Midterm No. 3, Opus 2 Flashcards
What enzyme is the rate limiter for the cholesterol synthesis pathway?
HMG-CoA
How does cholesterol move through the bloodstream?
HDL and LDL particles are solubilized by ApoA and ApoB respectively, then bound by carrier proteins to travel through the bloodstream
What protein binds and solubilizes LDL particles?
ApoB
What protein binds and solubilizes HDL particles?
ApoA
What makes up the apolar interior core of an LDL particle?
Cholesteryl ester
What makes up the polar surface of an LDL particle?
Phospholipids and unesterified cholesterol
pH of early sorting endosomes
5.9 - 6.0
pH of late endosomes
5.0 - 6.0
pH of lysosomes
4.0 - 5.0
pH of endocytic recycling compartments
6.4 - 6.5
What is the function of endocytic recycling compartments?
They recycle plasma membrane proteins back to the cell surface
What did Goldstein and Brown initially find in their studies on the genetics behind familial hypercholesterolemia?
They initially found the cause to be a single (autosomal dominant) gene mutation. They saw that both homozygous and heterozygous individuals with this mutation had the hypercholesterolemia phenotypes
Describe Goldstein and Brown’s experimental technique to study the genetics of familial hypercholesterolemia
Isolated and cultured patients’ cells
They generated LDL particles with radioactive 125I-tyrosine (I for iodine)
They did a pulse-chase at with the radioactive tyrosine in the patients’ cell cultures
The first part of the pulse-chase was done at 4 degrees C
Then they raised the temp to 37 degrees C
Why did Goldstein and Brown start their pulse-chase at 4 degrees C?
Because at that temp binding still happens, but energy-dependent process don’t occur.
Why did Goldstein and Brown include warming as part of their pulse-chase experiment?
At the low temp binding occurred, but no energy dependent processes happen
The slow warming allows them to get a better look at the endocytosis processes
What did Goldstein and Brown observe for their normal cell cultures (no hypercholesterolemia) in their initial pulse-chase experiments?
They saw high binding of the LDL particles at the beginning As the cells warmed more and more particles were internalized to endosomes, and eventually degraded in lysosomes
Goldstein and Brown’s proposed pathway of LDL particle endocytosis, for normal people
LDL particles are bound to receptors on the cell surface
Bound LDL particles enter a coated vesicle, vesicle eventually fuses with an endosome
In the endosome the cell surface receptors and LDL particles unbind, release
The now free cell surface receptors enter a recycling vesicle, to be recycled back to the cell surface
The endosome containing the LDL particles matures into a lysosome, and the LDL particles are eventually degraded
Individual cholesterol molecules are released from the lysosome, cell cholesterol levels subsequently rise
Droplets of cholesterol ester form in the cell
HMG CoA reductase is downregulated in response to the elevated cholesterol levels
LDL cell surface receptors are also downregulated in response to the elevated cholesterol levels
What cellular response(s) do normal people have for increased cellular cholesterol levels?
Downregulation of HMG CoA reductase and LDL cell surface receptors
Where does the downregulation information happen (in response to elevated cellular cholesterol levels)?
Happens in the ER
What was JD’s specific hypercholesterolemia phenotype?
His cells did bind to LDL, but not was well as normal patients, and what did bind wasn’t internalized
What caused JD’s funky hypercholesterolemia phenotype?
He had two different, equally crummy alleles
Mom was heterozygous, her cells bound to only 1/2 as many LDL particles as normal
Dad was also heterozygous, less than half of his bound LDL particles were internalized
What did Goldstein and Brown ultimately find as the cause(s) of familial hypercholesterolemia?
4 offending allele classes
1) mutation blocking LDL receptor synthesis
2) mutation blocking the LDL receptor’s transport form the ER to the golgi
3) mutation in the LDL receptor preventing the LDL particles from binding
4) mutation causing the LDL receptors to cluster in coated pits
What are coated pits?
Early formation phase of coated vesicles, on the cell surface
What are LAMPs?
Lysosomal membrane proteins
Describe the process of internal sorting of transmembrane proteins to be trashed
Transmembrane proteins are absorbed into the cell from a vesicle
The vesicle either fuses with or matures into an early endosome
The early endosome carrying the transmembrane proteins then either fuses with or matures into late endosomes
The late endosome’s membrane is studded with both the trash-transmembrane proteins and lysosomal (acid hydrolase) enzymes
Vesicles form in the late endosome that pinch off the body fo the late endosome, now called a multivesicular body
Trash-transmembrane proteins are part of the internal vesicles in the multivesicular body. Lysosomal enzymes are part of the multivesicular body’s outer membrane
The multivesicular body matures into a lysosome. Internal vesicles containing the trash-transmembrane proteins are degraded by the acid hydrolases
Which membrane in the multivesicular body are trash-transmembrane proteins part of?
