Chapter 8: Cell Signaling Systems Flashcards
What is signal transduction?
- The biochemical mechanism that transmits an extracellular signal across the plasma membrane and throughout the cell
- Conformational change due to binding of messenger
- Often involves covalent or noncovalent modification of intracellular target proteins
Examples: Flux through metabolic pathways and Gene expression
What are two ways to change protein conformation and activate signaling?
- Phosphorylation
- Signal comes in
- (Protein kinase) ATP -> ADP
- Signal goes out due to activation
- (Protein phosphatase) Substrate protein complex -> Protein + Pi
- cycle restarts
- GTP Binding
- Signal comes in
- GDP leaves protein and GTP binds
- GTP activates protein which causes signal to go out
- GTP hydrolysis releases Pi
- GDP is bound to protein
- cycle restarts
Outline the proteins and messengers involved in a generic signaling pathway.
- First messenger
- Receptor protein
- Upstream signaling protein
- Second messenger
- Downstream signaling protein
- Target protein
- First messengers are extracellular ligands that bind to receptor proteins
- Secondary messenger is an intracellular signaling molecule
What is a hormone?
A biologically active compound that is released into the circulatory system and comes into contact with a hormone receptor on a target cell
What are the three categories of hormones?
Endocrine - Send hormones to distant cells
Paracrine - Send hormone to local cells
Autocrine - Hormone illicits response in the same cell
Where is Epinephrine produced, where does it go, what does it do?
- Produced in adrenal medulla
- Targets Heart and Liver cells
- Increases heart rate and glycogen degradation
Where is Glucagon produced, what does it target, what does it do?
- Produced in pancreatic alpha cells
- Targets Liver cells
- Causes glycogen degradation
Where is Insulin produced, What does it target what does it do?
- Produced in pancreatic beta cells
- Targets muscle, liver,and adipose cells
- Increases glucose uptake
What is a secondary messenger? What are 4 examples?
- A small (non-protein) molecule that amplifies Receptor generated signal
Examples:
- cGMP (cyclic GMP)
- cAMP (cyclic AMP)
- DAG (Diacylglycerol) and IP3 (inositol 1,4,5-triphosphate)
- Ca2+
What does signal amplification look like?
Describe the Phosphoinositide Cascade.
- Receptor activates which causes PIP2 to be hydrolyzed by PLC(Phospholipase C)
- Newly generated DAG binds to PKC (protein kinase C) which phosphorylates downstream targets
- Secondary messenger IP3 activates Ca2+ channels on ER causing Ca2+ release into cytoplasm
- Ca2+ binds to Calmodulin which undergoes large conformational change
- PKC and Calmodulin activity regulated by Ca2+ binding
What are two major classes of receptor proteins found in Eukaryotes?
- G protein-coupled receptors
- Dissociation of heterotimeric G protein complex
- Adenylate cyclase and PLC are activated
- Receptor tyrosine kinases
- Phosphorylates Tyr residue in target protein to create docking site for intracellular signaling
How many transmembrane alpha helices does GPCR have?
7
What effect does Epinephrine binding have on GPCR?
- Epinephrine (ligand), a catecholamine, binds to the receptor and leads to conformational change on cytosylic side
Outline the steps in GPCR activation
- Ligand binds to GPCR causing conformational change
- GDP-GTP exchanged and subunit dissociates
- Regulation of downstream process
Describe the structure of a G-protein
- Heterotrimeric (αβγ) membrane associated proteins that bind GTP or GDP
What does Gβγ regulate? What does Gα
Gβγ
- phospholipase A
- Ion channels
- Receptor kinases
Gα
- Activate/inhibit Adenylate cyclase
- Regulate neuronal signaling
- stimulate phospholipases
- Stimulate phosphodiesterases
Give an overview of GPCR (7 steps)
- Epinephrine binds to receptor protein
- Hormone complex causes GDP bound to Gsα to be replaced by GTP which causes Gsα to activate
- Gsα separates from Gsβγ then moves to adenylyl cyclase and activates it (may activate many other Gsα subunits)
- Adenylyl cyclase catalyzes formation of cAMP
- cAMP activates PKA
- Phosphorylation of cellular proteins by PKA causes the cellular response to epinephrine
- cAMP degrades reversing the activation of PKA
Diagram the synthesis and breakdown of cAMP
ATP –(via adenylyl cyclase)–> cAMP –(hydrolysis via cAMP phosphodiesterase)–> AMP
What does the binding of cAMP do to protein Kinase A?
- cAMP binds to R2C2 tetramere (2 regulatory subunits and 2 catalytic subunits) then catalytic and cAMP-bound R subunit dissociate
What is the difference between Pseudo substrate sequence and substrate sequence in cAMP binding?
Pseudo Substrate Sequence (RRGAI)
- Fits well in active site
- Alanine cannot be phosphorylated
Substrate Sequence (RRGSI)
- Fits in active site
- Serine OH group on side chain can get phosphorylated
How does activation of protein Kinase C occur?
- cAMP binds to R subunit
- R subunit dissociates causing catalytic subunit to be unblocked
What three things can result from the phosphorylation of protein kinase A?
- Phosphorylation and inhibition of glycogen impedes glycogen synthesis
- Phosphorylation and activation of enzymes involved in glycogen degradation to produce glucose
- Phosphorylation and activation of enzymes involved in gluconeogenesis
How can G-protein signaling be inhibited using a GTPase activating protein?
GEF
- Guanine nucleotide exchange factor
- Promote GDP-GTP exchange
- Activate signaling
GAP
- GTPase activating protein
- Stimulates intrinsic GTP hydrolysis activity
- Inhibit signal transduction
Outline the steps of the termination of GPCR signaling
- Ligand activates GPCR signaling
- Gβγ binding to βARK recruits kinases to GPCR cytoplasmic tail
- βARK and PKA phosphorylate serine and threonine residues in GPCR cytoplasmic tail
- β-Arrestin binds to phosphorylated GPCR cytoplasmic tail
- β-Arrestin-GPCR complex is internalized by endocytic vesicle in cytoplasm
- β-Arrestin dissociates and rebinds to another GPCR
- GPCR is dephosphorylated and either degraded (lysosome) or recycled to plasma membrane
What is a receptor tyrosine kinase and what does it do?
- Receptor that transmits extracellular signal by ligand activation of a tyrosine kinase found in cytoplasmic tail of receptor
- Activated by RTKs are dimers
- Phosphorylate downstream signaling proteins that bind to RTK phosphotyrosines
Ex. Insulin receptor
What is Insulin and what does it do?
- Peptide hormone that is produced by β-cells of islets of Langerhans in pancreas
- Insulin is produced and released from pancreas in response to glucose
- Insulin reaches target cells such as liver, muscle, or fat via bloodstream
- Binding of insulin causes insulin receptor to cascade events which increase glucose uptake and metabolism
- Inability to make insulin results in diabetes
What does insulin do in the liver, fat, and muscle?
Liver:
- Decrease glucose synthesis
- Increase glycogen synthesis
Fat:
- Increase glucose metabolism
- Increase lipogenesis
- Decrease lipolysis
Muscle:
- Increase metabolism
- Increase glycogen synthesis
What is the precursor of Insulin
- Pre-proinsulin turned to pro insulin turned to insulin
What does insulin signaling cause?
- Induces conformational change
- Tyrosine kinase domain auto phosphorylates on 3 Tyr residues
- Active TK domain phosphorylates target substrate
How does PI-3K converted PIP2 to PIP3?
