Cell Signalling Flashcards
What level of structure are alpha-helices?
Secondary structure
What is a quartenary structure
Subunits
Paracrine signalling
A form of cell communication in which cells secrete paracrine agents that act on LOCAL TARGET CELLS.
The Paracrine agent diffuses through the interstitial fluid between cells to reach its destination.
Examples: Neurotransmission, Immune response
Endocrine signalling
A form of cell communication in which a specialised cell secretes hormones that travel in the blood circulation.
This allows them to reach cells at distant tissues.
Autocrine signalling
A form of communcation in which a cell secretes a chemical messenger (usuallly a hormone) that binds to receptors on THAT SAME CELL.
This causes changes within the cell.
Contact-dependent signalling
A form of cell communication that requires cells to be in direct membrane-membrane contanct.
Intercellular communication
Cell-to-cell communication that enables cells to influence other neighboring or distant cells to coordinate a response within the tissue/organ.
Intracellular communication
Communication that takes place INSIDE the cell once a signaling molecule has binded to either a receptor on the cell surface membrane or inside the cell.
Stages of cell signaling
- Release
- Reception
- Tranduction
- Response
What does cAMP do once synthesized?
It binds to the regulatory subunits of Protein kinase A and subsequently activates the catalytic subunit of Kinase A.
Kinase-A
Kinase A is a protein that consists of two regulatory subunits and two catalytic subunits. Protein Kinase A regulates other proteins through phosphorylation .
How is cAMP produced?
- Ligang binds to GPCR.
- G-protein activated.
- G-protein dissociates: α-subunit dissociates from the δ & γ-subunits.
- α-subunit binds to and activates the effector adenylyl cyclase .
- Adenylyl cyclase converts ATP to cAMP.
Second messenger
A molecule that amplifies and relays a signal from the receptor intracellularly.
Examples include…
* Diacylglycerol (DAG)
* Inositol 1,4,5-triphosphate (IP3)
* Cyclic adenosine monophosphate (cAMP)
Inositol 1,4,5-triphosphate
A secondary messenger synthesized by phospholipase C from PIP2 that acts on a ligand-gated ion channels in the Smooth ER.
This causes the release of Ca2+ ions from the smooth ER.
Porbol esters
They mimic Diacylglycerol and cause long lasting activation of PKC . This can cause cells to lose control of growth and behave like tumour cells.
Phosphoinositol signal pathway
- Phosphatidylinositol is phosphorylated into PI 4-phosphate [PI kinase]
- PI 4-phosphate is further phosphorylated into PI 4,5-biphosphate [PIP kinase].
- PI 4,5-biphosphate is hydrolysed by [phospholipase C] into DAG and IP3
PI
A glycerophospholipid known as Phosphatidylinositol
PI4-P
Phosphatidylinositol 4-phosphate
PIP2
Phosphatidylinositol 4,5-biphosphate
IP3
Inositol 1,4,5-triphosphate
Effector enzymes
Small molecules that bind onto proteins to regulate them.
Examples include:
* Adenylyl cyclase
* Phospolipase C β
Phospholipase C β
An enzyme that catalyses the hydrolysis of PIP2 into DAG and IP3.
How is phospolipase C β activated?
The activation of G-protein leads to the α-subunit activating the phospholipase C β
DAG
Diacylglycerol is a secondary messenger synthesized from the hydrolysis of PIP2 by phospholipase C β.
It causes the activation of protein kinase C
EF hands
Calcium-binding motifs found within a large family of proteins.
* They have a high affinity & selectivity for calcium ions.
CaM
Calmodulin
Calmodulin
A protein that contains four EF hand motifs that allow four calcium ions to bind and cause confirmational change to the protein.
Once activated it can bind and activate other proteins such as kinases.
CaM are activated through the increase of Ca2+ in the cytoplasm.
What other organelles store calcium ions?
- Smooth ER
- Lysosomes
- Mitochondria
CaMK
A Ca-CaM protein kinase complex.
As its name suggests it is a kinase that is bonded to and activated by Calmodulin (CaM).
What can calcium bind to?
- Calmodulin
- Kinase -> CaMK
- Troponin C
- Calcium-ATPase
How does calcium affect smooth muscle?
- Release of calcium ions into the cytoplasm.
- Bind to Troponin C
- Cause smooth muscle contraction
How is PKC activated?
- IP3 causes an increase in cytoplasmic conc. of calcium.
- DAG and calcium bind to C1 & C2 domains respectively.
- This causes an increase in affinity for phosphatidylserine at the C1 domain of PKC.
- The binding of phosphatidylserine allosterically ACTIVATES PKC.
How is the IP3 receptor regulated
It is regulated by calcium through a positive feedback loop.
Store depletion signal
When calcium ions are flowing out of the Smooth ER, the store is depleting. The Stim1 sensor notices this change and sends out a signal to the SOC .
SOC
Store-operated channel
It opens once it receives a store depletion signal from the Stim1 .
Stim1
Calcium ion sensor proteins that line the smooth ER.
They notice small changes in the concentration of calcium in the store and can send out a store depletion signal to the SOC .
What enzyme destroys cAMP
Phosphodiesterase
What does Phosphodiesterase do
They catalyze the hydrolysis of cAMP.
TRUE OR FALSE
A protein kinase transfers a phosphate group to a target protein onto a cysteine residue
FALSE
They transfer a phosphate group onto a threonine or serine residue.
Which amino acid do kinases phosphorylate
Threonine, serine or tyrosine residues
How is cAMP de-activated
- The Gi protein inhibits adenyl cyclase.
