Lipids

Question 1

Three classes of lipids in membranes and where they are synthesized.

Answer 1

There are three classes of lipids in a membrane and all
3 classes are amphipathic (contain hydrophilic and
hydrophobic domains). All are synthesized in the
endoplasmic reticulum (ER). The three classes are
phospholipids, sphingolipids, and cholesterol

Question 2

Most common types of phospholipids.

Answer 2

The
most common phospholipids are
phosphatidylethanolamine (PE), phosphatidylcholine
(PC), phosphatidylserine (PS), and
phosphatidylinositol (PI). All the lipid molecules
shown are derived from glycerol except for
sphingomyelin, which is derived from sphingosine.

Question 3

Structure of cholesterol and why its important

Answer 3

Cholesterol has a polar hydroxyl group, a rigid
steroid ring group, and a hydrocarbon tail (Fig.
6). Cholesterol is intercalated among membrane
phospholipids. The interaction of the steroid ring
with the hydrophobic tail of other phospholipids
tends to immobilize the lipid and decrease
fluidity. The structure of cholesterol is shown on
the right.
Lipids are forced to be straightened by
cholesterol. The thickness of a membrane
depends on the amount of cholesterol.

Question 4

mole % of cholesterol in diff membranes?

Answer 4

The
mole percentage of cholesterol roughly doubles from the ER (7%) to the Golgi (13%) and
again from the Golgi to the plasma membrane (26%).

Question 5

Distrubution of lipids in bilayer

Answer 5

Negatively charged phosphatidylserine (PS), phosphatidylethanolamine
(PE), and phosphatidylinositol (PI) are more abundant on the internal surface, while PC,
sphingomyelin, and glycolipids are more abundant on the external surface. Cholesterol is thought
to be distributed equally in the two leaflets/monolayers.

Question 6

two main mechs by which we get cholesterol synthesis

Answer 6

We get cholesterol from (1) ingestion and uptake and (2) synthesis by the liver. Uptake
depends on the low-density lipoprotein receptor (LDLR). The fundamental concept is that there
is negative feedback for cholesterol production; if you get enough in the diet, you decrease
synthesis and vice versa.

Question 7

First step of cholesterol pathway

Answer 7

The first and rate-limiting enzyme in this pathway is HMGCoA
reductase (3-hydroxy-3-methylglutaryl coenzyme A reductase); statins, used to lower
cholesterol, block this step.

Question 8

sterol regulatory

element binding protein (SREBP)

Answer 8

a protein containing a transcription factor that regulates both
LDLR and all 30 of the synthesis proteins. If cholesterol is low, the transcription factor is
released, moves to the nucleus and activates all these genes.

Question 9

Where is the amount of cholesterol measured?

Answer 9

? The sensor is in the ER membrane, where cholesterol is lowest
in the cell and changes would be easiest to detect

Question 10

structure and origin of transcription factor

Answer 10

The transcription factor is a basic
helix loop helix (bHLH) DNA- binding protein and is held inactive because it is part of a larger
transmembrane protein (SREBP). The transcription factor only becomes active when it is
cleaved from SREBP and then translocates to the nucleus.

Question 11

Where are the proteases that cleave

SREBP to release the bHLH?

Answer 11

? These proteases (there are two of them) are located in the Golgi
complex. 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.

Question 12

steps in cholesterol stuff?

Answer 12

1. When cholesterol levels are low SCAP-SREBP complex dissociates from Insig.
2. SCAP escorts SREBP to the Golgi by vesicular transport.
3. The bHLH transcription factor is released from SREBP by two step proteolysis- RIPRegulated
   Intramembrane Proteolysis
4. S1P is luminal, S2P is within the membrane – cleavage by both is required for activation
5. Nuclear bHLH SREBP moves to the nucleus, binds to DNA promoters, and activates many
   genes to produce more LDLR to bring cholesterol into the cell and to increase all the enzymes
   involved in cellular synthesis of cholesterol.

Question 13

tell me about insig. When does it bind scap? What happens when insig no longer binds scap?

Answer 13

Insig binds SCAP only when cholesterol is high and when Insig binds, it blocks a
signaling part of SCAP. This SCAP signal domain is recognized by a coat protein (COPII – more
about this in a later lecture) for vesicles that move from the ER to the Golgi. Thus, as cholesterol
concentration drops, Insig no longer binds SCAP and the SCAP/SREBP complex gets packaged
into vesicles to go to the Golgi

Question 14

rip

Answer 14

One cleavage that releases bHLH
cuts SREBP in the transmembrane domain. The prevailing dogma at the time of this discovery
was that proteases cut proteins in an aqueous environment, not within a membrane. 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.

Question 15

1. Describe the molecular components of a membrane.

Answer 15

1. Describe the molecular components of a membrane.
   • Cell membranes consist of lipid bilayers with embedded and associated proteins
   • Lipid bilayers are ~5 nm thick (50Å) and serve as a barrier to most water-soluble molecules
   • Proteins spanning the lipid bilayer mediate most of the functions of the of the membrane (~30% of the all proteins encoded in the genome are membrane proteins)
   • Lipid bilayers are dynamic and fluid structures; membrane fluidity depends on composition and temperature
   Composed of lipids, cholesterol, and proteins.

Question 16

2. Identify the parts of a phospholipid, sphingolipid, and cholesterol.

Answer 16

2. Identify the parts of a phospholipid, sphingolipid, and cholesterol.
   Phosphoglycerides: have a glycerol-3-phosphate backbone with 2 fatty acyl chains (either saturated or mono- or polyunsaturated) at one end and a polar head group at the other end. What type of head group determines which phosphatidylglycerol it is (ethanolamine, inositol, etc).
   Sphingolipids: have a ‘sphingosine’ (long acyl unit) with either of two polar head groups (which one it is distinguishes which type of sphingolipid it is) and an attached fatty acid chain.
   Sphingomyelin: phosphocholine head group.
   These are started to be made in the ER and finished in the Golgi apparatus
   Cholesterol: have a characteristic hydroxylated ring structure at one end, and a fatty acid chain on the other.
   There are some double-membrane (mitochondria) and some single-membrane (nucleus) organelles.

Question 17

3. List the different ways proteins associate with membranes.

Answer 17

3. List the different ways proteins associate with membranes.
   Integral proteins (fully or partially embedded in membrane)– can go multiple times back and forth through the membrane (multiple transmembrane domains).
   Notice that transmembrane domains are usually alpha-helices, but can also have beta-sheet barrel formations (usually bacterial).
   Can have transmembrane proteins that are only bound to one side of the bilayer.
   Can have proteins that are only attached at their ends to the membranes.
   Peripheral membrane proteins: covalently interacting with proteins bound in the membrane, but not themselves embedded in the membrane.

Question 18

5. Describe the concept of membrane fluidity.

Answer 18

5. Describe the concept of membrane fluidity.
   Membrane fluidity: the degree to which lateral motion is possible among adjacent phospholipids on a given side of the bilayer. Some lipids are anchored, some are free-floating within any given membrane.
   Notice that different physical compartments of a membrane can have different degrees of fluidity.
   Having acyl chains unsaturated makes the membrane more fluid (less tightly packed).
   Cholesterol, when intercalated in membranes, stiffens the membrane and makes it less fluid.
   Membrane composition determines thickness and curvature:
   Membranes high in cholesterol also thicken the membrane by straightening out the nonpolar, hydrophobic groups inside the bilayer.
   Curvature is dependent on the size of the polar head groups on both sides of the membranes.