Cell Signals and Signal Transduction Flashcards
1
Q
What are the three stages of cell signaling?
A
- Reception
- Transduction
- Response
2
Q
What are signals?
A
Signals are ions or molecules that directly or indirectly regulate protein function by altering their shape or conformation, thus activating or inactivating them.
3
Q
Where does epinephrine come from?
A
Epinephrine is secreted by the adrenal glands above the kidneys.
4
Q
Name two examples of cell communication via direct contact (local signaling).
A
- Gap junctions in animal cells.
- Plasmodesmata in plant cells.
5
Q
What are gap junctions and plasmodesmata?
A
Gap junctions and plasmodesmata are channels that join two cells so that you can have the free transfer of molecules from the cytoplasm of one cell to the other.
6
Q
Receptor-ligand interaction is an example of what type of cell communication?
A
Direct contact (local signaling).
7
Q
What is paracrine signaling?
A
A secreting cell acts on nearby target cells by secreting molecules that are local regulators (growth factors, nitric oxide, for example). Operates on the range of µm.
8
Q
What is synaptic signaling?
A
An electrical signal traveling across a nerve cell induces the release of neurotransmitters (serotonin, dopamine, for example) into a synapse, stimulating the target cell, such as a muscle or nerve cell. Operates on the range of nm.
9
Q
What are the two types of cell communication that do not require direct contact but must be nearby?
A
- Paracrine signaling
- Synaptic signaling
10
Q
What type of cell communication happens over long distances?
A
Endocrine (hormonal) signaling.
11
Q
How does endocrine signaling work?
A
Specialized endocrine cells (sender cells) secrete hormones (insulin, glucagon, etc) into body fluids, often blood. Hormones travel via the circulatory system (blood) and reach virtually all cells in the body, but are bound only by some cells (target cells).
12
Q
How does autocrine signaling work?
A
A secreted molecule diffuses locally and triggers a response in the cell that secretes it. The cell communicates with itself.
13
Q
How does neuroendocrine signaling work?
A
A nerve cell releases neurohormones that diffuse into the bloodstream and trigger responses in target cells elsewhere in the body (antidiuretic hormone, oxytocin, etc).
14
Q
How is neuroendocrine signaling related to endocrine signaling?
A
The process is essentially the same as endocrine signaling (release of a hormone to trigger a response), but the sending cell is a neuron that releases a neurohormone.
15
Q
In Sutherland’s experiment, what did he know prior to the experiment?
A
He knew epinephrine triggers the fight or flight response and that it interacts with the liver cells. In response to epinephrine, the liver cells secrete glucose. He also knew there was an enzyme in the liver called glycogen phosphorylase.
16
Q
What is the role of glycogen phosphorylase in the liver?
A
This enzyme is responsible for breaking down glycogen to secrete glucose into the bloodstream.
17
Q
Describe Sutherland’s first in vitro experiment.
A
First, he purified epinephrine and glycogen phosphorylase. He put the two with glycogen to see what would happen. When he mixed epinephrine and glycogen phosphorylase, nothing happened; the glycogen did not break down.
18
Q
Describe Sutherland’s second in vitro experiment.
A
Instead of mixing epinephrine with glycogen phosphorylase, he mixed it with adipocytes (liver cells he had purified). Since those adipocytes had glycogen phosphorylase on the inside, he saw glycogen breakdown.
19
Q
Interpret the results of Sutherland’s experiment.
A
The epinephrine was not interacting directly with glycogen phosphorylase. Instead, there have to be intermediate steps that were not present in the first mixture. Essentially, Sutherland discovered signal transduction.
20
Q
What two things do you need for reception?
A
- A signaling molecule (first messenger)
- A receptor that can specifically recognize the signaling molecule.
21
Q
How does reception occur?
A
The signaling molecule binds the receptor, which initiates a conformational change that triggers a response in the cell.
22
Q
What is a signal transduction cascade?
A
This is the response to reception inside the cell during signal transduction. One thing activates another, which activates another, and so on. If any intermediate step is removed, the process will not work.
