Membrane Structure, Composition of Cells, Part of Cell Volume Flashcards
What kind of phospholipid motility is seen in plasma membrane?
Lateral diffusion; flexion; rotation. Flip-flop rarely occurs and usually requires ATP driven Flippase
Name the 3 classes of lipids in a membrane? What do they all have in common?
Phospholipids; sphingolipids; cholesterol. All are amphipathic (have hydrophilic and hydrophic domains) and all are synthesized in ER
What are the most common phospholipids?
phosphatidylethanolamine (PE); phosphatidylcholine (PC); phosphatidylserine (PS); and phosphatidylinositol (PI).
Describe the structure of cholesterol
Has a polar hydroxyl group; a rigid steroid ring group; and a nonpolar hydrocarbon tail
What effects does cholesterol have on a membrane?
Interaction of the steroid ring with the hydrophobic tail of other phospholipids immobilizes the lipid and decreases fluidity. Cholesterol straightens lipids and determines membrane thickness. More cholesterol = thicker membranes. Thus; intracellular membranes have less cholesterol (are thinner) than plasma membranes
Describe the distribution of certain lipids among the two layers of the plasma membrane
Negatively charged phosphatidylserine (PS); phosphatidylethanolamine (PE) and phosphatidylinositol (PI) are more abundant on the internal surface. PC; sphingomyelin and glycolipids are more abundant on the external surface. Cholesterol is distributed equally btwn the two layers
What are 2 ways we get cholesterol?
Ingestion/uptake and synthesis by the liver
How is uptake/cholesterol synthesis regulated?
Negative feedback for cholesterol production. If you get enough in the diet; you decrease synthesis and vice versa. Uptake depends on low-density lipoprotein receptor (LDLR)
What is HMGCoA reductase?
The first and rate-limiting enzyme in cholesterol synthesis pathway.
What do statins do?
Lower cholesterol by blocking the HMGCoA reductase step.
What does the sterol regulatory element binding protein (SREBP) do?
Contains a transcription factor that regulates both uptake (via LDLR) and synthesis (via regulation of all 30 synthesis proteins) of cholesterol.
Where are the sensor and SREBP located? Why is the sensor there?
Sensor is in the ER membrane since that is where cholesterol is lowest and changes are easiest to detect.
Describe the transcription factor of SREBP
It is a basic helix-loop-helix (bHLH) DNA-binding protein. It is inactive when it is bound to SREBP and only becomes active when it is cleaved from SREBP (when cholesterol is low) and translocates to the nucleus to active all 30 cholesterol synthesis enzymes. Also activates genes to produce more LDLR to bring cholesterol into the cell
Where are the proteases that cleave SREBP to release the bHLH? What does this mean for SREBP?
Both are located in the Golgi. SREBP must be held in the ER until cholesterol is low and then SREBP must move to the Golgi where it gets cleaved and the bHLH released.
What does SCAP do?
It is a SREBP cleavage activating protein. It binds to both SREBP and sterols like cholesterol. When SCAP is bound to SREBP there is a SCAP signal domain that is recognized by a coat protein (COPII) for vesicles that move from the ER to the Golgi. This is how the SCAP/SREBP complex gets packaged into vesicles to go to the Golgi for cleavage to release the transcription factor
