chapter 23 Signal Transduction Mechanisms: II. Messengers and Receptors Flashcards
(175 cards)
1
Q
What does the study of “Signal Transduction Mechanisms: II. Messengers and Receptors” focus on?
A
It explores how cells respond to nonneuronal signals
2
Q
How do cells produce signals?
A
Cells produce signals by displaying molecules on their surfaces or by releasing chemical signals
3
Q
What is the role of chemical signaling in multicellular organisms?
A
To regulate and coordinate the various activities of cells and tissues.
4
Q
How do multicellular organisms control specialized cell activities?
A
Through the release of chemical messengers
5
Q
What are the two main classifications of signaling molecules based on the distance from the production site to the target?
A
- Endocrine signals: Produced far from the target tissues and reach them via the circulatory system.
- Paracrine signals: Diffusible and act over a short range.
6
Q
What are juxtacrine and autocrine signals?
A
Juxtacrine signals: Require physical contact between sending and receiving cells.
Autocrine signals: Act on the same cell that produces them.
7
Q
In the context of cell-to-cell signaling, what is the importance of hormones and local mediators?
A
They facilitate communication between cells by acting as signaling molecules
8
Q
What is a ligand, and where can it bind?
A
A ligand is a messenger molecule that binds to a receptor, either on the surface of the target cell or inside it
9
Q
How do ligands bind to receptors?
A
Ligands bind to receptors at a closely fitting binding site (or binding pocket) where the necessary amino acid side chains of the receptor form chemical bonds with the messenger.
10
Q
What does “The Overall Flow of Information During Cell Signaling” refer to?
A
The process by which information is transmitted from a ligand binding to its receptor, leading to cellular responses.
11
Q
What is the first step in cell-cell signaling?
A
Ligand binding to a receptor.
12
Q
What are second messengers in signal transduction?
A
Molecules or ions produced within the cell following ligand binding, initiating further signaling events.
13
Q
What is signal transduction?
A
The ability of a cell to respond to ligand-receptor binding by altering its behavior or gene expression.
14
Q
How does ligand binding alter receptors?
A
It can change the receptor’s conformation, cause receptors to cluster together, or both, triggering signal transduction events.
15
Q
What does “preprogrammed” mean in the context of cellular responses?
A
It refers to a cell’s repertoire of functions, some of which are unused until triggered by a specific signal.
16
Q
How does receptor-ligand binding resemble enzyme-substrate interaction?
A
A receptor binds to a ligand in a manner similar to an enzyme binding its substrate, and the receptor is considered “occupied” when bound.
17
Q
What happens as ligand concentration increases?
A
Saturation is reached when most receptors are occupied by the ligand.
18
Q
What is receptor affinity?
A
The relationship between ligand concentration in solution and the number of receptors occupied.
19
Q
Define the dissociation constant (Kd).
A
The concentration of free ligand required to occupy half of the available receptors.
20
Q
How is receptor affinity related to Kd?
A
High receptor affinity corresponds to a low Kd, and vice versa.
21
Q
What is the equilibrium constant (Ka) formula?
A
Ka=[HR] / [H][R], where HR is the hormone-receptor complex, H is the hormone concentration, and R is the receptor concentration.
22
Q
How is the dissociation constant (Kd) calculated?
A
Kd = [H][R] / [HR]
23
Q
What is fractional occupancy in receptor-ligand binding?
A
The fraction of receptors occupied by a ligand, given by
[𝐻𝑅] / [𝑅] + [𝐻𝑅]
24
Q
What is the fractional occupancy when ligand concentration ([H]) is zero?
A
the fractional occupancy is 0
25
What happens to fractional occupancy when ligand concentration greatly exceeds Kd?
fractional occupancy approaches 1
26
What is the relationship between Kd and receptor saturation?
When the receptor is half saturated, the ligand concentration ([H]1/2) equals Kd
27
How can Kd for a simple receptor-ligand system be visualized?
Through concentration dependence of ligand binding and reciprocal plots, such as the Scatchard equation.