- Phosphodiesterase hydrolyzes cAMP into AMP.
Both reduce the concentration of cAMP in the cell.
G-protein cycle
ACTIVATION-DEACTIVATION
- Ligand binds to the extracellular site causing conformation change.
- G-protein binds to GPRC intracellularly
- GDP on a-subunit is exchanged for a GTP [ACTIVATES]
- α-subunit acts as a GTPase and breakdowns GTP to GDP to DE-ACTIVATE the protein.
GEFs
Guanine nucleotide Exchange Factor is a GTPase switch protein.
They speed up the activation of G-protein.
GAPs
GTPase-activating protein is a switch protein.
They accelerate the hydrolysis of the GTP to GDP to DE-ACTIVATE the G-protein.
GAi
An α-subunit that inhibits adenylyl cyclase - decreases cAMP
GAs
A stimulatory α-subunit that ACTIVATES adenylate cyclase - Increases cAMP
G_βγ
An effector subunit that binds to and activates other proteins
IMPORTANT EXAMPLE: Phospholipase C
GPCRs
A receptor protein that consists of 7 transmembrane domains labelled TM1-TM7.
A G-protein is bonded to the GPCR intracellularly (cytosolic side).
TRUE OR FALSE
The C-terminus of GPCRs faces the extracellular side of the membrane
FALSE
C-terminus is intracellular and N-terminus is extracellular.
G-protein
A heterotrimeric protein that binds to the GPCR activating various different signalling pathways.
Activation of G-protein
- Ligand binds extracellularly
- Causes confirmational change intracellularly -> creates a ‘pocket’.
- G-protein binds into that pocket.
- Upon binding, GDP is exchanged for GTP at the α-subunit [GEFS speed up this process]
- α-subunit activates and dissociates from the β&γ subunits.
- α-subunit can activate its downstream effector molecules .
TRUE OR FALSE
GTP bound α-subunit is activated
TRUE
GAq
A certain subtype of the α-subunit that results in an increase of DAG and IP3 - increases Ca2+
Phosphatases
An enzyme that catalyses the removal of a phosphate group via hydrolysis.
Different types of kinases in terms of their targets
- Sereine/threonine kinases
- Tyrosine kinases
MAPKs
Mitogen-activated protein kinases
What are the major classes of enzyme-linked receptors
- Receptor tyrosine kinases (RTK)
- Receptor serine/threonine kinases (RSTK)
- Tyrosine kinase-associated receptors (TKAR)
SH2
Src homology 2 domain is a sequence-specific phosphotyrosine-binding module.
- Proteins that contain SH2 have a high affinity and selectivity for phosphotyrosine residues.
- Examples: Growth factor receptor-bound protein 2 (Grb2) and Signal transducer and activator of transcription (Stat)
Grb2
Growth factor receptor-bound protein that activates RAS through phosphorylation.
RTK activation
- Ligand binds to the extracellular receptor.
- Dimerization occurs between the two receptors.
- This causes the tyrosine kinase domain in one of the receptors to phosphorylate the tyrosine residues of the other receptor [VICE VERSA]
- The autophosphorylated tyrosine residues act as binding sites for SH2 domain containing proteins (such as phospholipase C).
Autophosphorylation
Is when a tyrosine kinase domain phosphorylates tyrosine residues on its own dimer.
RAS signalling pathway
- Activation of RTK.
- SH2 domain protein Grb2 binds to a phosphotyrosine residue (is phosphorylated in the process).
- Grb2 activates RAS. The activation of RAS causes a cascade of events.
- RAS exchanges GDP for GTP in RAF proteins.
- RAF is activated and phosphorylates Mek.
- Mek is activated and phosphorylates MAP kinase.
- MAP kinase is activated and causes the activation of transcription factors.
- Transcription factors enter the nucleus and alter gene expression.
What does the RAS pathway achieve
It alters gene expression through various downstream phosphorylation events.
What happens if RSTK signalling goes wrong?
It can cause…
* Gastrointestinal cancer
* Pancreatic cancer
* Pituitary cancer
* Aortic aneurysm
* Loeys-Dietz syndrome
PI3K
Phosphatidylinositol 3 kinase is a protein that catalyses the phosphorylation of glycophospholipids at carbon 3 of the inositol ring .
PI(4,5)P2
Phosphatidylinositol 4,5-biphosphate is a phospholipid that is phosphorylated by PI3K into PIP3 (phosphatidylinositol 3,4,5-triphosphate).
Explain the PI3K signalling pathway
- RTK activation
- Recruitment of PI3K at phosphotyrosine residues.
- PI3K phosphorylates PI(4,5)P2 into PI(3,4,5)P3.
- PI(3,4,5)P3 has a high affinity for PH domain proteins.
- Proteins Akt and PDK1 are recruited and activated by PIP3.
- Akt can regulate various downstream proteins.
Role of calcium in Skeletal muscle contraction
- Action potential propagation through T-tubule.
- Calcium is released from the SER.
- Calcium binds to troponin .
- This causes a physical repositioning of tropomyosin exposing the acting-binding sites.
- Myosin-actin bridges are formed
- Instigating a power stroke which leads to muscle contraction
Role of calcium in smooth muscle contraction
- Activation of GPRC coupled to GAq subunit.
- GAq activates phospholipase C
- Phospholipase C hydrolyses PIP2 into DAG and IP3
- IP3 binds to IP3 receptors on the SER allowing the release of calcium ions into the cytoplasm.
- Calcium binds to CaM (activates)
- CaM activates MLCK which increases GTPase activity of MYOSIN HEADS
- Increases powerstroke
