Flashcards in Membrane Structure Deck (16)
Lipid bilayers are dynamic and fluid structures; membrane fluidity depends on composition and temperature. Describe the movements of a typical lipid molecule.
exchanges places with its neighbors in a bilayer 10^7 times/second and diffuses several mm/second at 37°C within a lipid bilayer leaflet.
Phospholipids ____ (do/do not) spontaneously flip-flop in membranes.
do not. For specific functions, an ATP driven Flippase catalyzes flip-flop.
There are three classes of lipids in a membrane and all 3 classes are_____ . All are synthesized in the _____.
amphipathic; endoplasmic reticulum (ER)
The three classes are:
phospholipids, sphingolipids, and cholesterol.
The most common phospholipids are (4):
1. phosphatidylinositol (PI).
2. phosphatidylcholine (PC)
3. phosphatidylethanolamine (PE)
4. phosphatidylserine (PS)
All the lipid molecules shown are derived from ___ except for sphingomyelin, which is derived from sphingosine.
Cholesterol has a polar hydroxyl group, a rigid steroid ring group, and a hydrocarbon tail. Cholesterol is intercalated among membrane phospholipids. The interaction of the _____ tends to immobilize the lipid and____ fluidity.
steroid ring with the hydrophobic tail of other phospholipids; decrease
How does the mole percentage of cholesterol change from the ER/Golgi/PM?
Roughly doubles. ER (7%) to the Golgi (13%) and again from the Golgi to the plasma membrane (26%).
Which phospholipids inhabit the internal surface of the PM? The outside? Where does cholesterol go?
Negatively charged are abundant on the internal surface
iNies (inside Negative PI, PE, PS)
More abundant on the external surface:
Cholesterol is thought to be distributed equally in the two leaflets/monolayers.
We get cholesterol from? Cholesterol synthesis is regulated by a _____ mechanism.
(1) ingestion and uptake
(2) synthesis by the liver
The first and rate-limiting enzyme in the cholesterol production pathway is HMGCoA reductase (3-hydroxy-3-methylglutaryl coenzyme A reductase). _______ block this step.
The bHLH transcription factor is bound to ______, which is a transmembrane protein. Under ____ cholesterol conditions, SREBP relocates to the Golgi via _____, where the bHLH is cleaved and relocates to the nucleus where it ______ transcription of all 30 cholesterol synthesis proteins.
SREBP; low; vesicular transport; activates
The ____ protein binds SREBP under ____ cholesterol conditions, allowing SREBP to relocate to the golgi and be cleaved.
SCAP (SREBP Cleavage Activating Protein); low
Under ____ cholesterol conditions, ____ is bound by ______, which inactivates it, preventing relocation of the _____ complex to the Golgi.
High; SCAP; Insig; SCAP/SREBP
How does the SREBP/bHLH cleavage tie in to Alzheimer's disease?
One cleavage that releases bHLH cuts SREBP in the transmembrane domain. This cleavage within the membrane is now known as RIP, regulated intramembrane proteolysis. RIP has been shown to be critical also for Notch signaling in development and for cleavage of the amyloid precursor protein (APP) to produce the beta amyloid peptide in Alzheimer’s disease.