Biochemical Signaling: Pathways Flashcards
What does 7TM stand for?
Seven-transmembrane-helix receptors
What are the three most common types of membrane receptors that mediate information transfer into the cell?
1) Seven-transmembrane-helix receptors
2) Dimeric membrane receptors that recruit protein kinases
3) Dimeric protein receptors that are protein kinases
What is the largest class membrane receptors?
7TM
About what percentage of drugs in use alter 7TM receptors?
50%
What is the structure of 7TM receptors?
Seven helices spanning the membrane bilayer.
How do 7TM receptors work?
Binding of ligand on the part of the receptor outside of the cell induces a conformational change detectable within the cell.
What type of receptor is a beta-adrenergic receptor? What does it bind?
Seven-transmembrane-helix receptor (7TM); it binds epinephrine (also called adrenaline)
What receptor binds epinephrine? What kind of receptor is it?
beta-adrenergic receptor; 7TM
What does ligand binding to 7TM receptors lead to?
Conformational changes in the cytoplasmic domain of the receptor, leading to activation of a heterotrimeric GTP-binding protein (G protein).
What does an unactivated G-protein consist of?
1) An alpha subunit bound to GDP (most important)
2) A beta subunit
3) A gamma subunit
Why is an unactivated G-protein a heterotrimer?
It consists of three different subunits
What happens when a G-protein is activated?
1) GDP is exchanged for GTP
2) The alpha subunit dissociates from the beta and gamma subunits, forming G-alpha and G-beta, gamma units.
3) This dissociation transmits the signal that the ligand has bound to the receptor.
What does the “G” in G-protein stand for?
guanyl nucleotide
True or false: A single hormone-receptor complex can stimulate nucleotide exchange in only one G-protein heterotrimer.
False; It can stimulate nucleotide exchange in many G-protein heterotrimers, giving an amplified response.
What does the activated G protein do?
Stimulates activity of adenylate cyclase, a second messenger that increases the concentration of cAMP
What enzyme increases cyclic AMP by forming it from ATP?
adenylate cyclase
What does GPCR stand for?
G-protein coupled receptors
What does the effect of the increased concentration of cyclic AMP in the cell depend on?
The type of cell involved.
What are most effects of increasing the concentration of cAMP mediated by?
The activation of a protein kinase (an enzyme).
What enzymes phosphorylate a substrate at the expense of a molecule of ATP?
kinases
What does PKA stand for?
Protein kinase A
What does PKA consist of?
2 regulatory (R) chains and 2 catalytic (C) chains
What activates the R2C2 complex of protein kinase A?
cyclic AMP
Which chains of PKA does cAMP bind to?
The 2 regulatory chains
What happens when cAMP binds to the regulatory chains of PKA?
The 2 catalytic chains are released,
What does activated PKA do?
Phosphorylates specific serine and threonine residues in many target proteins, altering their activity.
What turns off the cAMP cascade?
cAMP phosphodiesterase
How does cAMP phosphodiesterase turn off the cAMP cascade?
It converts cAMP into AMP, which does not activate PKA.
What happens after the cAMP cascade is turned off by cAMP phosphodiesterase?
The C and R subunits of PKA rejoin to form the inactive enzyme.
How is the signal initiated by 7TM receptors switched off?
G-alpha subunits have intrinsic GTPase activity that hydrolyzes the bound GTP to GDP and Pi (inorganic orthophosphate) and thereby deactivates itself.
What happens as hormone concentration decreases?
The hormone dissociates from the receptor and the receptor returns to its initial unactivated form.
True or false: Ligand-receptor interaction is reversible.
True
True or false: The bound GTP on G alpha acts as a built in clock that spontaneously resets the G alpha subunit after a short period of time.
True
What does the likelihood of a receptor remaining in an inactive state depend on?
The concentration of hormone in its environment.
What are cholera and whooping cough due to?
Altered G-protein activity
What is the bacterial toxin that causes cholera?
