Chapter 9

Ca2+ as a second messenger

- stimulate contraction or secretion -binds to calmodulin to activate a protein kinase.

What are the effects of protein kinase A (PKA)?

-In kidney, the action of PKA leads to the production of water channels that can increase the permeability of tubule cells to water - In muscle cell: it activates an enzyme necessary to break down glycogen and inhibits another enzyme to synthesize glycogen. This leads to an increase in glucose available to the muscle.

Adenylyl cyclase is responsible for the production of which second-messenger molecule? a. Cyclic AMP b. Calcium c. IP3 d. Calmodulin

A

During a protein kinase signal cascade— a. sequential phosphorylation of different kinases leads to a change in gene expression b. multiple G proteins become activated c. phosphorylation of adapter proteins leads to the formation of second messengers d. the number of MAP kinase proteins present in the cytoplasm is amplified

A

In general, a signal transmitted via phosphorylation of a series of proteins A) brings a conformational change to each protein. B) requires binding of a hormone to a cytosol receptor. C) cannot occur in yeasts because they lack protein phosphatases. D) requires phosphorylase activity. E) allows target cells to change their shape and therefore their activity.

A

Of the following, a receptor protein in a membrane that recognizes a chemical signal is most similar to A) the active site of an allosteric enzyme that binds to a specific substrate. B) tRNA specifying which amino acids are in a polypeptide. C) a metabolic pathway operating within a specific organelle. D) an enzyme having an optimum pH and temperature for activity. E) an antibody in the immune system.

A

What is the function of the tight junctions in the formation of a tissue? a. Tight junctions connect one cell to the next, creating a barrier between the cells. b. Tight junctions form a strong anchor between two cells. c. Tight junctions allow for the movement of small molecules between cells. d. Tight junctions connect the cell to the extracellular matrix.

A

Which of the following best describes the immediate effect of ligand binding to a G protein-coupled receptor? a. the G protein trimer releases a GDP and binds a GTP b. the G protein trimer dissociates from the receptor c. the G protein trimer interacts with an effector protein d. the α subunit of the G protein becomes phosphorylated

A

Which of the following best describes the immediate effect of ligand binding to a G protein-coupled receptor? a. The G protein trimer releases a GDP and binds a GTP. b. The G protein trimer dissociates from the receptor. c. The G protein trimer interacts with an effector protein. d. The α subunit of the G protein becomes phosphorylated

A

Which of the following is true for the signaling system in an animal cell that lacks the ability to produce GTP? A) It would not be able to activate and inactivate the G protein on the cytoplasmic side of the plasma membrane. B) It could activate only the epinephrine system. C) It would be able to carry out reception and transduction but would not be able to respond to a signal. D) It would use ATP instead of GTP to activate and inactivate the G protein on the cytoplasmic side of the plasma membrane. E) It would employ a transduction pathway directly from an external messenger.

A

Which of the following most likely would be an immediate result of growth factor binding to its receptor? A) protein kinase activity B) adenylyl cyclase activity C) GTPase activity D) protein phosphatase activity E) phosphorylase activity

A

When epinephrine binds to its G-protein linked receptor, _____ is activated producing _____.

Adenyl cyclase, cAMP

How are proteins changed its composition?

By protein kinase (phosphorylation) and phosphatase (dephosphorylation)

How does the function of an intracellular receptor differ from that of a membrane receptor? a. The intracellular receptor binds a ligand. b. The intracellular receptor binds DNA. c. The intracellular receptor activates a kinase. d. The intracellular receptor functions as a second messenger

B

In the case of paracrine signaling the ligand is— a. produced by the cell itself b. secreted by neighboring cells c. present on the plasma membrane of neighboring cells d. secreted by distant cells

B

In which of the following ways do plant hormones differ from hormones in animals? A) Plant hormones interact primarily with intracellular receptors. B) Plant hormones may travel in air or through vascular systems. C) Animal hormones are found in much greater concentration. D) Plant hormones are synthesized from two or more distinct molecules. E) Animal hormones are primarily for mating and embryonic development.

B

Signal transduction pathways: a. are necessary for signals to cross the membrane b. include that intracellular events stimulated by an extracellular signal c. include extracellular events stimulated by an intracellular signal d. are only found in cases where the signal can cross the membrane

B

Since steroid receptors are located intracellularly, which of the following is true? A) The receptor molecules are themselves lipids or glycolipids. B) The steroid/receptor complex can cross the nuclear membrane. C) The unbound steroid receptors are quickly recycled by lysosomes. D) The concentration of steroid receptors must be relatively high in most cells. E) The receptor molecules are free to move in and out of most organelles.