Internal vesicle membranes
Which membrane in the multivesicular body are lysosomal enzymes (acid hydrolases) part of?
The multivesicular body’s outer membrane
(Briefly) How does the body initiate the fight or flight response?
Upon stimulus, epinephrine is released from the hypothalamus.
The epinephrine is absorbed into the bloodstream and sent throughout the body, triggering a whole body response
How do liver cells respond to epinephrine signals?
Epinephrine binds to cell surface beta2 receptors
A set of trimeric G proteins activate adenylyl cyclase
Adenylyl cyclase turns ATP into cAMP
cAMP activates Protein Kinase A (PKA)
PKA has a multipronged response, the goal of which is to convert the cell’s various sugar forms back into glucose
The glucose is released to provide fuel for the fight or flight response
At what pH do cell surface receptors bind to LDL particles?
~7
What pH facilitates unbinding/release of LDL particles from LDL receptors?
Lower, more acidic pH’s
The pH of the endosome (~5) is low enough to facilitate this release
What happens to LDL cell surface receptors in neutral pH’s (~7)?
The ligand-binding arm is free to bind to LDL particles
What happens to LDL cell surface receptors in acidic pH’s? (~5 or lower?)
The beta-propeller domain of the receptor becomes positively charged. This change in charge allows it to bind to the its ligand-bidning arm, stimulating release of any bound particles and preventing additional binding interactions with LDL particles
What does adenylyl cyclase do?
Converts ATP to cAMP
What does guanylyl cyclase do?
Converts GTP to cGMP
What’s the function of Protein Kinase A’s (PKA’s) multi-pronged resposne?
It’s activated by cAMP, and works to convert the cell’s various sugar forms back into glucose
The glucose is eventually released to provide fuel for the fight or flight response
Does a single signal elicit only one response per cell?
No(t always). A single signal can elicit many different distinct responses in each cell
What is the nature of a cell’s responses to a signal dependent on?
Which effector(s) are present in the cell, and the cell’s history
What receptor does epinephrine bind to in liver cells, and what is the cell’s main response?
beta2
Stimulates glucose release
What receptor does epinephrine bind to in fat cells, and what is the cell’s main response?
beta2
Stimulates release of fatty acids
What receptor does epinephrine bind to in lung cells, and what is the cell’s main response?
beta2
Stimulates bronchodilation
What receptor does epinephrine bind to in heart muscle cells, and what is the cell’s main response?
beta1
Stimulates increased rate and force of contraction
What receptor does epinephrine bind to in intestinal blood vessel cells, and what is the cell’s main response?
alpha2
Stimulates vessel constriction
How are the genes for the beta2, beta1, and alpha2 receptors related?
The genes for beta2 and beta1 are closely related (though they’re still distinct genes)
The gene for alpha2 is more distantly related to the genes for beta2 and beta1
What happens downstream immediately after epinephrine binds to a beta receptor?
G(alpha)s is activated
(adenylate cyclase is turned ON, cAMP levels rise)
What happens downstream immediately after epinephrine binds to a alpha receptor?
G(alpha)i is activated
(adenylate cyclase is turned OFF, cAMP levels decrease)
Does G(alpha)s turn adenylate cyclase on or off?
ON
Does G(alpha)i turn adenylate cyclase on or off?
OFF
Does G(alpha)s activation increase or decrease cAMP levels?
Increase
Does G(alpha)i activation increase or decrease cAMP levels?
Decrease
What kind of drug is terbutaline (found in asthma inhalers)?
Agonist
It mimics a natural compound (epinephrine)
What does terbutaline (found in asthma inhalers) do?
It stimulates bronchiole cell’s beta2 receptors, opens the bronchioles
Why might oral terbutaline (pill form) have adverse side effects for patients with underlying cardiac disorders?
If you have a naturally weak heart, excessive stimulation of the heart’s beta1 receptors could overstress it, causing issues
Why might oral terbutaline (pill form) have adverse effects for patients with diabetes?
Additional stimulation of liver beta2 receptors increases the blood glucose levels, elevating sugar release. This make sit hard for a diabetic to manage their sugar levels and may further stress their pancreas
Where do pharma drugs’ side effects come from?
The drug binding to a diverse variety of receptors, both within a single cell type and within multiple cell types
A single receptor causing multiple cellular responses
What kind of drug is propanol (the beta-blocker)
Antagonist
Binds to beta1 receptors, prevents other ligands (epinephrine) from binding
How do beta-blockers work?
They target cardiac muscle. They bind to beta1 receptors and prevent other ligands (like epinephrine) from binding
This prevents epinephrine’s cellular responses
What kinds of patients would be prescribed propanol?
Ones at risk of heart attacks or ones with severe nervousness
Patients that don’t want the rate and force of their heart muscle contractions to increase
3 basics of cell signaling: what do cells need to survive?