Choleragen
What is the result of choleragen infection?
Choleragen modifies the G-alpha-s (s=stimulatory) protein so that it is trapped in the active GTP-bound form. This causes loss of NaCl and water into the intestine (causes cholera).
What is the result of pertussis infection?
Pertussis modifies the G-alphai (I=inactive) protein, trapping it in the inactive form, rendering it inactive (causes whooping cough)
What does the hormone vasopressin regulate?
water retention
What does PIP2 stand for? What is it?
Phosphatidylinositol 4, 5-bisphosphate; a phospholipid present in cell membranes.
What does the binding of vasopressin to its 7TM receptor lead to?
Activation of phospholipase C, which cleaves PIP2 into two second messengers, IP3 and DAG
What is the Ga protein that activates phospholipase C called?
Gaq
What does activated phospholipase C enzyme do?
It cleaves PIP2, forming 2 second messengers: 1) Diacylglycerol (DAG) and 2) Inositol 1, 4, 5 triphosphate (IP3)
What is a soluble molecule that can diffuse from the membrane that is released when an activated phospholipase C enzyme cleaves PIP2?
Inositol 1, 4, 5 triphosphate (IP3)
What is a second messenger that stays in the membrane that is released when phospholipase C enzyme cleaves PIP2?
Diacylglycerol (DAG)
True or false: IP3 causes a cascade of phosphorylation to elicit a response from the cell.
False
What does IP3 cause?
Rapid release of Ca2+ from the ER and SR (sarcoplasmic reticulum–in muscle) into the cell cytoplasm.
What membrane protein does IP3 associate to allow the flow of Ca2+ from the ER into the cytoplasm?
IP3-gated channel or IP3 receptor
What is the key difference between the actions of the activated G Protein when epinephrine and vasopressin are received by 7TM receptors?
With epinephrine, the Gas protein activates adenolate cyclase; with vasopressin, the Gaq protein activates phospholipase C.
What types of processes does the release of Ca2+ from the ER and SR trigger?
1) Smooth muscle contraction
2) Glycogen breakdown
True or false: The half-life of IP3 is very short.
True
What do DAG and Ca2+ activate?
Protein kinase C (PKC)
What does protein kinase C (PKC) do?
Phosphorylates serine and threonine residues in target protons
How is IP3 turned off?
It has a short half-life and is rapidly converted to derivatives that have no effect on IP3-gated channels.
How is it theorized that lithium ion works to treat bipolar disorder?
May inhibit the recycling of IP3.
Which kinase requires both DAG and Ca2+ to be activated?
Protein kinase C (PKC)
What level of structure in 7TM membranes is changed by ligand binding?
Tertiary structure
What level of structure in dimerizing receptors is changed by ligand binding?
Quaternary structure
How does a growth hormone receptor exist in the absence of a bound hormone?
As a monomer
What is the activated form of a growth hormone receptor?
A dimer
How is the dimer formed when the growth hormone binds?
The growth hormone binds to two monomeric receptors, creating the dimer.
What is the structure of a GH (growth hormone) receptor?
Single, membrane spanning helix
What causes 2 JAK2 proteins to phosphorylate each other when GH binds to its receptor?
GH binds two monomeric receptors, drawing them together into a dimer; this pulls each receptor’s JAK2 proteins together inside the cell and allows them to phosphorylate each other.
What does JAK2 stand for?
Janus kinase (Tyrosine protein kinase)
When does JAK2 become activated?
When it is cross-phosphorylated by its partner JAK2
What is the structure of insulin?
It is a polypeptide hormone that consists of two monomeric chains linked by two disulfide bonds.
What class of membrane proteins does an insulin receptor belong to?
The tyrosine kinase class
What does each insulin receptor consist of?
1) An extracellular alpha subunit
2) An intracellular beta subunit that spans the membrane with a single transmembrane protein
3) The two subunits are linked by two disulfide bonds
What causes the two subunits in an insulin receptor to dimerize?