B

The action of steroid hormones is often longer-lived than that of peptide hormones. This is because they: a. enter the cell and act like enzymes for a longer period of time b. they turn on gene expression to produce proteins that persist in the cell c. result in the production of second messengers that act directly on cellular processes d. stimulate G proteins that act directly on cellular processes

B

The amplification of a cellular signal requires all of the following except— a. a ligand b. DNA c. a second messenger d. a protein kinase

B

The response to signaling through G protein-coupled receptors can vary in different cells because— a. all receptors act through the same G protein b. different isoforms of a receptor bind the same ligand but activate different effectors c. the amount of receptors in the membrane differs in different cell types d. different receptors can activate the same effector

B

Which of the following is NOT an example of how differential gene regulation affects cellular response? A. The expression of proteins that are controlled by signal transduction pathways may vary in different cell types. B. Cell junctions passing ions between the cytosol of one cell to another cell. C. A cell may or may not express a receptor for a particular signaling molecule. D. The expression of proteins involved in intracellular signal transduction pathways may vary in different cell types E. Different cell types have different cell surface receptors that recognize the same signaling molecules.

B

Which of the following statements about steroid receptors is FALSE? A. The receptors undergo a conformational change after binding the steroid. B. The receptors are localized to the plasma membrane. C. Once the steroid binds the receptor, the steroid/receptor complexes dimerize. D. Steroid receptors act as transcriptional factors, regulating the expression of certain genes and the subsequent production of proteins. E. Steroid receptors are localized to either the cytoplasm or nucleus

B

A neurotransmitter functions as a ligand in which type of signaling? a. Direct contact b. Endocrine c. Synaptic signaling d. Autocrine

C

Adenylyl cyclase has the opposite effect of which of the following? A) protein kinase B) protein phosphatase C) phosphodiesterase D) phosphorylase E) GTPase

C

Cadherins and intermediate filament proteins are associated with __________, whereas connexons are associated with ____________ a. tight junctions; anchoring junctions b. cell surface markers; tight junctions c. desmosomes; gap junctions d. adherens junctions; plasmodesmata

C

Caffeine is an inhibitor of phosphodiesterase. Therefore, the cells of a person who has recently consumed coffee would have increased levels of A) phosphorylated proteins. B) GTP. C) cAMP. D) adenylyl cyclase. E) activated G proteins.

C

Different receptors can have the same effect on a cell. One reason for this is that: a. most receptors produce the same second messenger b. different isoforms of receptors bind different ligands, but stimulate the same signaling pathway c. signal transduction pathways intersect: the same pathway can be stimulated by different receptors d. all receptors converge on the same signal transduction pathways

C

In the formation of biofilms, such as those forming on unbrushed teeth, cell signaling serves which function? A) formation of mating complexes B) secretion of apoptotic signals C) aggregation of bacteria that can cause cavities D) secretion of substances that inhibit foreign bacteria E) digestion of unwanted parasite populations

C

Receptor tyrosine kinases (RTKs) are found at high levels on various cancer cells. A protein, Herceptin, has been found to bind to an RTK known as HER2. This information can now be utilized in breast cancer treatment if which of the following is true? A) If Herceptin is found in the breast lymph nodes of the patient. B) If HER2, administered by injection, is in sufficient concentration. C) If the patient's cancer cells have detectable HER2. D) If the patient's genome codes for the HER2 receptor. E) If the patient's genome codes for the manufacture of Herceptin.

C

The function of a ________ is to add phosphates to proteins, whereas a _________ functions to remove the phosphates. a. tyrosine; serine b. protein phosphatase; protein dephosphatase c. protein kinase; protein phosphatase d. receptor; ligand

C

The ion Ca2+ can act as a second messenger because it is: a. produced by the enzyme calcium synthase b. normally at high levels in the cytoplasm c. normally at a low level in the cytoplasm d. stored in the cytoplasm

C

The receptors for steroid hormones and peptide hormones are fundamentally different because: a. of the great difference in size of the molecule b. peptides are one of the four major polymers and steroids are simple ringed structures c. peptides are hydrophilic and steroids are hydrophobic d. peptides are hydrophobic and steroids are hydrophilic