28
Why is measuring real-world Kd values challenging?
Because it is technically difficult and often done indirectly.
29
30
What are agonists and their function?
Agonists are drugs that activate the receptor they bind to.
31
What are antagonists and their function?
Antagonists bind receptors without triggering a change and prevent natural messengers from activating the receptor.
32
What are two common ways cells shut down receptor signaling?
1. Reducing the amount of free ligand.
2. Reducing receptor sensitivity or receptor numbers.
33
What is receptor desensitization?
A process where prolonged receptor occupation causes the cell to adapt and no longer respond to the ligand.
34
How can cells reduce receptor activity biochemically?
By adding or removing phosphate groups to/from amino acids, altering receptor affinity.
35
Provide an example of signal amplification.
A single epinephrine ligand binding to a liver cell receptor can release hundreds of millions of glucose molecules from glycogen.
36
What does signal transduction pathway amplification achieve?
It amplifies the cellular response to an external signal.
37
How do cell-cell signals act through receptors and signal transduction pathways?
Cells use a limited number of basic signaling pathways and receptors to process signals.
38
What types of ligands bind to transmembrane receptors?
Hydrophilic ligands, which include proteins, small peptides, amino acids and their derivatives, and nucleotides or nucleosides.
39
Where do hydrophobic ligands act, and what is their function?
Hydrophobic ligands bind to receptors in the cytosol and can enter the nucleus to regulate transcription of specific genes.
40
What are some examples of hydrophobic ligands?
Steroid hormones and retinoids.
41
What are the key types of signaling pathways for hydrophilic ligands?
1. Ligand-gated ion channels.
2. G protein−coupled receptors relying on GTP hydrolysis.
3. Receptors activating cytosolic enzymes like protein kinases.
42
What are G protein−coupled receptors (GPCRs)?
Receptors that, upon ligand binding, change conformation to activate a specific G protein (guanine-nucleotide binding protein).
43
How do GPCRs act via hydrolysis of GTP?
An activated G protein binds a target protein (enzyme or channel protein), altering its activity.
44
Give examples of G protein−coupled receptor targets.
Olfactory receptors, β-adrenergic receptors, and hormone receptors, including clinically important opioid receptors.
45
Describe the structure of GPCRs.
GPCRs have seven transmembrane α helices connected by cytosolic or extracellular loops. The extracellular portion contains a unique messenger-binding site.
46
How is the cytosolic portion of a GPCR significant?
It allows the receptor to interact with specific types of G proteins.
47
How are GPCRs regulated through phosphorylation?
G protein−coupled receptor kinases (GRKs) phosphorylate amino acids in the receptor's cytosolic domain, targeting activated receptors.
48
How is Protein Kinase A (PKA) involved in GPCR signaling?
PKA is activated by G protein-mediated signaling and can phosphorylate amino acids on the receptor, inhibiting it. This serves as an example of negative feedback during cell signaling.
49
How do G proteins act in signal transduction?
G proteins act as molecular switches, with "on" and "off" states depending on whether they are bound to guanosine triphosphate (GTP) or guanosine diphosphate (GDP).
50
What are the two types of G proteins, and what are their roles?
1. Heterotrimeric G proteins: Mediate signal transduction through GPCRs and have Gα, Gβ, and Gγ subunits.
2. Monomeric G proteins: Involved in smaller signaling pathways.
51
What happens when Gα binds to GTP?
Gα detaches from the Gβγ subunits and initiates signal transduction, depending on the G protein and cell type.
52
What is the function of Gβγ subunits in G protein signaling?
Gβγ subunits associate with GPCRs and can initiate signaling events, including opening ion channels or activating other proteins.
53
Describe the steps in the activation of G proteins.
1. Gα binds to GDP and detaches from Gβγ.
2. Gα binds to GTP and activates signal transduction by binding to target enzymes or proteins.
3. Gα hydrolyzes GTP to GDP, reassociating with Gβγ, shutting down the signaling pathway.
54
What regulates the activity of G proteins?