23
Q
Name an example of a cellular response to signal transduction.
A
- Activate or deactivate a protein or enzyme.
- Alter gene expression (turn on/off specific genes).
24
Q
Describe the process of a secreted signal that cannot cross the plasma membrane.
A
The signal (often a water-soluble hormone) is secreted via exocytosis by the sending cell. If endocrine, it enters the bloodstream. If paracrine, it does not. Then the signal binds a cell-surface receptor.
25
Describe the process of a secreted signal that can cross the plasma membrane.
Small hydrophobic molecules can cross the plasma membrane because it is lipid-based. Then they bind a receptor inside the cell. This allows them to alter gene expression, making them essentially transcription factors.
26
Why do hydrophobic endocrine signals need to use transport proteins in the blood?
Blood is water-based, making it difficult for hydrophobic signals to move in them due to their hydrophobicity.
27
What are the three main kinds of cell-surface receptors?
1. G protein-coupled receptors (GPCRs)
2. Receptor tyrosine kinases (RTKs)
3. Ion channel receptors.
28
What is the most common type of cell surface receptor?
GPCRs. The human genome encodes for about 800 of them. That means about 4% of the genome is for GPCRs.
29
How many RTKs does the human genome encode for?
58.
30
How many genes encode for ion channel receptors?
About 300 split between ligand-gated and voltage-gated channels.
31
Describe the basic characteristics of GPCRs.
GPCRs contain seven transmembrane domains and function by interacting with G-proteins.
32
Describe the basic characteristics of G proteins.
They are membrane proteins that bind energy-rich guanine nucleotides: GDP and GTP. They are composed of an a, B, and y subunit. Found inside the cell.
33
Which G protein subunit binds GDP/GTP?
The a subunit.
34
How does a first messenger interact with a GPCR?
It binds the binding site outside the cell and never enters the cytoplasm. This causes a conformational change inside the cell which binds the G protein.
35
Which G protein subunits are anchored to the membrane and which are not?
The a and y subunits are anchored but the B subunit is not.
36
Describe the G protein in its inactive state.
In its inactive state, the a subunit is coupled to GDP.
37
What causes the a subunit to displace the GDP it is coupled to with a GTP?
The first messenger binds the GPCR outside the cell, triggering a conformational change in the G protein. This change causes the G protein to displace the GDP it was bound to and exchange it for a GTP. This activates the G protein.
38
What happens to the a subunit when the G protein binds a GTP?
The a subunit, bound to the GTP, dissociates from the B and y subunits. This frees the a subunit to diffuse along the membrane to activate its next target by binding to a specific enzyme.
39
What happens to the enzyme bound by the a subunit?
It changes conformation and becomes active, leading to a cellular response.
40
How does the fact that the G protein has its own enzymatic activity affect the inactivation of the signal cascade?
This allows the signal cascade to be inactivated. The a subunit of the G protein hydrolyzes GTP to GDP, inactivating both itself and the enzyme. It then dissociates from the enzyme.
41
After dissociating from the inactivated enzyme, what does the G protein do?
It binds to its partners: the B and y subunits of the G protein. This allows it to be used again.
42
Describe the basic characteristics of RTKs.
1. Plasma membrane receptors with enzymatic activity.
2. RTKs catalyze the transfer of a phosphate group from ATP to tyrosine residues on a substrate protein.
3. RTKs can simultaneously activate multiple signal transduction pathways.
43
How do RTKs differ from GPCRs?
RTKs are enzymes while GPCRs are not. They are less common than GPCRs, but they can activate multiple pathways at the same time.
44
What do kinases do?
They phosphorylate other proteins.
45
What is the inactive state of RTKs?
When the RTK is in its monomer form, it is inactive.
46
Describe the monomers of RTKs.
Each monomer has a ligand binding site, a membrane-spanning a-helix, and an intracellular tail with tyrosine residues.
47
What activates RTKs?