C

Which method of communication is most often observed between nerve cells? (p. 167) A) Paracrine B) Direct contact C) Synaptic D) Endocrine E) Two of the above

C

Which of the following is LEAST likely involved in calcium signaling? A. IP3 B. calmodulin C. protein kinase A D. protein kinase C E. phospolipase C

C

Which of the following is characterized by a cell releasing a signal molecule into the environment, followed by a number of cells in the immediate vicinity responding? A) hormonal signaling B) autocrine signaling C) paracrine signaling D) endocrine signaling E) synaptic signaling

C

Which of the following is the best explanation for the inability of a specific animal cell to reduce the Ca2⁺ concentration in its cytosol compared with the extracellular fluid? A) blockage of the synaptic signal B) loss of transcription factors C) insufficient ATP levels in the cytoplasm D) low oxygen concentration around the cell E) low levels of protein kinase in the cell

C

An inhibitor of phosphodiesterase activity would have which of the following effects? A) block the response of epinephrine B) decrease the amount of cAMP in the cytoplasm C) block the activation of G proteins in response to epinephrine binding to its receptor D) prolong the effect of epinephrine by maintaining elevated cAMP levels in the cytoplasm E) block the activation of protein kinase A

D

In comparing small G proteins like Ras and GPCR proteins, we can say that: a. both proteins have intrinsic GTPase activity that stops signaling b. both proteins are active bound to GTP c. Ras is active bound to GDP and GPCRs are active bound to GTP d. both a and b are true

D

In yeast signal transduction, the yeast cells A) must physically and directly interact. B) produce signal molecules that change themselves so they can interact with one another. C) produce response molecules that diffuse to other yeast cells. D) secrete molecules that result in response by other yeast cells. E) mate, after which the new cells secrete hybrid signals.

D

One of the major categories of receptors in the plasma membrane reacts by forming dimers, adding phosphate groups, and then activating relay proteins. Which type does this? A) G protein-coupled receptors B) ligand-gated ion channels C) steroid receptors D) receptor tyrosine kinases

D

Signaling through receptor tyrosine kinases often: a. leads to the production of the second messenger cAMP b. leads to the production of the second messenger IP3 c. stimulates gene expression directly d. leads to the activation of a cascade of kinase enzymes

D

The receptors for a group of signaling molecules known as growth factors are often A) ligand-gated ion channels. B) G protein-coupled receptors. C) cyclic AMP. D) receptor tyrosine kinases. E) neurotransmitters.

D

The same signal can have different effects in different cells because there: a. are different receptor subtypes that initiate different signal transduction pathways b. may be different coactivators in different cells c. may be different target proteins in different cells' signal transduction pathways d. all of the choices are correct

D

What is a ligand? a. An integral membrane protein associated with G proteins b. A DNA-binding protein that alters gene expression c. A cytoplasmic second-messenger molecule d. A molecule or protein that can bind to a receptor

D

What is a ligand? (p. 168) A) Something that crosses the plasma membrane B) A molecule which infuses itself into the fatty acid region of a phospholipids bilayer C) Structures which contain a lot of energy D) A substance which binds to receptors on the surface of a cell E) A chemical which alters the primary structure of a protein

D

What is the function of Ras during tyrosine kinase cell signaling? a. It activates the opening of channel-linked receptors. b. It synthesizes the formation of second messengers. c. It phosphorylates other enzymes as part of a pathway. d. It links the receptor protein to the MAP kinase pathway.

D

Which of the following is true of transcription factors? A) They regulate the synthesis of DNA in response to a signal. B) They transcribe ATP into cAMP. C) They initiate the epinephrine response in animal cells. D) They control gene expression. E) They regulate the synthesis of lipids in the cytoplasm.

D

Which of the following receptor types is not a membrane receptor? a. Channel-linked receptor b. Enzymatic receptor c. G protein-coupled receptor d. Steroid hormone receptors

D

Which of the following type(s) of membrane receptors functions by changing the phosphorylation state of proteins in the cell? a. Channel-linked receptor b. Enzymatic receptor c. G protein-coupled receptor d. Both b and c

D

All of the following are general components of the growth factor signaling pathway EXCEPT A. tyrosine kinase-linked receptor. B. GRB2-Sos-Ras complexes. C. phosphorylated transcription factors. D. Raf-1. E. IP3 and Ca2

E

Because most receptors are membrane proteins, which of the following is usually true? A) They lead to changes in intracellular ion concentration. B) They open and close in response to protein signals. C) They are only attached to one membrane surface: exterior or interior. D) They preferentially bind with lipid or glycolipid signal molecules. E) They change their conformation after binding with signal polypeptides.