G protein activity is enhanced by regulators of G protein signaling (RGS) proteins and GTPase-activating proteins (GAPs), which promote GTP hydrolysis.
55
What is the primary function of G proteins in signal transduction?
To release or form second messengers, which amplify the cellular response.
56
How do the α and βγ subunits of G proteins engage in signaling?
The α subunits interact with enzymes like adenylyl cyclase and phospholipase C, while βγ subunits can activate other proteins, such as G protein-coupled receptor kinases.
57
What happens when acetylcholine binds the muscarinic acetylcholine receptor?
The βγ subunits of the G protein cause the opening of potassium channels in the membrane, which close again when acetylcholine is no longer present.
58
What are second messengers in G protein-mediated signaling?
Second messengers are molecules like cyclic AMP (cAMP) and calcium ions, which are crucial in mediating signal transduction.
59
How is cyclic AMP (cAMP) formed?
cAMP is formed from cytosolic ATP by the enzyme adenylyl cyclase, which is activated when bound to GTP-Gsα.
60
What activates adenylyl cyclase to produce cAMP?
Adenylyl cyclase is activated when GTP-Gsα binds to it after receptor-ligand binding, causing the release of Gsβγ.
61
How long do G proteins remain active, and why is this important?
G proteins remain active for a very short time, which allows them to quickly respond to changing conditions in the cell.
62
What happens when Gsα becomes inactive?
When Gsα becomes inactive, adenylyl cyclase stops producing cAMP, and the remaining cAMP is degraded by phosphodiesterase.
63
What is the main function of cAMP in signal transduction?
cAMP regulates protein kinase A (PKA) by separating its regulatory and catalytic subunits, which allows PKA to phosphorylate proteins.
64
What does PKA phosphorylate, and what is the energy source for this process?
PKA phosphorylates proteins on serine or threonine residues using ATP as the phosphate source.
65
How does an increase in cAMP concentration affect cells?
The effects of increased cAMP concentration vary between different cell types, affecting various cellular functions.
66
What disease is caused by the disruption of G protein signaling due to cholera toxin?
Cholera is caused by cholera toxin, which modifies Gs so that it cannot hydrolyze GTP, disrupting salt and fluid secretion in the intestine and leading to dehydration.
67
How does the pertussis toxin affect G protein signaling in whooping cough?
Pertussis toxin modifies Gi so it cannot inhibit adenylyl cyclase, causing fluid accumulation in the lungs and the persistent cough characteristic of whooping cough.
68
What is the role of inositol-1,4,5-trisphosphate (IP3) in signal transduction?
IP3 acts as a second messenger, generated from PIP2 when phospholipase C is activated, and plays a key role in the release of calcium ions.
69
How are inositol trisphosphate (IP3) and diacylglycerol (DAG) produced?
They are produced when phospholipase C cleaves PIP2 (phosphatidylinositol-4,5-bisphosphate), generating both IP3 and DAG.
70
What are some cell functions regulated by inositol trisphosphate (IP3) and diacylglycerol (DAG)?
Functions include cell growth, regulation of ion channels, changes in the cytoskeleton, increases in cellular pH, and effects on secretion of proteins and other substances.
71
What happens when a ligand binds its membrane receptor in the IP3 and DAG signaling pathway?
The binding activates a G protein called Gq, which then activates phospholipase C (Cβ), generating both IP3 and DAG.
72
What is the role of IP3 in signaling?
IP3 diffuses through the cytosol and binds to the IP3 receptor channel, causing calcium to be released into the cytosol, triggering a cellular response.
73
How do calcium and diacylglycerol (DAG) contribute to signaling?
They activate members of the protein kinase C (PKC) family, which then phosphorylate target proteins on serine and threonine residues, leading to various cellular effects.
74
What is the primary role of calcium in signaling?
Calcium plays an essential role in regulating various cellular functions, including muscle contraction, neurotransmitter release, and gene expression.
75
How are calcium concentrations regulated in the cell?
Calcium concentrations are maintained at low levels through calcium ATPases in the plasma membrane and endoplasmic reticulum (ER), which transport calcium out of the cytosol.
