57.The answer is e.(Alberts, pp 868–870. Junqueira, pp 28–30.)Albuterol
binds to the β-receptor initiating a cyclic AMP (cAMP) signal transduction
cascade. The structure labeled B is the stimulatory G protein (GS). The figure
illustrates the response of the β-adrenergic receptor to ligand binding. β-
receptors mediate the tissue effects of epinephrine and norepinephrine and
respond to pharmacological agents such as albuterol, a β-adrenergic agonist.
Signal transduction following ligand binding involves a specific G protein
[shorthand for guanosine-triphosphate (GTP)–binding regulatory protein]. G
proteins associated with increasing cAMP levels in the cell are known as stim-
ulatory G proteins (GS) because of their role in enzyme activation. In the inac-
tive state, GSis bound to GDP. After ligand binding, a GSbinding site is
exposed and the GSprotein (B in the figure) binds to the βreceptor. The
resulting complex is capable of binding GTP in exchange for GDP, activating
the G protein. The αsubunit of the activated GSprotein exchanges GDP for
GTP and activates adenylate cyclase (Ain the figure). Intrinsic GTPase activ-
ity of the α-subunit is increased resulting in a short activation time for the
complex and recycling of the subunits to the inactive state. The inactive
cAMP-dependent kinase is labeled Cin the figure. It is the phosphorylating
action of cAMP-dependent protein kinase (kinase A), stimulated by increased
intracellular cAMP concentration, that affects many aspects of intracellular
metabolism and function. Phosphorylation (E)stimulates exocytosis and
induces nuclear changes, including transcriptional events. The star in the
figure delineates the site of ligand-binding-induced conformational change,
exposing the GS-binding site. (See table in the answer for question 36.)
58.The answer is c.(Alberts, pp 859–860.)Bethanechol binds to mus-
carinic receptors, Molecule A in the figure is phospholipase C that catalyzes
the formation of diacylglycerol (DAG) and inositol triphosphate (IP 3 ) from
phosphatidylinositol 4,5–bisphosphate (PIP 2 ). The phosphoinositide (PI)
cycle illustrated in the figure is based on the formation of PIP 2 in the inner
leaflet of the plasma membrane. Breakdown of PIP 2 leads to the formation
of the key functional agents of the PI cycle. Binding of a ligand to its G
protein–linked receptor on the cell surface (Gq) activates a phosphoinosi-
tide-specific phospholipase C. PI-specific phospholipase C hydrolyzes PIP 2
to form DAG and IP 3. Those two molecules function differently to regulate
intracellular function. IP 3 functions in the mobilization of calcium, while DAG
activates protein kinase C (answer a),leading to multiple phosphorylations
Cell Biology: Intracellular Trafficking Answers 135