E

Which of the following are common secondary messengers? (p. 170) A) ATP B) cAMP C) Calcium D) All of the above E) Two of the above

E

Which of the following receptor and signal transduction molecules are mismatched? A. sensor kinase receptor (NarQ): NarL B. G-protein coupled receptor: adenylyl cyclase C. G-protein coupled receptor: phospholipase C D. enzyme linked receptors: protein kinase E. steroid receptor: calcium

E

Which of the following would be inhibited by a drug that specifically blocks the addition of phosphate groups to proteins? A) G protein-coupled receptor signaling B) ligand-gated ion channel signaling C) adenylyl cyclase activity D) phosphatase activity E) receptor tyrosine kinase activity

E

When epidermal growth factor binds to its_____ the receptor phosphorylates itself triggering a signal transduction pathway.

Enzyme linked receptor

What is cholera and why is it classified as a disease?

Now that we know about the role of cAMP in G protein signaling pathways, we can explain in molecular detail how certain microbes cause disease. Consider cholera, a disease that is frequently epidemic in places where the water supply is contaminated with human feces. People acquire the cholera bacterium, Vibrio cholerae, by drinking contaminated water. The bacteria form a biofilm on the lining of the small intestine and produce a toxin. The cholera toxin is an enzyme that chemically modifies a G protein involved in regulating salt and water secretion. Because the modified G protein is unable to hydrolyze GTP to GDP, it remains stuck in its active form, continuously stimulating adenylyl cyclase to make cAMP. The resulting high concentration of cAMP causes the intestinal cells to secrete large amounts of salts into the intestines, where water following by osmosis. An infected person quickly develops profuse diarrhea and if left untreated can soon die from the loss of water and salts.

PIP3 as a second messenger

PIP3 moves into cytosol and PIP3 triggers release Ca2+ at the same time

Following its initial activation, how does a G protein become inactivated?

The G-protein alpha subunit GTP is hydrolyzed to GDP allowing reassociation of the G-protein alpha subunit and beta-gamma subunit

How does an animal cell maintain its calcium ion concentration.

The calcium concentration in the cytosol is usually much lower than in the extracellular fluid and ER> Protein pumps in the plasma membrane and the ER membrane, driven by ATP, move Ca2+ from the cytosol into the extracellular fluid and into the lumen of the ER. Mitochondrial pumps, driven by chemiosmosis, move Ca2+ into mitochondria when the calcium level in the cytosol rises significantly.

How does cAMP work as a second messenger in a G protein signaling pathway?

The first messenger activates a G protein-coupled receptor, which activates a specific G protein. In turn, the G protein activates adenylyl cyclase, which catalyzes the conversion of ATP to cAMP. The cAMP then acts as a second messenger and activates another protein, usually protein kinase A, leading to cellular responses.

Function of signal transduction

The function of signal transduction is to change the behavior or nature of the cell. This function require the change in the composition of protein

Why is calcium considered a third messenger sometimes?

Why is calcium considered a third messenger sometimes? In response to a signal relayed by a signal transduction pathway, the cytosolic calcium level may rise, usually by a mechanism that releases calcium from the cell's ER. The pathways leading to calcium release involve still other messengers, inositol triphosphate (IP3) and diacylglycerol (DAG). These two messengers are produced by cleavage of a certain kind of phospholipid in the plasma membrane. Because IP3 acts before calcium in these pathways, calcium could be considered a "third messenger." However, scientists use the term second messenger for all small, nonprotein components of signal transduction pathways.

Testosterone functions inside a cell by A) acting as a signal receptor that activates tyrosine kinases. B) binding with a receptor protein that enters the nucleus and activates specific genes. C) acting as a steroid signal receptor that activates ion channel proteins. D) becoming a second messenger that inhibits adenylyl cyclase. E) coordinating a phosphorylation cascade that increases spermatogenesis.

b

What type of signaling occurs when insulin is secreted from the pancreas and acts on muscle cells to increase glucose uptake?

endocrine

DAG as a second messenger

remains in the membrane and activates protein kinase C.