L14: Cholesterol Metabolism and Membranes

Question 1

Cholesterol Synthesis

Summarize the 4 steps of cholestoral synthesis

Answer 1

1) Synthesis of mevalonate from acetate
2) Activated isoprene production
3) Condensation of activated isoprene units
4) Cyclization of squalene

Question 2

Cholesterol Synthesis

4 Steps of Cholesterol Biosynthesis

Answer 2

Question 3

Cholesterol Synthesis

What is Oxidosqualene cyclase?
What are it’s 2 properties?

Answer 3

1) Catalyzes Cyclization reaction

2) Membrane insertion of product

Question 4

Cholesterol Synthesis

Within the Cholesterol Metabolism sequence what enzyme is a major point of regulation?

What can this enzyme help with?

Answer 4

Major point of regulation: HMG-CoA reductase

Target of many cholesterol-lowering drugs, which act as competitive inhibitors

Question 5

Cholesterol Synthesis

What is Cholesterol metabolism stimulated by?

What is Cholesterol metabolism inhibited by?

Answer 5

Stimulated by Insulin:

Inhibited by: Low ATP, glucagon, oxysterol

Question 6

Cholesterol Synthesis

What are the fates of cholesterol?

Answer 6

Cholesterol ester and hormones

Question 7

Cholesterol Synthesis

What are the metabolic fates of cholesterol?

Answer 7

Cholesterol ester and hormones

Question 8

Lipid Energetics

Describe the Dispersion of lipids in H20?

Answer 8

• Make the water more ordered
• Entropy DECREASED
• Unfavorable because nature always pushes towards more disorder (aka Entropy INCREASING)

Question 9

Lipid Energetics

What are the 3 formations that lipids take in water?

Answer 9

• Micelle
• Bilayer
• Vesicle

Question 10

Membrane composition

How do the composition of lipids vary?

How do they affect the fluidity of the membrane?

Answer 10

• More saturated = tighter compaction -> less fluidity and higher melting point
• Longer fatty acid -> higher melting point
• Membrane fluidity is physiologically regulated

Question 11

Membrane composition

Describe sterols in the context of membrane composition
- What do they interfere with

Answer 11

Sterols are membrane plasticizers
- Interfere with motion of FA side chains
- Prevent highly ordered packing

Question 12

Membrane composition

What happens to membrane composition at high temperatures?

What raises the melting point of membrane?

Answer 12

• Membrane starts to melt
• Cholesterol raises the MP of membrane
• Addition of unsaturated phospholipids

Question 13

Membrane composition

What happens to membrane composition at high temperatures?

What does cholesterol prevent?

Answer 13

• Membrane starts to freeze
• Cholesterol PREVENTS ordered packing and INCREASES fluidity

Question 14

Lipid Diffusion

Describe the relationship between lipids and diffusion within the planes of membranes?

What is lipid diffusion driven by?

Answer 14

Lipids diffuse rapidly LATERALLY within the plane of membranes

Lipids diffuse very slowly VERTICALLY across membranes

Driven by Monomeric, Oligomeric, ATP-driven

Question 15

Lipid Diffusion

What are the enzymes that catalyze VERTICAL lipid diffusion across membranes?
- What are their mechanisms?
- What are they dependent on?

Answer 15

• Flippase: outside to in; ATP dependent
• Floppase: inside to outside; ATP dependent
• Scramblase: either direction; ATP independent

Question 16

Lipid Diffusion

Describe the process of FRAP (Fluorescence Recovery After Photobleaching)

Answer 16

1) Stain the whole bilayer with florescence
2) Use a high-power laster to get rid of the florescence at a specific area of the bilayer
- Observe lateral diffusion of phospholipids because florescence will migrate to the “photobleached” area of the bilayer

Question 17

Membrane Proteins

What is an Integral protein?

Answer 17

An integral protein is inserted into a large part of the membrane and can span thru the membrane

Question 18

Membrane Proteins

How are Integral proteins removed?