Insulin binding
What causes the insulin-receptor to become an activated tyrosine kinase?
Cross-phosphorylation of beta subunits when the dimer is formed due to insulin binding
How are docking sites for insulin receptor substrates (IRS) formed on the insulin receptor?
By phosphorylation of additional sites on the receptor when insulin binds to create the dimer
What happens to the IRS proteins that dock on the insulin-receptor tyrosine kinsase?
They are phosphorylated
What do phosphorylated IRS proteins do?
Act as adaptor proteins (i.e., they are recognized by other proteins, most importantly phosphoinositide 3-kinase)
What is the most important protein that recognizes phosphorylated IRS proteins?
Phosphoinositide 3-kinase
What does phosphoinositide 3-kinase do when it recognizes phosphorylated IRS proteins?
Phosphorylates membrane-bound PIP2 to form membrane-bound PIP3
What does membrane-bound PIP3 do?
Activates PDK1 (PIP3 Dependent Protein Kinase)
What does activated PDK1 do?
Phosphorylates and activates PKB (also called Akt), causing it to detach from the membrane.
What does Akt (PKB) do once it is detached from the membrane?
Moves through the cell phosphorylating enzymes to 1) stimulate glycogen synthesis and 2) move GLUT 4 transporters to the cell surface to allow glucose entry into the cell.
What is GLUT 4?
A glucose transporter
How is insulin signaling terminated?
By protein phosphatases that remove phosphates and deactivate enzymes
What three types of phosphatases are responsible for terminating an insulin signal?
1) Protein tyrosine phosphatase
2) Lipid phosphatases
3) Protein serine phosphatases
What activates the enzymes that terminate an insulin signal?
Insulin binding to the receptor (both activates and starts a cascade for deactivation)
What enzymes hydrolyze PIP3 back to PIP2 to help terminate the insulin signal?
Lipid phostatases
What enzymes dephosphorylate activated protein kinases such as Akt in the insulin signal pathway?
Protein serine phosphatases
What enzymes remove phosphoryl groups from tyrosine in the insulin receptor to help terminate the signal?
Protein tyrosine phosphatases
What are transmembrane receptors that have tyrosine kinase domains within their intracellular domains?
Receptor tyrosine kinases (RTK’s)
What stimulates the growth of epidermal and epithelial cells?
Epidermal growth factor
What does binding of EGF to the extracellular domain of its receptor cause?
1) Dimerization
2) Cross-phosphorylation and activation
Which adapter protein links the cross-phosphorylation of an EGF receptor to chains of phosphorylation that stimulate cell growth?
Grb-2
What causes Grb-2 to bind to receptor proteins of tyrosine kinase on the intracellular domain of EGF receptors?
Cross-phosphorylation of the EGF receptor
What does Grb-2 do when it binds to the EGF receptor?
Recruits Sos protein
What does Sos do when it is activated by Grb-2?
Binds to Ras and activates it.
What family of proteins is Ras a member of?
small G Proteins (or small GTPases)
How are small G proteins (or GTPases) like heterotrimeric G proteins?
They cycle between a GDP inactive and a GTP active form and cause the final phosphorylation events.
How do G proteins (or GTPases) differ from heterotrimeric G proteins?
They are smaller and monomeric
What do activated GTPase proteins stimulate?
Cell growth and differentiation
By what mechanism does Sos activate Ras?
Sos opens up the nucleotide binding pocket of Ras, allowing GDP to escape and GTP to enter.
What does GEF stand for? Which protein in the EGF signal pathway falls in this category?
Guanine-nucleotide exchange factor; Sos
True or false: Ras, like the Ga protein in the 7TM pathway, contains intrinsic GTPase activity that terminates the signal and returns the system to the inactive state.
True
What terminates the signal in the EGF pathway?
GTPase activity that is begun when Ras is activated