Ligands bind the receptors of the monomers, causing them to dimerize. Dimerization activates the tyrosine kinase region of each monomer, activating enzymatic activity.
48
How does the enzymatic activity of RTKs work?
Each tyrosine kinase adds a phosphate from an ATP to a tyrosine on the tail of the partner monomer. This turns hydrophobic areas into more hydrophilic areas. This allows for the binding and activation of other proteins.
49
How do RTKs trigger a cellular response?
The activated receptor is recognized by specific relay proteins inside the cell. Each relay protein binds a specific phosphorylated tyrosine, inducing a conformational change that activates the protein. This induces a signal transduction pathway that leads to a cellular response.
50
Describe the basic characteristics of ligand-gated ion channel receptors.
Transmembrane receptor that contains a pore that opens and closes in response to a signaling molecule, allowing or blocking the flow of specific ions. Also known as ionotropic receptors.
51
Describe the basic characteristics of voltage-gated ion channel receptors.
Transmembrane receptor that contains a pore that opens or closes in response to changes in membrane potential (electrical current), allowing or blocking the flow of specific ions.
52
What determines what is activated on an RTK?
The location of the tyrosine, the three-dimensional structure of the protein, and whatever relay molecules are inside determine what gets activated.
53
Where can ligand-gated ion channel receptors be located?
The plasma membrane and interior membranes (such as the ER membrane).
54
What is the inactive state of a ligand-gated ion channel?
In this state, the gate is closed and no ligand is bound to the receptor.
55
How is a ligand-gated ion channel activated?
A ligand binds to the receptor and the gate opens. Specific ions flow through the channel, inducing a cellular response.
56
How is a ligand-gated ion channel inactivated?
The ligand dissociates from the receptor, the gate closes, and ions no longer enter the cell.
57
How does signal transduction function?
Signal transduction functions via the activation of proteins by the addition of removal of phosphate groups, and/or the release of second messenger molecules.
58
What are the advantages of signal transduction?
1. Amplification
2. Tight regulation
3. Activation of additional pathways.
59
Which type of signal transduction leads to cell division?
Signal transduction involving RAS, MEK, MAPK, and others.
60
What can RAS/MAPK proteins be compared to?
They are similar to the a-subunit of the G protein. They are tethered to the membrane and important in cell cycle regulation.
61
How is RAS activated in the RAS/MAPK pathway?
Similar to the a-subunits of G-proteins, RAS is activated by exchanging GDP to GTP. There is a receptor that activates a guanine exchange factor (GEF). GEF then adds a GTP to RAS, thus activating it.
62
After RAS is activated, what happens next?
RAS activates RAF, which then activates MEK, when then activates MAPK, which then activates transcription factors that lead to the production of proteins that are involved in cell division.
63
If there is a mutation on RAS that makes it always coupled to GTP, what happens?
If RAS is always coupled to GTP, there is increased cell division. In many cases, this uncontrolled division leads to cancer.
64
What are second messengers?
Second messengers are water-soluble molecules or ions that readily spread throughout the cell and activate cellular responses. Second messengers participate in pathways that are initiated by both GPCRs and RTKs. They are often Ca ions, nitric oxide, cyclic AMP, but NOT PROTEINS.
65
Describe how cyclic AMP is used as a second messenger.
There is an activated GPCR that activates a G protein by binding it to ATP. The G protein activates an enzyme called adenylyl cyclase, which then produces cyclic AMP.
66
What is the role of phosphodiesterase?
Phosphodiesterase hydrolyzes cyclic AMP to AMP to inactivate the second messenger.
67
In Sutherland's experiment, what was the first messenger?
Epinephrine bound to a GPCR.
68
In Sutherland's experiment, what enzyme did the a-subunit of the G protein activate?
Adenylyl cyclase.
69
What is the role of adenylyl cyclase in Sutherland's experiment?
It takes ATP and makes cyclic AMP, which is the second messenger.
70
What does cyclic AMP activate in Sutherland's experiment?