76
What is the function of calcium ATPases?
Calcium ATPases transport calcium out of the cell or sequester it in the lumen of the ER, helping maintain low cytosolic calcium concentrations.
77
How do some cells further reduce cytosolic calcium concentration?
Some cells use sodium-calcium exchangers to help reduce the cytosolic calcium concentration.
78
How is calcium transported within the cell?
Mitochondria transport calcium into the mitochondrial matrix to regulate calcium levels.
79
What causes the release of calcium ions in signaling processes?
Calcium ions can be released by opening calcium channels in the plasma membrane, as seen in neuronal signaling, or through activation of GPCRs and other receptors.
80
How does calcium-induced calcium release work?
A rapid increase in calcium concentration can open IP3 receptor channels and ryanodine receptor channels in the ER, releasing more calcium into the cytoplasm
81
How can calcium levels be measured in cells?
Calcium-sensitive fluorescent dyes (calcium indicators) are used to demonstrate the importance of calcium release in signaling.
82
What effect do calcium ionophores have on cells?
Calcium ionophores release calcium from internal stores in the absence of a stimulus, further implicating calcium as an intermediary in IP3 signaling.
83
How is calcium released following the fertilization of animal eggs?
Sperm activation triggers the release of calcium from internal stores, starting at the site of sperm entry and spreading across the egg surface like a wave.
84
What are the two key roles of calcium release during fertilization?
1. Stimulating cortical granules to release their contents and alter the vitelline envelope, preventing polyspermy.
2. Inducing egg activation, resuming metabolic processes and reorganizing egg contents for embryonic development.
85
How does calcium bind to effector proteins?
Calcium can directly bind to effector proteins, altering their activity, with calmodulin being a key mediator in calcium-activated processes.
86
Describe the structure and function of calmodulin.
Calmodulin has an arm-like structure with two "hands," each capable of binding two calcium ions. The binding induces a conformational change, activating calmodulin to bind proteins with calmodulin-binding sites, dramatically altering their activity.
87
What is the role of calmodulin in signal transduction?
Calmodulin mediates calcium-activated processes by binding to proteins and altering their activity, helping to regulate various cellular functions.
88
What are enzyme-coupled receptors, and how do they function?
Enzyme-coupled receptors are a family of receptors that are enzymes, typically kinases. Ligand binding stimulates their kinase activity, initiating a phosphorylation cascade.
89
What types of kinases are involved in enzyme-coupled receptor signaling?
Kinases involved are either tyrosine kinases or serine/threonine kinases.
90
How do growth factors relate to receptor kinases?
Growth factors bind to receptor kinases, stimulating cellular growth. These include factors like insulin and fibroblast growth factor (FGF).
91
What is the difference between plasma and serum in blood?
Plasma is whole blood without red and white blood cells, including platelets. Serum is the clear fluid remaining after blood clots, containing growth factors secreted by platelets.
92
What is platelet-derived growth factor (PDGF), and what does it do?
PDGF is a growth factor secreted by platelets that stimulates fibroblasts to form new connective tissue.
93
What type of receptor does PDGF bind to?
PDGF binds to receptor tyrosine kinases.
94
Name some growth factors that stimulate receptor tyrosine kinases.
- Insulin
- Insulin-like growth factor-1
- Fibroblast growth factor
- Epidermal growth factor
- Nerve growth factor
95
What are growth factor families?
Groups of signaling molecules that stimulate cell growth, division, and differentiation.
96
Name three examples of growth factors and their receptor types.
- Epidermal growth factor (EGF) → Tyrosine kinase receptor
- Platelet-derived growth factor (PDGF) → Tyrosine kinase receptor
- Fibroblast growth factor (FGF) → Tyrosine kinase receptor
97
What is a common structural feature of receptor tyrosine kinases?
A single polypeptide chain with one transmembrane segment.
98
What are the two primary domains of receptor tyrosine kinases?
The extracellular ligand-binding domain and the cytosolic tyrosine kinase domain.
