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Chemical Signaling


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steroid hormone receptors -- describe the pertinent features of the C terminal
relatively conservative
involved with HSP 90
steroid binding
steroid hormone receptors -- describe the pertinent features of the N terminal
least conserved
required for the maximum gene transcription activity
steroid hormone receptors -- describe the pertinent features of the central domain
cystein rich
folded into two zinc finger motifs
highly conserved
down regulation
receptor disappears from the cell surface
cell surface receptors: what are the three main groups (classified according to output)
(1) enzyme-linked (intrinsic or associated tyrosine kinase activity)

(2) those linked to heterotrimeric G proteins

(3) those that form ion channels
what are four examples of PTKs (receptors with intrinsic tyrosine kinase activity)?
Insulin, IGF-1, EGF, and PDGF receptors
give the general structure of the receptors with intrinsic tyrosine kinase activity
N-terminal extracellular domain (hormone binding)

single transmembrane helix

C-terminal cytoplasmic domain (tyrosine kinase activity and several phophorylation sites)
what are the three AAs that can be phosphorylated?
threonine, tyrosine, serine
receptors with intrinsic tyrosine kinase activity
Group I
- characteristics
- e.g.
no subunits
receptors with intrinsic tyrosine kinase activity
Group II
- characteristics
- e.g.
two subunits:
N-terminal alpha
C-terminal beta
two form tetramer

e.g. Insulin and IGF-1
receptors with intrinsic tyrosine kinase activity
Group III
- characteristics
- e.g.
no subunits
describe the extracellular region of the PDGF receptor?
five immunoglobin-like domains (N-terminal)
receptors having associated tyrosine kinase activity
receptors for prolactin, growth hormone, erythropoietin, interferons (IFNs) and interleukins 2, 7, 10, 13, and 15

extracellular domain with four cysteine moieties for disulfide bonding and a potential site for glycosylation
Janus kinases
receptors having associated tyrosine kinase activity often work through these, which phosphorylate various target proteins when the ligands bind its receptor
receptors linked to heterotrimeric G proteins:
Beta 1 and beta 2-adrenergic receptors
- fxn
activate adenylyl cyclase
receptors linked to heterotrimeric G proteins:
alpha 2-adrenergic receptors
- fxn
inihibit adenylyl cyclase
receptors linked to heterotrimeric G proteins:
alpha 1-adrenergic receptors
- fxn
activates phospholipase C
what is an example of a receptor that forms a sodium channel?
nicotine acetylcholine receptor
what are two examples of receptors that form a chloride channel?
gamma-amino butyric acid (GABA) and glycine receptors
describe the structure of the nicotine acetylcholine receptor
five subunits (alpha, beta, gamma, sigma, epsilon)

each has an extracellular domain, four or five transmembrane helices, and a large cytoplasmic domain
what are the six major signal transduction pathways?
1. cAMP pathway
2. Calcium-Phospholipid pathway
3. cGMP pathway
4. G-Ras protein-linked pathway
5. JAK-STAT pathway
6. Cytoplasmic serine phosphorylation and / or proteolysis pathways
stimulatory G protein (Gs)
- how does it help make cAMP?
ligand binds to receptor, which is linked to a heterotrimer G-protein

causes conformational change that allows GTP to replace GDP on alpha subunit

alpha subunit disengages and attaches to adenyl cyclase, which activates the enzyme to produce cAMP from ATP
what effect do GRFs have on stimulatory G proteins?
GRFs = guanosine nucleotide releasing factors

these help GDP dissociate from alpha subunit, so GTP can rapidly bind
what effect do GAPs have on stimulatory G proteins?
GAPs = GTPase activating proteins

facilitate the hydrolysis of GTP to GDP by intrinsic ATPase of the alpha subunit
once cAMP is formed, what is the next step?
cAMP binds to the two regulatory (R2) subunits of PKA

R2 dissociates from C2 (catalytic subunits)

C2 phosphorylate various proteins and transfer the terminal phosphate group from ATP to specific serine or threonin residues, causing a cascade of cellular events
once the cAMP cascade has been activated, how does the cell return to normal?
dephosphorylation of phosphorylated serines and threonine by phosphatases

metabolizing cAMP to inactive 5' AMP by one of more cAMP phosphodiesterases (PDE)
Are cAMP levels maintained by changing the phosphodiesterase activity?
no, levels are maintained by activating or inhibiting adenylyl cyclase
how does the cholera toxin work?
catalyzes the transfer of ADP-ribose from NAD+ to the alpha subunit of Gs, locking GTP into an unhydrolyzable state -- too much cAMP is made, leads to Na+ and water efflux into the gut, diarhhea
how does inhibitory G protein work?
same as stimulatory G protein except the GTP-bound form of the alpha subunit inhibits adenyl cyclase
how does the pertussis toxin (whooping cough) work?
ADP-ribosylation of inhibitory alpha G protein, meaning that it can't inhibit adenylyl cyclase
name three ways that cytosolic Ca2+ can be kept low
(1) Ca2+-ATPase pump to ECF
(2) Na+/Ca2+ exchanger (uses Na+ gradient)
(3) Ca2+-ATPase pump to intracellular compartment
Name the two ways cytosolic Ca2+ levels can be raised from 10-7 to 5x10-6
voltage-gated Ca2+ channels in electrically excitable tissues

receptor-activated Ca2+ channels (calcium-phospholipid pathway)
4 good things to know about calmodulin
(1) found in plants and animals
(2) highly conserved)
(3) multifxnal
(4) single peptide
does calmodulin have enzymatic activity?
no direct enzymatic activity, but it activates other proteins by binding to them
what do ANPs do?
ANP = atrial natriuretic peptide

secreted by muscle cells in the atrium of the heart when BP rises, increase Na+ and water excretion and enhance relaxation of smooth muscle cells of the blood vessels, thus lowering BP
is there G-protein involvement in the ANP pathway?
no, it is a direct linkage between ligand binding and activation of guanylyl cyclase
show the ANP pathway
ANP binds to extracellular domain
up guanylyl cyclase
up cGMP
up G-kinase
up phosphorylation of serines and threonines
up biological responses
how are Na+ channels regulated in retinal rod cells?
in absence of light Na+ channels are open

when light hits rod cells, rhodopson activates Gt (G protein transducin)

the alpa subunit dissociates and activates cyclic GMP phosphodiesterase

this hydrolyses channel bound cGMP, which closes the Na+ channels
chart the Ras pathway
growth factors such as EGF and PDGF, insulin, and IGF-1

- binding of ligand to receptor results on dimerization and autophosphorylation
- phosphotyrosine residues cause receptors to bind to GRB-2
- through SOS activates G-Ras by GTP-GDP exchange
- G-Ras acts on protein kinases such as MAPKs
- gene expression

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