Lipids, Biological Membranes, and Membrane Transport Chapters 9 and 10

1
Lipids, Biological Membranes, and Membrane
TransportChapters 9 and 10
2
The lipid bilayer

• The thickness of a bilayer is usually up to
  around 60 Å.
• Is the barrier that keeps ions, proteins and
  other molecules where they are needed.
• Are impermeable to most water-soluble
  (hydrophilic) molecules.
• Particularly impermeable to ions, which allows
  cells to regulate salt concentrations and pH by
  pumping ions across their membranes using
  proteins called ion pumps.

3
Biological Membranes

• Typically include several types of lipids other
  than phospholipids.
• A particularly important example in animal cells
  is cholesterol, which helps strengthen the
  bilayer and decrease its permeability
• When closed into bubbles', bilayers provide a
  barrier between inside' and outside' i.e. they
  define closed compartments'.
• Large bubbles (microns in diameter) are often
  called vesicles'

4
The Plasma Membrane

• Composed of a phospholipid bilayer and proteins.
• The phospholipid sets up the bilayer structure
• Phospholipids have
• hydrophilic heads and fatty acid tails.
• The plasma membrane is fluid--that is proteins
  move in a fluid lipid background

5
The Fluid Mosaic Model

• Originally proposed by S. Jonathan Singer and
  Garth Nicolson in 1972.
• Allows for dynamic nature of membrane
• Little transition of lipids can take place
  without specific enzymes to mediate transfer -
  flippase.

6
Flippase

• Enzymes located in the membrane responsible for
  aiding the movement of phospholipid molecules
  between the two leaflets that compose a cell's
  membrane
• Two types
• Transverse
• Lateral

7
Transverse Diffusion

• Or flip-flop involves the movement of a lipid or
  protein from one membrane surface to the other.
• Is a fairly slow process due to the fact that a
  relatively significant amount of energy is
  required for flip-flopping to occur.

8
Transverse Diffusion

• Most large proteins do not flip-flop due to their
  extensive polar regions, which are unfavorable in
  the hydrophobic core of a membrane bilayer.
• This allows the asymmetry of membranes to be
  retained for long periods, which is an important
  aspect of cell regulation.

9
Lateral Diffusion

• Refers to the lateral movement of lipids and
  proteins found in the membrane. 
• Membrane lipids and proteins are generally free
  to move laterally if they are not restricted by
  certain interactions.
• Is a fairly quick and spontaneous process

10
The Endo-membrane system

• Proteins or lipids made in the ER contained in
  transport vesicles fuse with the Golgi.
• The Golgi modifies proteins and lipids from the
  ER, sorts them and packages them into transport
  vesicles.
• This transport vesicle buds off and moves to
  the cytoplasm.
• Fuse with plasma membrane.

11
Flippase

• Potential role of ATP-dependent lipid flippases
  in vesicle formation.
• ATP-dependent lipid translocation might help
  deform the membrane by moving lipid mass towards
  the cytoplasmic leaflet

12
Flippase

• This area asymmetry will increase the spontaneous
  curvature of the bilayer, and may thus help
  deform the membrane during vesicle budding.
• Lem3-Cdc50 proteins regulate the localization and
  activity of P4-ATPases.
• P4-ATPases play a pivotal role in the biogenesis
  of intracellular transport vesicles, polarized
  protein transport and protein maturation.

13
Flippase

• Interaction of P4-ATPases with peripheral guanine
  nucleotide-exchange factors (GEFs) might cause
  activation of small GTPases.
• GTPases subsequently bind to the membrane and
  facilitate the assembly of coat proteins (if
  required)
• And thus, the endo-membrane system allows gene
  expression, post-translational modification, and
  secretion to occur!

14
Membrane Structure and Dynamics

• Membrane functions - physical barrier from entry
  and exit form cell and organelles

15

• Phospholipids
• Two fatty acids covalently linked to a glycerol,
  which is linked to a phosphate.
• All attached to a head group, such as choline,
  an amino acid.
• Head group POLAR so hydrophilic (loves water)
• Tail is non-polar hydrophobic
• The tail varies in length from 14 to 28 carbons.

