a. cyclic AMP
b. Calcium ions and inositol
triphosphate.
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