99
What happens when receptor tyrosine kinases bind their ligands?
Signal transduction is initiated, involving receptor dimerization or clustering and subsequent autophosphorylation.
100
What is autophosphorylation?
A process where receptor tyrosine kinases phosphorylate themselves on tyrosine residues following ligand binding.
101
What occurs during the activation of fibroblast growth factor (FGF) receptors?
The receptors dimerize and phosphorylate each other.
102
What occurs during the activation of epidermal growth factor (EGF) receptors?
Ligand binding leads to receptor clustering.
103
What role does autophosphorylation play in signaling?
It creates binding sites for cytosolic proteins that initiate downstream signaling pathways.
104
What proteins are recruited by receptor tyrosine kinases after autophosphorylation?
Cytosolic proteins with SH2 domains, which recognize phosphotyrosine residues.
105
What is the SH2 domain?
A specific protein domain that binds phosphotyrosine residues and facilitates recruitment to activated receptors.
106
What signaling cascade is initiated by receptor tyrosine kinases?
A Ras-MAP kinase cascade, among other pathways, leading to cellular responses such as proliferation or differentiation
107
What is Ras, and why is it important?
Ras is a small monomeric G protein that regulates cell proliferation. It is active only when bound to GTP.
108
How does Ras acquire GTP to become active?
Ras requires assistance from a guanine-nucleotide exchange factor (GEF) called Sos, which helps it release GDP and bind GTP.
109
How does Sos become active?
Sos binds the receptor through GRB2, which has an SH2 domain, forming a complex with the receptor.
110
What happens after Ras is activated?
Activated Ras triggers a series of phosphorylations, starting with Raf, a protein kinase.
111
What does Raf do in the Ras signaling pathway?
Raf phosphorylates serine and threonine residues in MEK, another protein kinase.
112
What is MEK's role in Ras signaling?
MEK phosphorylates threonine and tyrosine residues in mitogen-activated protein kinases (MAPKs).
113
What do MAPKs do?
MAPKs activate transcription factors (e.g., Jun and Ets family members) that regulate genes for cell growth and division.
114
How is Ras inactivated?
A GTPase activating protein (GAP) facilitates GTP hydrolysis, converting active GTP-bound Ras into inactive GDP-bound Ras.
115
What is a dominant negative mutation in RTKs?
It occurs when mutant receptors dimerize with normal receptors but fail to autophosphorylate, interfering with normal receptor function.
116
What is a constitutively active mutation?
A mutation that keeps receptors active even without ligand binding, such as the mutation in FGFR-3 that causes achondroplasia.
117
How were RTK signaling pathways studied?
Genetic analysis, such as studying Ras mutations in the Drosophila compound eye, linked specific proteins to signaling pathways.
118
What is the structure of a normal Drosophila compound eye?
A closely formed smooth mosaic structure with 6 structures and an R7 photoreceptor at the center of each ommatidium.
119
What happens to the Drosophila eye in a sevenless mutant?
The R7 photoreceptor is absent, leaving only six cells per ommatidium.
120
What is the function of the Sevenless receptor in Drosophila?
A: It is activated by the Boss ligand on the neighboring R8 cell, which is crucial for R7 photoreceptor development.
121
Q: What was discovered by studying mutations in a sensitized background?
A: Key components of the Sevenless (sev) signaling pathway were identified, which are essential for RTK signaling.
122
Q: How does the sev pathway connect to Ras?
A: The sev pathway activates Ras, as evidenced by R7 development in double mutants lacking functional Sevenless but with a dominant active Ras mutation.
123
Q: What other signaling pathways do receptor tyrosine kinases (RTKs) activate?
A: RTKs can activate phospholipase Cγ (producing IP3 and DAG) and PI 3-kinase (phosphatidylinositol-3-kinase).
124
Q: How does phospholipase Cγ interact with RTKs?
A: It contains an SH2 domain and binds to the RTK to become activated.
125
Q: Which growth factors signal through receptor serine-threonine kinases?
A: The transforming growth factor β (TGF β) family of growth factors.