Cells must respond, and respond adequately, to external stimuli
Cells must be able to trigger a diverse array of responses
Cells must be able to transmit the information of extracellular signals across their plasma membranes
Generally, how many cell surface receptors do humans have?
A fuck ton
They’re currently sorted into ~20 different classes, 5-6 of which are more prominent
List 3 types of effector molecules
Metabolic enzymes
Transcription regulatory proteins
Cytoskeletal proteins
What do metabolic enzyme effectors do?
Alter metabolism
What do transcription regulatory protein effectors do?
Alter gene expression
What do cytoskeletal protein effectors do?
Alter cell shape or movement
Of the 3 types of effectors, which is the slowest?
Transcription regulatory proteins
Of the 3 types of effectors, which is the fastest?
Metabolic enzymes and cytoskeletal proteins
Of the 3 types of effectors, which is the longest lasting?
Transcription regulatory proteins
Of the 3 types of effectors, which is the most transient/least lasting?
Metabolic enzymes and cytoskeletal proteins
What are the two main types of intracellular signaling?
Signaling by phosphorylation
Signaling by GTP binding
Are kinases intracellular signaling proteins?
YES
They can be regulated, turned on and off
Why are intracellular signaling processes so complex?
It allows the cell to have different levels of regulation
Creates different side paths, allowing for coordination of multiple things at once
Allows for signal amplification
Gives the cell potential for diverse effector molecules/proteins, and thus diverse responses
List 6 things that can be a signaling molecule (ligand)
Proteins and peptides
Small water-soluble molecules
Lipids
Gasses
Photons
Mechanosensory triggers
Can proteins/peptides be a signaling molecule?
Yes
Can small water-soluble molecules be a signaling molecule?
Yes
Can lipids be a signaling molecule?
Yes
Can gasses be a signaling molecule?
Yes
Can photons be a signaling molecule?
Yes
Can mechanosensory triggers be a signaling molecule?
Yes
Can cells integrate their cellular responses?
Yes
They can interpret competing signals to output a single response
Is it true that some cells that normally live in community will die if they stop receiving certain signals?
Yes, they will undergo apoptosis
This is a cancer prophylactic
(Briefly) describe how signaling by phosphorylation works
Signal occurs
A kinase hydrolyzes ATP –> ADP + Pi
Some effector is phosphorylated, either activating or deactivating it
Signal is turned off by a phosphatase that chews the Pi off the effector
(Briefly) describe how signaling by GTP binding works
Signal occurs
GTP binds to an effector protein
Signal is turned off by GTP hydrolysis of the GTP bound to the effector (GTP –> GDP + Pi)
List the 4 distances/types of cell signaling we discussed
Contact-dependent
Autocrine/Paracrine
Synaptic
Endocrine
Which of the 4 distances/types of cell signaling is the most long-distance?
Endocrine
Which of the 4 distances/types of cell signaling is the most neighborly?
Contact-dependent
Briefly describe contact-dependent cell signaling
Signal cell has the ligand bound in its plasma membrane
Target cell receives the ligand with a membrane-bound receptor
Briefly describe autocrine/paracrine cell signaling
A signal cell sends out ligands via local mediators
Receptor cells receive the ligands with their membrane-bound receptors
Briefly describe synaptic cell signaling
A neuron releases neurotransmitters (ligands) in a synapse
The receptor cell receives the ligands with membrane-bound receptors in the synapse
Briefly describe endocrine cell signaling
A hormone (ligand) is secreted from an endocrine (signal) cell
The hormone is carried by a carrier protein through the bloodstream to the target cell
The target cell receives the hormone with a membrane-bound receptor, or with intracellular receptors
What is the (general) function of G proteins?
On/off switches for intracellular signaling pathways
How do G proteins function as on/off switches for intracellular signaling pathways?
They activate and/or inactivate ion channels or effector enzymes
The ion channels and effectors generate second messengers
Do G proteins generate second messenger molecules?
No, the activate/inactivate ion channels and effector enzymes. The ion channels and effectors are what generates the second messengers
How many genes do humans have for GPCRs? What percent of human protein coding genes is this?
Around 8000, 4% of all human protein coding genes
What percent of pharma drugs target GPCRs?
Around 35%
What does GPCR stand for?
G Protein Coupled Receptor
In the basic example we studied of a GPCR path, what made up the trimeric G protein?
Galpha, Gbeta, and Ggamma
Who in a GPCR path acts like a GEF?
The active receptor protein
Who in a GPCR path acts like a GAP?
The active effector protein
How are Galpha and Ggamma attached to the membrane?
They’re lipid linked (no TMDs)
How is the trimeric G protein attached to the membrane?
Its Galpha and Ggamma subunits are lipid linked to the membrane
