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
What is the effect of fructose-2,6-bisphosphate on PFK-1?
in the absence of this enzyme the activity of PFK-1 is VERY low
How is hexokinase generated in the muscle?
hexokinase will be operating close to its Vmax under almost any circumstances bc the normal blood glucose concentration is about 5 mM - so this enzyme is operating close to its Vmax at all times
Why isn’t glucokinase subject to the same level of regulation as hexokinase?
regulated by changes in glucose concentration - goes up and down depending on glucose con. Not the same as hexokinase
Km is 5-10 mM - this is the range of blood glucose concentrations that we experience on a daily basis
What is the difference between hexokinase & glucokinase in regard to glucose concentrations?
hexokinase actually has a higher affinity for glucose (low Km), which means that it really wants to bind to glucose and do its job. Glucokinase, then, has a low affinity (high Km), so it won’t bind quite so easily or quickly
The reduced affinity for glucose allows the activity of glucokinase to differ under physiological conditions according to the amount of glucose present.
What are the allosteric inhibitors of pyruvate kinase?
In both liver and muscle:
ATP
alanine (alanine is one of the products generated from pyruvate)
What is the allosteric activator of pyruvate kinase?
F-1,6-bisP
pyruvate kinase down at the bottom of glycolysis senses the presence of high concentrations of important substrates up at the top
What is a difference between muscle and liver pyruvate kinase?
covalent modification is another important way of regulating enzyme activity (IN THE LIVER ONLY)
What is covalent modification?
In covalent modification, a functional group is transferred from one molecule onto the enzyme or protein, thereby turning the enzyme either on or off.
Major fuel source for brain
glucose
ketone bodies during starvation – (>3 days) & danger occurs when [glucose] < 2.2 mM
Fuel STORE for the brain
None
relies on constant supply of blood glucose (GLUT3, Km ~ 1 mM)
- this Km is way below the normal blood glucose concentration of 5 mMol
- so GLUT3 will always and at a high velocity transfer glucose to brain tissue
- bc brain is high priority
Glut 3 has . . .
a very high affinity for glucose compared to other GLUT transporters
Resting conditions for the brain
consumed 60% of total GNG glucose = 120g/day
Major fuel source for muscle
glucose
fatty acids
ketone bodies
Fuel stores of muscle
stores 75% of glycogen in body
only provides energy LOCALLY (just to the muscle cell where it is stored)
Resting conditions of muscle
FAs as major fuel in resting state
heart muscle uses one of the ketone bodies, ACETOACETATE, in preference to glucose
How are muscle and liver metabolite connected?
via the Cori cycle
Fuel source of adipose
glucose synthesizes and stores triacylglycerol, which is immobilized during fasting
Fuel stores of adipose
adipose stores >80% of total available energy
Resting conditions of adipose
highly active during starvation
decreased insulin activates hormone-sensitive lipase which breaks down TAG
Fuel source of liver
glucose
fatty acids
ketone bodies
amino acids
prefers a-keto acids derived from degradation of amino acids in preference to glucose
Fuel stores of liver
stores 1/4 of total body glycogen
uses LACTATE and ALANINE from muscle, GLYCEROL from adipose, and GLUCOGENIC amino acids from diet to make ~200 g of glucose per day via gluconeogenesis
Resting conditions of liver
highly active during starvation making glucose via GNG to maintain blood glucose primarily for brain + RBCss
oxidizes FAs for energy & formation of KBs for brain, heart, muscle and other tissues
What are some other functions of the liver?
makes heme
makes TAGs, PLs, cholesterol
secretes cholesterol as VLDL for lipoprotein transport
an ALTRUISTIC organ