Cyclic AMP activates protein kinase A, which then leads to a cellular response.
71
Describe the composition of protein kinase A.
Protein kinase A is composed of four subunits. Two are regulatory subunits and two are catalytic subunits. Each regulatory subunit has two binding sites.
72
What happens when cyclic AMP occupies the binding sites on protein kinase A?
It release the catalytic subunits from the regulatory subunits and activates them.
73
What happens when protein kinase A is hydrolyzed?
The regulatory and catalytic subunits come back together, inactivating protein kinase A.
74
What causes cholera?
Cholera is caused by the bacterium, Vibrio cholera. Humans become infected by drinking contaminated water.
75
What is the role of the B subunit of cholera toxin?
The B subunit binds to membrane ganglioside in the cell membrane of crypt cells of the small intestine.
76
What is the role of the A subunit of cholera toxin?
The A subunit enters the cell, takes in a G protein, activates it, and keeps it in its active form so that it cannot hydrolyze GTP to GDP anymore.
77
What happens when the G protein is in a constant state of activation? (cholera example)
Because the G protein cannot hydrolyze GTP to GDP, the adenylyl cyclase it activates will also remain activated, continually producing cAMP. This cAMP activates protein kinases.
78
What happens when cAMP activates protein kinases in great quantities? (cholera example)
This activates the opening of chloride ion channels on the surface of the enterocyte, and chloride ions flow out to the lumen of the intestine. This causes the lumen to become negative.
79
What happens when the lumen of the intestine becomes negative? (cholera example)
This disrupts the electrochemical gradient. To counteract the negativity, sodium ion channels are established to pump Na+ into the lumen. Then, because of the high outflux of ions, water also flows to the lumen due to osmosis.
80
How does the water that flows to the lumen of the intestine factor into the symptoms of cholera?
This causes the person to experience diarrhea and get dehydrated.
81
Where are calcium ions most highly concentrated?
The ER, mitochondria, and outside the cell. There is very little in the cytoplasm. There is about 10,000 times more calcium outside the cell than inside the cell.
82
Calcium ions are a second messenger in what types of pathways?
GPCR and RTK pathways.
83
What processes are calcium ions involved in?
Muscle contraction, secretion of molecules, cell division, and many other processes.
84
Describe the initiation of the process that uses calcium ions as its second messenger.
There is an activated GPCR that then activates a G protein, which activates phospholipase C.
85
What is the role of phospholipase C in the process that uses calcium ions as its second messenger?
The G protein activates phospholipase C (a membrane bound enzyme), which then cleaves the phospholipid PIP2 into two parts: DAG (a second messenger that activates PKC) and IP3.
86
What is the role of IP3 in the process that uses calcium ions as its second messenger?
IP3 binds to calcium ion channels (which are ligand-gated) in the ER, opening them. Calcium ions flow into the cytosol, activating various proteins and inducing signal transduction pathways and cellular responses.
87
What is an example of an important protein that calcium ions bind when they rush out of the ER in response to IP3?
Calmodulin, which activates other types of cellular responses.
88
What is the first messenger in the example relating calcium ions, nitric oxide, cGMP, and vasodilation?
Acetylcholine, which binds a GPCR.
89
What does calmodulin bind to?
Nitric oxide synthase.
90
What is the role of nitric oxide synthase?
It takes arginine and converts it into citrulline and nitric oxide.
91
What is the role of nitric oxide?
It activates guanylyl cyclase.
92
What is the role of guanylyl cyclase?
It creates a new second messenger by converting GTP to cGMP.
93
What is the role of cGMP?
It activates protein kinase G.
94
What does the activation of protein kinase G do?
It leads to muscle relaxation and in turn vasodilation, which increases blood flow.
95
How is vasodilation halted?
A phosphodiesterase takes cGMP and converts it to GMP so that it is no longer active.
96
When acetylcholine is the first messenger to a GPCR, what are the second messengers that work in the following cascade? (name them in order)
IP3, calcium ions, nitric oxide, and cGMP.