126
Q: What initiates TGF β signaling?
A: The binding of TGF β to its transmembrane receptor, involving type I and type II receptors.
127
Q: What happens after TGF β binds its receptors?
A: The type II receptor phosphorylates the type I receptor, initiating a signal transduction cascade by phosphorylating Smads.
128
Q: What are the three types of Smads?
A:
- R-Smads (receptor-regulated Smads): Phosphorylated by the receptor complex.
- Smad4: Forms a multiprotein complex with phosphorylated R-Smads to enter the nucleus.
- Inhibitory Smads: Regulate and inhibit the TGF β pathway.
129
Q: What is the function of the Smad complex?
A: It enters the nucleus to regulate gene expression in response to TGF β signaling
130
Q: What is the primary function of the tyrosine phosphatase receptor family?
A: They remove phosphate groups from tyrosine receptors to regulate tyrosine receptor kinases.
131
Q: What does the guanylyl cyclase receptor family produce?
A: Cyclic GMP (cGMP).
132
Q: Where is cGMP signaling important?
A: In photoreceptor cells of the retina, fluid regulation in the gut, and vasodilation.
133
Q: Do signal transduction pathways act in isolation?
A: No, cells coordinate responses to multiple simultaneous signals.
134
Q: What mechanisms help coordinate simultaneous signaling pathways?
A: Localizing signaling complexes via scaffolds and integration of different signaling pathways.
135
Q: What is the role of scaffolding complexes in signaling?
A: They assemble signaling components, such as those in the Ras pathway, into multiprotein complexes to improve efficiency.
136
Q: How does scaffolding work in yeast mating signaling?
A: The Ste5 scaffolding protein recruits all kinases in the cascade into a large complex at the plasma membrane, mediated by activated Gβγ subunits.
137
Q: What large-scale changes occur in yeast mating signaling?
A: Polarized secretion, cytoskeletal changes, and gene expression changes.
138
Q: What is signaling crosstalk?
A: When activated components from one pathway affect components of another pathway.
139
Q: Why is signaling considered a network rather than a linear process?
A: Because multiple pathways can interact, integrate, and converge on the same molecules or influence each other.
140
Q: Can a single receptor activate multiple pathways?
A: Yes, single receptors can activate multiple pathways, and multiple pathways can converge
141
Q: What are the two key outcomes of many signaling pathways?
A: Production of second messengers (e.g., IP3 and calcium ions) and phosphorylation of target proteins.
142
Q: Why is there significant opportunity for crosstalk in signaling?
A: Because many pathways intersect and share common second messengers and phosphorylation targets.
143
Q: What are hormones in plants and animals used for?
A: Coordinating signals across different tissues and organs.
144
Q: How do endocrine hormones travel to target tissues?
A: Via the circulatory system, after being secreted by endocrine tissues directly into the bloodstream.
145
Q: How long can endocrine hormones persist in the bloodstream?
A: Their life spans range from a few seconds to many hours.
146
Q: What are the four chemical categories of endocrine hormones?
- Amino acid derivatives (e.g., epinephrine)
- Peptides (e.g., vasopressin)
- Proteins (e.g., insulin)
- Lipid-like hormones (e.g., steroids such as testosterone)
147
Q: What is the role of adrenergic hormones in stress?
A: They redirect resources to the heart and skeletal muscles by stimulating glycogen breakdown to supply glucose to muscles.
148
Q: Which glands produce adrenergic hormones?
A: The adrenal glands
149
: What are the two types of adrenergic receptors and their primary ligands?
- α-adrenergic receptors: Bind epinephrine and norepinephrine; regulate flow to visceral organs.
- β-adrenergic receptors: Bind epinephrine better than norepinephrine; regulate flow to heart, lungs, and skeletal muscles.
150
Q: How are α- and β-adrenergic receptors linked to G proteins?
A:
- α-adrenergic receptors: Linked to Gq proteins.
- β-adrenergic receptors: Linked to Gs proteins, activating the cAMP pathway.
