Module 7 Flashcards
What types of signals do cells receive?
o Survive
o Grow and divide
o Differentiate
o Dies (apoptosis) when it doesn’t receive the signals that maintain the cells to be alive
Briefly explain how a signal is received by a cell.
- ligand binds to transmembrane protein
- conformational change (intracellular portion)
- called SIGNAL TRANSDUCTION
- affects intracellular signaling proteins
- can go on to change phenotype of cell (changing EFFECCTOR proteins)
Different signals operate over different distances. Give some examples.
o Contact-dependent
o Paracrine
o Synaptic
o Endocrine
What are the 4 characteristics of signaling pathways?
o SPECIFICITY
o AMPLIFICATION
o MODULARITY
o TRANSIENT (expressed and degraded very quickly)
What are the 3 different actions of acetylcholine?
o Binds to receptor on heart muscle –> decreased rate and force of contraction
o Binds to receptor on skeletal muscle cell –> contraction
o Binds to receptor on salivary gland cell –> secretion
What are some common molecular switches (signaling by phosphorylation)?
o Having a protein in an OFF state that becomes phosphorylated to turn ON (& increase its activity)
o Signal is received & activates a protein kinase
o This transfers a phosphate from ATP to the signalling molecule and turns it ON
o Switching on can be reversed by a protein phosphatase
o The opposite can also happen
GTP and signalling
o Signalling molecule is OFF when GDP is bound
o Signal is received
o GDP is exchanged for GTP
o Protein is switched ON when GTP binds
o Can self-hydrolyze the GTP back into GDP to switch itself OFF
What are GPCRs? What is their role?
- integral membrane proteins
- play a role in senses of sight, smell, taste
- contain 7 transmembrane helices
- also contain an extracellular portion that interacts w ligand
- and an intracellular portion that interacts with G protein
What are G proteins?
family of proteins that act as molecular switches inside cells
What are the features of G proteins?
- composed of alpha, gamma, and beta subunits
- alpha subunit has the GDP binding site
- alpha has GTPase activity (can hydrolyze GTP –> GDP)
Explain how B-adrenergic receptors work
Epinephrine binds to GPCR
Conformational change to intracellular part
G protein activated
GDP –> GTP
a-subunit moves another receptor
G protein interacts w receptor adenylyl cyclase
ATP –> cAMP
cAMP activates enzyme PKA
PKA phosphorylates a host of downstream signaling targets
response it turned off when cAMP is hydrolyzed to AMP
What are receptor tyrosine kinases?
enzyme-coupled cell-surface receptors
What are some features of RTKs?
- modular proteins
- have alpha chains with ligand binding regions
- have beta chains with kinase domains
- protein kinases phosphorylate amino acid side chains with hydroxyl groups
- serine + threonine ALSO phosphorylate RTKs
- many of these RTKs respond to the binding of GROWTH factors
Why are RTKs really important in cancer?
- overactive cell growth is triggered by aberrant cell signalling
- defects of downstream signalling often stimulate cell survival, growth, and proliferation (hallmarks of cancer)
Outline normal RTK activation (ligand-induced receptor dimerization)
o We have 2 copies of inactive RTKs
o When the ligand interacts it brings them into close proximity with each other
o The tyrosine kinase domain of one receptor can phosphorylate the other – cross phosphorylation or trans autophosphorylation
Outline domain-negative inhibition by mutant RTK
if one copy of the dimer is mutated and lacks activity, we can’t get signalling downstream
How does an inactive tyrosine kinase domain become ACTIVE?
o Activation loop block access to active site
o Tyr is blocking the hole where substrate tyrosines would be able to bind
o If this loop becomes phosphorylated, it can now no longer do that
o The active size would become exposed, and the target protein can now bind.
What are some recurring motifs in regulation?
4 + 5 = hormonal reg
- compartmentalisation
- allosteric regulation (enzymes catalyzing committed and usually irreversible steps)
- specialisation of organs
- covalent regulation
- enzyme levels
Allostery is a feature of enzymes that have quaternary structure. Define allosteric.
Of or involving a change in the shape and activity of an enzyme that results from the binding of a REGULATORY molecule at a site other than the active site
What are the 3 irreversible steps (control points) of glycolysis?
1) GLUCOSE –> G-6-P
Hexokinase - inhibited by G-6-P
2) F-6-P –> F-1,6-bisP
PFK-1 (has both inhibitors and activators)
3) PEP –> pyruvate
L-PK (has both activators & inhibitors)
What are the 2 allosteric inhibitors of PFK-1?
ATP & citrate
citrate is a product of the Krebs cycle & if we have a lot of it, it means glycolysis has been occurring at a sufficient rate to provide enough substrate for Krebs cycle
ATP CAN ALSO BE THE SUBSTRATE AND BIND AT THE ACTIVE SITE along w fructose-6-phosphate & be the phosphate DONOR to generate the product
How does ATP inhibit PFK-1?
While ATP binds at the active site equally well in both R and T states, it preferentially binds the allosteric site of the T state
This preferential binding causes a shift from equilibrium of the two states, to a greater amount of T state, which decreases the affinity for F6P.
What are the 3 allosteric activators of PFK-1?
AMP, ADP, & F-2,6-bisP
- ADP & AMP are the hydrolysis products of ATP (if we have a lot of ADP and AMP, it means our glycolysis stores are low so we need more glycolysis)
Describe the structure of PFK-1.
- homotetramer of 4 copies of the same type of subunit
- each subunit has an active + regulatory site
- binding of molecules to regulatory site of one subunit can change the conformation of neighbouring subunits