97
How does the drug Viagra work?
It inhibits the penile isoform of cGMP phosphodiesterase, meaning the active form is maintained, which allows for vasodilation and increased blood flow.
98
How does cell signaling regulate transcription?
In the nucleus, signaling pathways may end with the activation of transcription factors that turn on or off the transcription of specific genes.
99
When regulating transcription, what does protein kinase A phosphorylate?
Transcription factor CREB (cyclic AMP response element binding protein).
100
What does CREB bind to?
CREB binds CRE (cAMP response element) in DNA to activate transcription.
101
What is glycogen?
A polymer of glucose. In essence, a large chain of glucose molecules.
102
How does amplification occur in the example with epinephrine and the liver cells?
Epinephrine (the first messenger) binds a GPCR, causing it to activate the G protein and send the a-subunit to activate adenylyl cyclase. But then, because the GPCR is no longer occupied by the G protein but still has the epinephrine associated with it, This means the GPCR can then activate another G protein.
103
About how many G proteins can epinephrine activate, and what is this an example of?
It can activate about 100 G proteins. This is an example of amplification.
104
About how many cAMPs can one adenylyl cyclase bound to a G protein make?
100. However, because there were 100 G proteins activated from epinephrine, and 100 adenylyl cyclases activated, in reality there were 10,000 cAMP molecules activated.
105
How many phosphorylated kinases can cAMP activate?
About 10.
106
How many glycogen phosphorylases can be activated by each activated phosphorylase kinase?
About 10.
107
How many glucose molecules can each glycogen phosphorylase release?
100.
108
Overall, how many glucose molecules does one epinephrine molecule release?
100 million glucose molecules are released into the blood.
109
Why is amplification associated with epinephrine so important?
Epinephrine comes from the adrenal gland (along the kidneys) and enters the bloodstream. It needs to reach the liver, but the majority of it will end up elsewhere because blood takes it everywhere. But, when epinephrine does reach the liver, the liver can react very quickly.
110
What does it mean to say that signal transduction pathways are tightly regulated?
The many steps in a multistep pathway provide control points where the cell's response can be further regulated. This allows for specificity and coordination with other pathways. The overall efficiency of the response is enhanced by the presence of scaffolding proteins. All signals must be terminated.
111
What does specificity mean in the context of cell signaling?
It means the same signal can induce different responses in different cells because different types of cells have different collections of proteins, which lead to different cellular events.
112
How do the responses of liver and heart cells to epinephrine differ?
When epinephrine binds in a liver cell, it sends the message to break down glycogen into glucose. When it binds in a heart cell, it sends the message to contract more rapidly.
113
What are scaffolding proteins?
Scaffolding proteins are large relay proteins to which several other relay proteins are simultaneously attached. They increase speed and accuracy of signal transduction events.
114
Give an example of how scaffolding proteins work.
When a receptor is bound by a first messenger and initiates a response inside the cell, that response can bind to a scaffolding protein that is already bound to three different protein kinases (for example), thus eliminating the need for it to bind each kinase separately.
115
What are the six ways in which a signal is terminated?
1. The concentration of the first messenger decreases.
2. For GPCRs, the G protein hydrolyzes its bound GTP to GDP.
3. Second messengers are degraded or sequestered (for example, phosphodiesterase: cAMP -> AMP)
4. Protein phosphatases inactivate phosphorylated kinases.
5. Receptors become desensitized via phosphorylation by G-protein receptor kinases (GRKs).
6. Receptors are internalized.
116
What is the role of the insulin pathway?
It regulates glucose levels in the blood via a receptor tyrosine kinase (RTK).
117
What are the roles of insulin?
1. Activates glucose transporters.
2. Initiates glycolysis
3. Inhibits glycogen breakdown
4. Promotes glycogen synthesis.
5. Alters gene transcription.
6. Initiates the synthesis of new proteins.
118
After its creation, where does nitric oxide go, and where did it come from?
It moves from the endothelial cell to the smooth muscle cell through the plasma membrane.
