BIOL 2112 Chapter Notes - Chapter 11: Signal Transduction, Cell Surface Receptor, Protein Kinase A
3 May 2018
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Lectures 20-21 Study Guide – Cell Communication
1) What orgais ats as a odel syste for ell sigalig? Unique match between
mating factor and receptor is key to ensuring mating only among cells of the same species
of yeast. Evidence supports a model in which changes in the genes encoding receptors and
mating factor proteins can lead to establishment of new species. 3) A. List three
examples of local cell-to-cell signaling. Cell junctions, cell-cell recognition, paracrine
Signaling (signaling cell acts on nearby target cells by secreting molecules of a local
regulator), Synaptic Signaling (nerve cell releases neurotransmitter molecules into a
synapse stimulating the target cell, such as a muscle or nerve cell) B. What is the
signaling molecule used in long-distance signaling? Hormones secreted by endocrine
cells. Hormones reach most body cells, but are bound by and affect only some cells. C.
What is the primary difference between paracrine and hormonal signaling? Signaling
molecules that are secreted by a signaling molecule act on nearby target cells in paracrine
signaling. In hormonal signaling, specialized endocrine cells secrete hormones into body
fluids, usually blood. Hormones reach most cells but affect only some. 4) What are the
three stages of cell signaling? Reception – target ell’s detetio of sigalig oleule
oig fro outside the ell; sigalig oleule is oud to reeptor protei o ell’s
surface. Transduction – binding of signaling molecule changes receptor protein in some
way initiating transduction; converts signal to a form that can bring about a specific
cellular response; often requires signal transduction pathway (molecules in pathway
called relay molecules). Response – transduced signal triggers specific cellular response;
catalysis by an enzyme; rearrangement of cytoskeleton; activation of specific genes in
nucleus. 5) What is a ligand and what is the effect of ligand binding on a receptor
protein? A signaling molecule acts as a ligand – a molecule that specifically binds to
another (often larger) molecule. Ligand binding generally causes a receptor protein to
undergo a change in shape. For many receptors, this shape change directly activates the
receptor, enabling it to interact with other cellular molecules. For other kinds of receptors,
the ligand binding causes the aggregation of two or more receptor proteins which leads to
further molecular events inside the cell. Most signal receptors are plasma membrane
proteins but others are located inside the cell. 6) There are two primary types of signal-
transduction membrane receptors, G-protein-linked receptors and Receptor tyrosine
kinases. A. How do G-protein-linked receptors function? A G protein-coupled receptor
(GPCR) is a cell-surface transmembrane receptor that works with the help of a G protein,
a protein that binds the energy rich GTP molecule. Anchored to the cytoplasmic side of the
membrane, a G protein functions as a molecular switch that is either on or off depending
on whether GDP or GTP is attached. When GDP is bound, the G protein is inactive. The
receptor and G protein work together with another protein (usually an enzyme). When
appropriate signaling molecule binds to extracellular side of receptor, receptor is
activated and changes shape. Its cytoplasmic side binds the inactive G protein, causing a
GTP to displace the GDP activating the G protein. Activated G protein dissociates from
receptor, diffuses along the membrane and then binds to an enzyme, altering the
enzymes shape and activity. Once activated, enzyme can trigger next step leading to
cellular response. Binding of ligands is reversible. Ligand concentration outside cell
determines how often a ligand is bound and initiates signaling. Changes in the enzyme
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protein are only temporary because G protein also functions as a GTPase enzyme –
hydrolyzes its bound GTP to GDP and P. Now inactive again, G protein leaves enzyme
which returns to its original state and G protein is now available for reuse. GTPase
function of G protein allows the pathway to shut down rapidly when the signaling
molecule is no longer present. B. Where do many G-protein-coupled receptors function
in humans? Cells in a multicellular organism usually communicate via signaling molecules