16
Membrane components -

• 60 to 70 of mammalian lipids are phospholipids
• Bacteria have almost no PC and are mostly PE
• Neuronal tissue (myelin) PI gt PC
• Alterations in lipid composition - permeability,
  fluidity, exocytosis, neural transmission and
  signaling potential

17
Membrane Asymmetry

• P-ethanolamine and P-serine predominately faces
  inside of cell
• P-choline faces outside of membrane and inside of
  organelles
• carbohydrates of glycoproteins facing outside
• During apoptosis there is a re-arraignment of
  lipids where phosphatidyl serine moves to the
  exterior face of the membrane.
• One of the key signals of cell death

18

• Proteins - Add function and structure to membrane
• Extrinsic proteins (peripheral)
• Loosely attached to membrane
• ionic bonds with polar head groups and
  carbohydrates
• hydrophobic bonds with lipid
• proteins have lipids tails
• easily displaced from membrane
• salt, pH, sonication

19
Integral proteins

• - tightly bound to membrane - span both sides
• Protein has both polar and hydrophobic sections
  removed only through disrupting membrane with
  detergents
• detergents disrupt lipid bilayer and incorporate
  proteins and some lipids into detergent micelles
• allows for purification of membrane proteins
• reconstitute into specific vesicles for study

20
Transmembrane proteins

• So designated because they are both structurally
  and functionally an integral component of a
  membrane.
• Example
• Human erythrocyte glycophorin A
• Involved in interactions with the Red Blood Cell
  cytoskeleton that may modulate membrane rigidity.
• The extracellular portion of the protein also
  serves as the receptor for the influenza virus

21
Transmembrane proteins

• Has a total molecular weight of about 31,000 and
  is approximately 40 protein and 60
  carbohydrate.
• The primary structure consists of a segment of 19
  hydrophobic amino acid residues with a short
  hydrophilic sequence on one end and a longer
  hydrophilic sequence on the other end.
• The 19-residue sequence is just the right length
  to span the cell membrane if it is coiled in the
  shape of an a-helix.
• The large hydrophilic sequence includes the amino
  terminal residue of the polypeptide chain. 

22
Transmembrane proteins

• General Rules of thumb
• takes about 20 aa to cross membrane
• many proteins cross many times
• odd of transmembrane regions,
• -COOH terminal usually cytosolic
• -NH3 terminal extracellular
• can be predicted by amino acid sequence
• high of side chains will be hydrophobic

23
membrane transport

• The term refers to the collection of mechanisms
  that regulate the passage of solutes such
  as ions and small molecules through biological
  membranes namely lipid bilayers that
  contain proteins embedded in them.
• The regulation of passage through the membrane is
  due to selective membrane permeability.
• The movements of most solutes through the
  membrane are mediated by membrane transport
  proteins which are specialized to varying degrees
  in the transport of specific molecules.

24
An example of membrane transport

• Cholesterol 
• waxy steroid metabolite found in the cell
  membranes and transported in the blood plasma of
  all animals.
• Essential structural component of mammalian cell
  membranes, required to establish proper membrane
  permeability and fluidity
• transported in the circulatory system
  within lipoproteins
• LDL molecules are the major carriers of
  cholesterol in the blood
•  each one contains approximately 1,500 molecules
  of cholesterol
• Recognized by the LDL receptor

25
An example of membrane transport

• Upon binding many LDL receptors become localized
  in clathrin-coated pits.
• Both the LDL and its receptor are internalized
  by endocytosis to form a vesicle within the cell.
• The vesicle then fuses with a lysosome, which
  has an enzyme called lysosomal acid lipase that
  hydrolyzes the cholesterol.
• Now within the cell, the cholesterol can be used
  for membrane biosynthesis or esterified and
  stored within the cell, so as to not interfere
  with cell membranes.

26
Cholesterol Uptake

• Cells destined to take up cholesterol possess
  surface receptors for the LDL particle.
• Receptor Binding Activation that LDL receptor
  binds to Apo-B protein on the LDL particle
• Coated Pit Formation
• Clathrin forms cage around forming endosome
• Clathrin-Coated Vesicle Budding
• Uncoating of the Vesicle

27
Cholesterol Uptake

• Early Endosome associates with other vesicle
• Formation of CURL (Compartment for Uncoupling of
  Ligand and Receptor) or Late Endosome
• Recycling of the Receptor to the cell surface
• Fusion of Transport Vesicle with Lysosome
• Digestion of the LDL to Release Cholesterol

28
CURL Formation Lysosome Digestion

• CURL  Compartment for Uncoupling of Receptor
  Ligand
• pH drops to acidic (pH 5)
• Conformational change in Receptor releases LDL
• Receptor recycles to cell membrane
• Late Endosome fuses with lysosomal vesicles
• LDL is degraded Esterases digest esters
  Cholesterol is released into the cytoplasm

29

• Familial Hypercholesterolemia (FH) high levels
  of blood cholesterol and other characteristics
• Leads to an increase in blood LDL (cholesterol)
• Risk of Atherosclerosis Heart Disease
• Atherosclerosis buildup of cholesterol deposits
  lead to plaques clog arteries
• Contributes to heart attacks at early age
• One human mutation is due to a defect in LDL
  receptor (e.g., in adapter binding site can't
  form coated pit for LDL uptake) which causes the
  buildup of LDL particles at the cell surface
  leading to plaque formation.