Answer 18

With detergents or organic solvents (which disrupts the hydrophobic interactions)

Question 19

Membrane Proteins

Describe Amphitropic proteins?

Answer 19

NOT embedded in the membrane

Question 20

Membrane Proteins

How are Amphitropic proteins removed?

Answer 20

Removed by pH, chelating agents, other mild treatments

Question 21

Membrane Proteins

What does a Hydropathy plot do?

What is on the Y axis?

What is on the X axis?

Answer 21

Shows hydrophobic vs. Hydrophilic character of side chains given a protein sequence

Y axis: Hyrdropathy index
(pos= hydrophilic) (neg= hydrophobic)

X axis: residue numbers

Question 22

Membrane Fusion

What are the 3 steps in Membrane fusion?

Answer 22

1) Joining
- Vsnares with NTs interact with t-SNARES plasma membrane

2) Hemifusion
- Inner leaflets come into contact as tension builds i on bilayers due to zipping of SNARES

3) Complete Fusion
- Pore widens, NTMs released

Question 23

Membrane Fusion

What are the 3 Models for protein-induced membrane curvature?

Answer 23

Model 1: Protein with intrinsic curvature

Model 2: Protein with one or several amphiphatic helices that force the membrane to bend

Model 3: Proteins with BAR domains polymerize into superstructures that force the membrane to bend

Question 24

Solute Transport

What are the two ways that molecules can travel though membrane channels?

Describe the mechanism behind each

Answer 24

Uniport: one molecule travels though channel

Cotransport: two molecules traveling at once

Question 25

Solute Transport

What are the two modes of Co transport?

Describe them

Answer 25

Symport - same direction

Antiport - opposite directions

Question 26

Solute Transport

What are the six different types of solute transport?

Answer 26

1) Simple diffusion
2) Facilitated diffusion
3) Primary active transport
4) Secondary active transport
5) Ion channel
6) Ionphore-mediated ion transport

Question 27

Solute Transport

Describe Simple Diffusion
- Mechanism
- Accepted compounds

Answer 27

Down concentration gradient; no help needed
- non polar compounds only

Question 28

Solute Transport

Describe Facilitated Diffusion
- Mechanism
- Accepted compounds

Answer 28

Down concentration gradient w/ help of channel
- Polar/ charged compounds

Question 29

Solute Transport

Describe Primary active transport
- Mechanism
- Drive

Answer 29

Against gradient; driven by ATP

Question 30

Solute Transport

Describe Secondary Active transport
- Mechanism
- Drive

Answer 30

Against gradient; driven by other molecules

Question 31

Solute Transport

Describe ion channel
- Mechanism
- Selectivity

Answer 31

Down electrochemical gradient; selective; may be gated

Question 32

Solute Transport

Describe Ionophore-mediated ion transport
- Mechanism
- Selectivity

Answer 32

down electrochemical gradient

Question 33

Solute Transport

What are active transporters?

Answer 33

Primary & Secondary

Question 34

Solute Transport

What are the passive transporters?

Answer 34

Facilitated diffusion, ionophore, ion channels

Question 35

Lipoproteins

Describe the interaction between lipids and hydrophobic fluids?

Answer 35

It is hard to transport lipids though hydrophobic fluids

Lipoproteins can get around this

Question 36

Lipoproteins

What are lipoproteins?

Answer 36

They transfer lipids
- Lipids are hydrophobic and thus are not soluble in hydrophobic fluids like blood serum, therefore they need carriers to help them transport

Question 37

Lipoproteins

How many classes of lipo proteins are there?

Answer 37

4

Question 38

Lipoproteins

How are Lipoproteins uptaken?

Answer 38

Endocytosis: LDL particles bind to receptor and become engulfed by the membrane to form an endosome

Endosome fuses with lysosome for degradation of LDL into AAs, TAG, and cholesterol