L14: Cholesterol Metabolism and Membranes Flashcards
Cholesterol Synthesis
Summarize the 4 steps of cholestoral synthesis
1) Synthesis of mevalonate from acetate
2) Activated isoprene production
3) Condensation of activated isoprene units
4) Cyclization of squalene
Cholesterol Synthesis
4 Steps of Cholesterol Biosynthesis
Cholesterol Synthesis
What is Oxidosqualene cyclase?
What are it’s 2 properties?
1) Catalyzes Cyclization reaction
2) Membrane insertion of product
Cholesterol Synthesis
Within the Cholesterol Metabolism sequence what enzyme is a major point of regulation?
What can this enzyme help with?
Major point of regulation: HMG-CoA reductase
Target of many cholesterol-lowering drugs, which act as competitive inhibitors
Cholesterol Synthesis
What is Cholesterol metabolism stimulated by?
What is Cholesterol metabolism inhibited by?
Stimulated by Insulin:
Inhibited by: Low ATP, glucagon, oxysterol
Cholesterol Synthesis
What are the fates of cholesterol?
Cholesterol ester and hormones
Cholesterol Synthesis
What are the metabolic fates of cholesterol?
Cholesterol ester and hormones
Lipid Energetics
Describe the Dispersion of lipids in H20?
- Make the water more ordered
- Entropy DECREASED
- Unfavorable because nature always pushes towards more disorder (aka Entropy INCREASING)
Lipid Energetics
What are the 3 formations that lipids take in water?
- Micelle
- Bilayer
- Vesicle
Membrane composition
How do the composition of lipids vary?
How do they affect the fluidity of the membrane?
- More saturated = tighter compaction -> less fluidity and higher melting point
- Longer fatty acid -> higher melting point
- Membrane fluidity is physiologically regulated
Membrane composition
Describe sterols in the context of membrane composition
- What do they interfere with
Sterols are membrane plasticizers
- Interfere with motion of FA side chains
- Prevent highly ordered packing
Membrane composition
What happens to membrane composition at high temperatures?
What raises the melting point of membrane?
- Membrane starts to melt
- Cholesterol raises the MP of membrane
- Addition of unsaturated phospholipids
Membrane composition
What happens to membrane composition at high temperatures?
What does cholesterol prevent?
- Membrane starts to freeze
- Cholesterol PREVENTS ordered packing and INCREASES fluidity
Lipid Diffusion
Describe the relationship between lipids and diffusion within the planes of membranes?
What is lipid diffusion driven by?
Lipids diffuse rapidly LATERALLY within the plane of membranes
Lipids diffuse very slowly VERTICALLY across membranes
Driven by Monomeric, Oligomeric, ATP-driven
Lipid Diffusion
What are the enzymes that catalyze VERTICAL lipid diffusion across membranes?
- What are their mechanisms?
- What are they dependent on?
- Flippase: outside to in; ATP dependent
- Floppase: inside to outside; ATP dependent
- Scramblase: either direction; ATP independent
Lipid Diffusion
Describe the process of FRAP (Fluorescence Recovery After Photobleaching)
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
Membrane Proteins
What is an Integral protein?
An integral protein is inserted into a large part of the membrane and can span thru the membrane
Membrane Proteins
How are Integral proteins removed?
With detergents or organic solvents (which disrupts the hydrophobic interactions)
Membrane Proteins
Describe Amphitropic proteins?
NOT embedded in the membrane
Membrane Proteins
How are Amphitropic proteins removed?
Removed by pH, chelating agents, other mild treatments
Membrane Proteins
What does a Hydropathy plot do?
What is on the Y axis?
What is on the X axis?
Shows hydrophobic vs. Hydrophilic character of side chains given a protein sequence
Y axis: Hyrdropathy index
(pos= hydrophilic) (neg= hydrophobic)
X axis: residue numbers
Membrane Fusion
What are the 3 steps in Membrane fusion?
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
Membrane Fusion
What are the 3 Models for protein-induced membrane curvature?
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
Solute Transport
What are the two ways that molecules can travel though membrane channels?
Describe the mechanism behind each
Uniport: one molecule travels though channel
Cotransport: two molecules traveling at once
Solute Transport
What are the two modes of Co transport?
Describe them
Symport - same direction
Antiport - opposite directions
Solute Transport
What are the six different types of solute transport?
1) Simple diffusion
2) Facilitated diffusion
3) Primary active transport
4) Secondary active transport
5) Ion channel
6) Ionphore-mediated ion transport
Solute Transport
Describe Simple Diffusion
- Mechanism
- Accepted compounds
Down concentration gradient; no help needed
- non polar compounds only
Solute Transport
Describe Facilitated Diffusion
- Mechanism
- Accepted compounds
Down concentration gradient w/ help of channel
- Polar/ charged compounds
Solute Transport
Describe Primary active transport
- Mechanism
- Drive
Against gradient; driven by ATP
Solute Transport
Describe Secondary Active transport
- Mechanism
- Drive
Against gradient; driven by other molecules
Solute Transport
Describe ion channel
- Mechanism
- Selectivity
Down electrochemical gradient; selective; may be gated
Solute Transport
Describe Ionophore-mediated ion transport
- Mechanism
- Selectivity
down electrochemical gradient
Solute Transport
What are active transporters?
Primary & Secondary
Solute Transport
What are the passive transporters?
Facilitated diffusion, ionophore, ion channels
Lipoproteins
Describe the interaction between lipids and hydrophobic fluids?
It is hard to transport lipids though hydrophobic fluids
Lipoproteins can get around this
Lipoproteins
What are lipoproteins?
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
Lipoproteins
How many classes of lipo proteins are there?
4
Lipoproteins
How are Lipoproteins uptaken?
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