30

• Other types of Membrane Transport

31
Summary of membrane transport

• Three types of membrane transporters enhance the
  movement of solutes across plant cell membranes
• Channels passive transport
• Carriers passive transport
• Pumps- active transport

32
Channels

• Transmembrane proteins that work as selective
  pores
• Transport through these passive
• The size of the pore determines its transport
  specifity
• Movement down the gradient in electrochemical
  potential
• Unidirectional
• Very fast transport
• Limited to ions and water

33
Channels

• Sometimes channel transport involves transient
  binding of the solute to the channel protein
• Channel proteins have structures called gates.
• Open and close pore in response to signals
• Light
• Hormone binding
• Only potassium can diffuse either inward or
  outward
• All others must be expelled by active transport.

34
The aquaporin channel protein

• There is some diffusion of water directly across
  the bi-lipid membrane.
• Aquaporins Integral membrane proteins that form
  water selective channels allows water to
  diffuse faster
• Facilitates water movement in plants
• Alters the rate of water flow across the plant
  cell membrane NOT direction

35
Carriers

• Do not have pores that extend completely across
  membrane
• Substance being transported is initially bound to
  a specific site on the carrier protein
• Carriers are specialized to carry a specific
  organic compound
• Binding of a molecule causes the carrier protein
  to change shape
• This exposes the molecule to the solution on the
  other side of the membrane
• Transport complete after dissociation of molecule
  and carrier protein

36
Carriers

• Moderate speed
• Slower than in a channel
• Binding to carrier protein is like enzyme binding
  site action
• Can be either active or passive
• Passive action is sometimes called facilitated
  diffusion
• Unidirectional

37
Example GLUT1 glucose carrier

• GLUT1 is a large integral protein, predicted via
  hydropathy plots to include 12 transmembrane
  a-helices
• Transporter exists in 2 conformations, T1 with
  glucose binding site exposed on outer surface of
  plasma membrane, and T2, with binding site
  exposed on inner surface.
• D-Glc binding on T1 triggers change to T2.
• Glc is released into cytosol, triggering
  conformational change back to T1, ready to pick
  up another glucose from the outside.
• Process is fully reversible, and as
  Sin approaches Sout, rates of entry and exit
  become equal.

38
Active transport

• To carry out active transport
• The membrane transporter must couple the uphill
  transport of a molecule with an energy releasing
  event
• This is called Primary active transport
• Energy source can be
• The electron transport chain of mitochondria
• The electron transport chain of chloroplasts
• Absorption of light by the membrane transporter
• Such membrane transporters are called PUMPS

39
Primary active transport- Pumps

• Movement against the electrochemical gradient
• Unidirectional
• Very slow
• Significant interaction with solute
• Direct energy expenditure

40
pump-mediated transport against the gradient
(secondary active transport)

• Involves the coupling of the uphill transport of
  a molecule with the downhill transport of another
• (A) the initial conformation allows a proton from
  outside to bind to pump protein
• (B) Proton binding alters the shape of the
  protein to allow the molecule S to bind

41
pump-mediated transport against the gradient
(secondary active transport)

• (C) The binding of the molecule S again alters
  the shape of the pump protein. This exposes the
  both binding sites, and the proton and molecule
  S to the inside of the cell
• (D) This release restores both pump proteins to
  their original conformation and the cycle begins
  again

42
pump-mediated transport against the gradient
(secondary active transport)

• Two types
• (A) Symport
• Both substances move in the same direction across
  membrane
• (B) Antiport
• Coupled transport in which the downhill movement
  of a proton drives the active (uphill) movement
  of a molecule

43
Example Lactose permease H symport carrier
44
pump-mediated transport against the gradient
(secondary active transport)

• The proton gradient required for secondary active
  transport is provided by the activity of the
  electrogenic pumps
• Membrane potential contributes to secondary
  active transport
• Passive transport with respect to H (proton)

45
Summary
46
The end