17.2 G proteins and Cyclic AMP Signaling

Question 1

process of transmitting signals from a receptor to a series of intracellular targets

Answer 1

signal transduction

Question 2

signal transduction is used by cell surface receptors to regulate the activity of ()

Answer 2

intracellular target enzymes

Question 3

targets of signal transduction frequently include ()

Answer 3

transcription factors that function to regulate gene expression

Question 4

largest family of cell surface receptors that transmits signals to intracellular targets via intermediary action of G proteins

Answer 4

G protein-coupled receptors

Question 5

guanine nucleotide-binding proteins

Answer 5

G proteins

Question 6

G protein-coupled receptors are structurally and functionally related proteins characterized by ()

Answer 6

7 membrane-spanning alpha helices

Question 7

give an overview of the action of G protein-coupled receptors

Answer 7

1. binding of ligands to extracellular domains of receptor induces conformational changes
2. cytosolic domain of the receptor activates a G protein associated with the inner place of the PM
3. activated G protein dissociates from the receptor and carries the signal to an intracellular target (e.g. enzyme or ion channel)

Question 8

G proteins consist of 3 subunits, designated ()

Answer 8

alpha, beta, and gamma

Question 9

G proteins are frequently called () to distinguish them from other guanine nucleotide-binding proteins (e.g. Ras, Ran, Arf, Rab, etc.)

Answer 9

heteromeric G proteins

Question 10

how do G protein-coupled receptors activate associated G proteins

Answer 10

stimulating the exchange of GDP for GTP

Question 11

in G proteins associated to receptors, () binds to guanine nucleotides that regulate G protein activity

Answer 11

alpha subunit

Question 12

overview of G protein regulation

Answer 12

1. In the inactive state, the alpha subunit is bound to GDP in a complex with beta and gamma
2. hormone binding stimulates the exchange of GDP for GTP
3. activated GTP-bound alpha subunit dissociates from beta-gamma complex to interact with its targets
4. activity of the alpha subunit is terminated by hydrolysis of bound GTP, stimulated by RGS (regulator of G protein signaling) proteins
5. inactive GDP-bound alpha subunit reassociates with the beta-gamma complex

Question 13

overview of regulation of glycogen breakdown by epinephrine (via G protein-coupled receptors)

Answer 13

1. receptor stimulation by epinephrine leads to G protein-mediated activation of adenyl cyclase → cAMP synthesis
2. cAMP activates protein kinase A; binding of cAMP to regulatory subunits induces conformational change that leads to dissociation of catalytic subunits (enzymatically active)
3. protein kinase A activates phosphorylates kinase → phosphorylase kinase activate glycogen phosphorylase
4. glycogen phosphorylase catalyzes breakdown of glycogen to glucose-1-phosphate

Question 14

G proteins act as physiological switches that regulate the activities of a variety of intracellular targets in response to extracellular signals; they are intermediaries in () activation

Answer 14

adenylyl cyclase

Question 15

an important second messenger that mediates cellular responses to a variety of hormones (e.g. epinephrine)

Answer 15

cyclic AMP (cAMP)

Question 16

cAMP usually acts by stimulating (), which phosphorylates and activates downstream targets

Answer 16

protein kinase A (aka cAMP-dependent protein kinase)

Question 17

protein kinase A is a tetramer consisting of ()

Answer 17

2 regulatory and 2 catalytic subunits

Question 18

ive an overview of protein kinase A stimulation

Answer 18

1. cAMP binds to regulatory subunits
2. regulatory subunits dissociate from the catalytic subunits
3. now free, the catalytic subunits are enzymatically active and able to phosphorylate serine residues on their target proteins

Question 19

enzyme responsible for cAMP formation from ATP

Answer 19

adenylyl cyclase

Question 20

cyclic ATP is degraded to cAMP by ()

Answer 20

cAMP phosphodiesterase

Question 21

in many cells, increases in cAMP activate the transcription of specific target genes that contain a regulatory sequence called the ()

Answer 21

cAMP response element (CRE)

Question 22

in the nucleus, protein kinase A phosphorylates a transcription factor called ()

Answer 22

CREB (CRE-binding proteins)

Question 23

protein phosphorylation is rapidly reversed by the action of () → remove phosphate groups from either phosphorylated tyrosine or serine/threonin residues

Answer 23

protein phosphatases

Question 24

protein phosphatases serve to (…)

Answer 24

terminate the responses initiated by receptor activation of protein kinases

Question 25

levels of phosphorylation of protein kinase A substrates (e.g. phosphorylase kinase, CREB) are determined by a (…)

Answer 25

balance between intracellular activities of protein kinase A and protein phosphatases

Question 26

overview of gene regulation by cAMP-dependent protein kinase (protein kinase A)

Answer 26

1. increases in cAMP concentration trigger the activation of target genes containing CRE