151
Q: What enzyme facilitates glycogen breakdown, and what is the product?
A: Glycogen phosphorylase facilitates breakdown, releasing glucose-1-phosphate.
152
Q: What initiates glycogen degradation in response to adrenergic hormones?
A: Binding of epinephrine to a β-adrenergic receptor on liver or muscle cells, activating the Gs protein and adenylyl cyclase
153
Q: What enzyme does adenylyl cyclase generate and what does it activate?
A: Adenylyl cyclase generates cAMP from ATP, which activates protein kinase A (PKA).
154
Q: What does PKA phosphorylate to activate glycogen breakdown?
A: PKA phosphorylates phosphorylase kinase, converting glycogen phosphorylase from its less active b form to the active a form.
155
Q: How does cAMP regulate glycogen synthesis?
A: cAMP stimulates the inactivation of glycogen synthesis by phosphorylating and inactivating glycogen synthase via PKA.
156
Q: How do α1-adrenergic receptors affect smooth muscle?
A: They stimulate the IP3 pathway, increasing intracellular Ca²⁺ levels, leading to smooth muscle contraction and reduced blood flow.
157
Q: What happens to blood vessels when α1-adrenergic receptors are activated?
A: Blood vessels constrict, reducing blood flow.
158
Q: What hormones regulate glucose levels, and what are their roles?
A:
- Glucagon: Increases blood glucose through glycogen breakdown.
- Insulin: Reduces blood glucose by stimulating glucose uptake and glycogen synthesis.
159
Q: How do glucagon and epinephrine act on cells?
A: Both act via G protein-coupled receptors to stimulate glycogen breakdown.
160
Q: What is the cause and treatment of Type I diabetes?
A: It is caused by a loss of insulin-producing cells in the pancreas and can be treated with insulin.
161
Q: Why is Type II diabetes not effectively treated with insulin?
A: It results from resistance to insulin rather than a lack of it.
162
Q: What kind of receptor does insulin bind to?
A: Receptor tyrosine kinases with two α and two β subunits.
163
Q: What happens when insulin binds to its receptor?
A: The β subunits phosphorylate insulin receptor substrate 1 (IRS-1), initiating two pathways.
164
Q: What are the two pathways activated by phosphorylated IRS-1?
A:
Ras pathway activation via GRB2 recruitment.
PI3K pathway activation, converting PIP2 into PIP3.
165
Q: What enzyme opposes PI3K and how?
A: PTEN, a phosphatase, removes a phosphate from PIP3, preventing Akt activation
166
Q: What does PIP3 activate, and what enzyme opposes this activation?
A: PIP3 activates Akt (protein kinase B), while PTEN opposes this activation.
167
Q: What are the two key consequences of Akt activation?
A:
GLUT4 movement to the plasma membrane, allowing glucose uptake.
Phosphorylation of glycogen synthase kinase 3 (GSK3), enhancing glycogen synthesis.
168
Q: What does Figure 23.27 illustrate?
A: The Insulin Signaling Pathway, showing the cascade initiated by insulin binding its receptor.
169
Q: Where do steroid hormones bind, and what do they influence?
A: Steroid hormones bind to cytosolic receptors, primarily acting in the nucleus to mediate gene transcription.
170
Q: Name examples of steroid hormones.
A: Progesterone, estrogen, testosterone, and glucocorticoids.
171
Q: How do steroid hormones enter the cell?
A: Being hydrophobic, they pass easily through the plasma membrane.
172
Q: What happens after a steroid hormone binds its receptor?
A: The receptor-hormone complex activates or inhibits the transcription of specific target genes.
173
Q: What does Figure 23.28 depict?
A: The activation of gene transcription by steroid hormone receptors.
174
Q: What are examples of dissolved gases that serve as signals in animals and plants?
A:
- Animals: Oxygen (O₂) and carbon dioxide (CO₂) in respiration.
- Plants: Ethylene gas in fruit ripening.
175
Q: What is nitric oxide (NO), and where is it especially important?
A: Nitric oxide is a gaseous signal, especially important in the nervous system.
