d4: signaling II (cyclic-amp, g-proteins, and ADP-ribosylations
Terms in this set (46)
effectors frequently regulate cell functions by interacting with _____ in the cell membrane which then generates an intracellular signal termed _____.
Examining regulation of glycogenolysis by the ________ illustrates principles applicable to this type of system.
- G protein coupled receptor
- second messenger
second messenger cyclic-AMP (cAMP
Many animal cells use cAMP as a second
messenger to promote _______ of the cell which re-
quires _____. For example, in many glands,
cAMP promotes the __________
- a function associated
with a specialized function
release of a hormone from
processes that supply energy to cells such as glycogenolysis and lipolysis
promotes release of glucose from the liver and
free fatty acids from adipose tissue to supply
energy to the body as a whole.
two major hormones promoting hepatic glycogenolysis?
*1) epinephrine (from adrenal gland)
2) glucagon (from pancreas)*
effector regulation via cAMP is characterized by
2) coordinated control
3) regulation of multiple functions
each effector molecule
leads to many cAMP molecules. cAMP activates a cAMP-dependent protein kinase
which activates many additional kinases which in turn phosphorylate many targets.
promotes glycogenolysis while
diminishes the flow of glucose to storage as glycogen.
regulation of multiple functions
the kinases (and other mechanism) act
not only to mobilize energy but regulate other cell functions, including specialized functions of the cell.
Allosteric regulation of enzyme activity controls cell function independently of _________
via covalent modifications, effectors may alter
allosteric regulation to regulate cell functions.
These alterations may be detected as changes in dependence or sensitivity to allosteric effectors in ex vivo assays.
__________ are now frequently used to characterize effector action rather than assessing the allosteric regulatory properties of an enzyme. However, it is important to recognize that covalent modifications may not _________
Direct structure-based measurements of phosphorylation
simply turn enzyme activity on or off but alter internal regulatory mechanisms.
Ultimately, regulation of a cellular process is a balance between
external signals (e.g. a hormone) and internal cellular control mechanisms.
For example, by promoting glycogenolysis, cAMP may __________. Glycogen inhibits _________. Thus, glycogen depletion leads to ____________ in the absence of an external signal to restore glycogen content.
- deplete cell glycogen depots
- glycogen synthase phosphatase
- dephosphorylation and activation of glycogen synthase
Soon after the discovery of cAMP came the demonstration that hormones (e.g. epinephrine and glucagon) promoted its synthesis in cell extracts. The enzyme catalyzing the conversion of ATP to cAMP was termed _____ ______. For 20 years attempts to purify a hormone-sensitive adenylate cyclase failed. The reason was the involvement of a third protein in addition to the hormone receptor and the catalytic protein. The third protein was termed a _________
- adenylate cyclase
- G protein.
regulate adenylate cyclase by progressing through the G protein cycle
A key feature of the cycle is that
the binding, not hydrolysis of GTP to G protein produces an effect (mediates effector action)
Hydrolysis of _______ terminates the effect
GTP to GDP
What's required to start a new cycle?
release of GDP (promoted by an effector-receptor complex)
Effectors that stimulate adenylate cyclase - promote the release of GDP from the alpha subunit of Gs, the subsequent binding of GTP and the dissociation of the alpha subunit from beta-gamma subunits
The GTP-Gsα complex then stimulates
the catalytic (C) subunit to
what stops the synthesis of cAMP?
Hydrolysis of GTP
thus, effectors that stimulate adenylate cyclase activity also...
stimulate a GTPase activity
the turnover rate of GTP is
the various permutations of subunit combinations making up G proteins form
4 main families
different ligands, termed...
biased agonists may induce GPCRs to interact with different types of G proteins
GPCRs are a pharmaceutical target with
opportunities for specificity either in the extracellular domain binding the effector, or in the 7 regions spanning the membrane. Although GPCRs share the motif of 7 regions spanning the cell
membrane these regions are not _____.
Effector regulation mediated by GPCRs in general, and certainly those linked to cAMP, frequently
display homologous and/or heterologous desensitization.
a mechanism contributing to homologous desensitization is
agonist-dependent phosphorylation of GPCRs.
Productive interaction with signaling mechanisms and internalization of the GPCR.
Internalized receptor may be
dephosphorylated and recycled to the cell membrane or degraded
Heterologous desensitization of regulation mediated by cAMP may arise from
• An increased rate of degradation of cAMP by the enzyme
• An increased rate of extrusion of cAMP from the cell by active pumping or facilitated diffusion.
• A decrease in the effective activity of cAMP-dependent protein kinase. The free catalytic subunit
of cAMP-dependent protein kinase has a much shorter intracellular half-life than the subunit
associated with the R subunit.
• Generation of inhibitors of PKA
clinical relevance - drugs
It has been estimated that ~40% of pharmaceuticals interact with GPCRs. Numerous drugs act by influencing cAMP metabolism. Some mimic effectors acting via either Gs or Gi, others are competitive antagonists.
G protein deficiencies
*Mutations can lead to reduced expression of G proteins leading to diminished responses to effectors acting via the G protein. An example is diminished expression of Gsα which leads to markedly diminished responses to parathyroid hormone.
Mutations increasing cAMP or the effects of cAMP can cause
via the persistent signal to perform the cell's differentiated function which leads to cell division
Tumors with only this type of
mutation would not be expected
to lose localization function and metastasize.
Such mutations include Gs-
proteins with a reduced rate of hydrolysis of GTP, constitutive activation of GPCRs linked to Gs, and constitutive activation of cAMP-dependent protein kinase. Collectively , these types of mutations are termed
Viruses may encode
constitutively active analogs of
GPCRs or downstream steps
The G-proteins that regulate adenylate cyclase are the targets of bacterial toxins that act by catalyzing the _______. The reaction transfers the ADP- ribose moiety of NAD to the target protein
mono-ADP-ribosylation of the G-protein α subunits
Cholera - some strains of ____ will produce a similar toxin to that of Vibrio cholera, termed _____, which causes ____
heat-labile enterotoxin, which causes a milder disease sometimes termed traveler's diarrhea
. Both toxins act by ADP-ribosylating Gsα, which dramatically retards the hydrolysis of GTP
the result is
persistent activation of adenylate cyclase
causes whooping cough
Pertussis toxin ADP-ribosylates Giα, retards the dissociation of GDP from Giα, prevents Giα from binding
GTP (Figure 8), and blocks hormone inhibition of .
as a result, hormones that stimulate adenylate cyclase
become more potent, and their normal effects are exaggerated.
Pertussis toxin blocks actions...
with blocking inhibition of adenylate cyclase, demonstrating that Giα participates in transmembrane signaling systems not involving cAMP.
Both cholera and pertussis toxins block
the GTPase activity of a G-protein.
Recombinant analogs of these and other toxins lacking ADP-ribosyltransferase activity are included as
antigens in vaccines to protect against disease.