targeted for cells that may not be immediately adjacent. Eukaryotic cells may
communicate by direct contact. Plants and animal cells have cell junctions that, where
preset, diretly oet the ytoplas’s of adjaet ells. I these ases, sigalig
substances dissolved in the cytosol can pass freely between neighboring cells. Animal cells
may communicate via direct contact between membrane-bound cell-surface molecules in
a process called cell-cell recognition. This sort of local signaling is important in embryonic
development and the immune response, and also sensory reception – vision, smell, and
taste depend on GPCRs. Malfunctions of the associated G proteins themselves are
involved in many human diseases including bacterial infections. C. How is G-protein
signaling halted? Changes in the enzyme protein are only temporary because G protein
also functions as a GTPase enzyme – hydrolyzes its bound GTP to GDP and P. Now inactive
again, G protein leaves enzyme which returns to its original state and G protein is now
available for reuse. GTPase function of G protein allows the pathway to shut down rapidly
when the signaling molecule is no longer present. C. How do receptor tyrosine kinases
function? RTKs belong to a major class of plasma membrane receptors characterized by
having enzymatic activity. An RTK is a protein kinase – an enzyme that catalyzes the
transfer of phosphate groups from ATP to another protein. Before signaling molecule
binds, receptors exist as individual units referred to as monomers. Each monomer has an
extracellular ligand-binding site, an
helix spanning the membrane, and an intracellular
tail containing many tyrosine. Binding of a signaling molecule causes two receptor
monomers to associate closely with each other, forming a complex called a dimer, in a
process called dimerization. Dimerization activates tyrosine kinase region of each
monomer; each tyrosine kinase adds a phosphate from an ATP molecule to a tyrosine that
is part of the tail of the other monomer. Now the receptor protein is activated, and it is
recognized by specific relay proteins inside the cell. Each such protein binds to a specific
phosphorylated tyrosine, undergoing a structural change that activates the bound relay
protein. Each activated protein triggers a transduction pathway, leading to a cellular
response D. Describe an important difference between many G-protein-coupled
receptors and receptor-tyrosine kinases. The part of the receptor protein extending into
the cytoplasm functions more specifically as a tyrosine kinase, an enzyme that catalyzes
the transfer of a phosphate group from ATP to the amino acid tyrosine of a substrate
protein. RTKs are membrane receptors that attach phosphates to tyrosines. Upon binding
a ligand, one RTK can activate 10+ different transduction pathways and cellular
responses. More than one signal transduction pathway can be triggered at once, helping
cell regulate and coordinate many aspects of cell growth and reproduction. GPCRs usually
activate a single transduction pathway. Abnormal RTKs that function even in absence of
signaling molecules are associated with many kinds of cancer. 7) A. Describe the
mechanism of a ligand-gated ion channel. A ligand-gated ion channel is a membrane
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Document Summary
Unique match between mating factor and receptor is key to ensuring mating only among cells of the same species of yeast. Evidence supports a model in which changes in the genes encoding receptors and mating factor proteins can lead to establishment of new species. Signaling (signaling cell acts on nearby target cells by secreting molecules of a local regulator), synaptic signaling (nerve cell releases neurotransmitter molecules into a synapse stimulating the target cell, such as a muscle or nerve cell) b. Hormones reach most body cells, but are bound by and affect only some cells. Signaling molecules that are secreted by a signaling molecule act on nearby target cells in paracrine signaling. In hormonal signaling, specialized endocrine cells secrete hormones into body fluids, usually blood. Hormones reach most cells but affect only some. Reception target (cid:272)ell"s dete(cid:272)tio(cid:374) of sig(cid:374)ali(cid:374)g (cid:373)ole(cid:272)ule (cid:272)o(cid:373)i(cid:374)g fro(cid:373) outside the (cid:272)ell; sig(cid:374)ali(cid:374)g (cid:373)ole(cid:272)ule is (cid:271)ou(cid:374)d to re(cid:272)eptor protei(cid:374) o(cid:374) (cid:272)ell"s surface.
