Transcript Medical Biochemistry Membranes: Bilayer Properties, Transport Lecture 71 Membrane function • Serve as barriers to separate contents of cell from external environment or contents of.

Medical Biochemistry
Membranes: Bilayer Properties,
Transport
Lecture 71
Membrane function
• Serve as barriers to separate contents of cell from
external environment or contents of organelles
form remainder of the cell
• Proteins in cell membrane have many functions
– transport of substances across the membrane
– enzymes that catalyze biochemical reactions
– receptors on exterior surface that bind external ligands
(e.g., hormones, growth factors)
– mediators that aid ligand-receptor complex in triggering
sequence of events (second messengers that alter
metabolism are produced inside the cell)
Plasma membrane has selective
permeabilities
• Channels and pumps
– for ions and substrates
• Specific receptors
– for signals (e.g., hormones)
• Exchange materials with extracellular
environment
– exocytosis and endocytosis
Membranes form specialized
compartments
• Organelles with specialized functions
– e.g., mitochondria, ER, Golgi complex
• Localize enzymes
• Excitation-response coupling
• Energy Transduction
– photosynthesis, oxidative
phosphorylation
Internal Water Is Compartmentalized
• Intracellular Fluid (2/3 of total water)
– rich in K+ and Mg2+, phosphate major anion
– protein higher
• Extracellular Fluid (1/3 of total water)
– high Na+ and Ca+, chloride major anion
– glucose higher
Composition of membranes
varies within and between cells
• Major lipids in mammalian membranes
– Phospholipids
– Glycosphingolipids
– Cholesterol
• Phospholipids - two major classes
1. phosphoglycerides are more common
• glycerol backbone
• two fatty acids in ester linkage
– usually even-numbered carbons (C16,
C18)
– unbranched, either saturated or unsaturated
• C18 or 20:4,5,8,11,14
• phosphorylated alcohol
– phosphatidic acid (1,2-diacylglycerol 3phosphate) is simplest -- key intermediate
in formation of all other phospholipids
• Phospholipids - two major classes
2.sphingomyelins
• sphingosine backbone (rather than glycerol)
• fatty acid attached by amide linkage
• primary hydroxyl group of sphingosine esterified to
phosphocholine
• prominent in myelin sheaths
• Glycosphingolipids
– sugar-containing lipids
• e.g., cerebrosides and gangliosides
• also derived from sphingosine
• differ from sphingomyelin in group attached
to primary hydroxyl group of sphingosine
– sphingomyelin - phosphocholine
– cerebroside - single hexose (glucose or galactose)
– ganglioside - chain of 3 or more sugars (at least
one is sialic acid)
• Sterols
– most common sterol  cholesterol
• almost exclusively in plasma membrane
– lesser amounts in mitochondria, Golgi, nuclear membranes
– generally more abundant toward outside of plasma
membrane
• intercalates among phospholipids of membrane with
its hydroxyl group at aqueous interface and
remainder of molecule within leaflet
Membrane lipids are amphipathic
• Contain both hydrophobic and hydrophilic
regions (like detergents)
– polar head group
– nonpolar tails
• Saturated fatty acids - straight tails
• Unsaturated fatty acids (generally cis) kinked tails
What is the effect of unsaturated
fatty acids?
What is the effect of unsaturated
fatty acids?
• as more kinks added, membrane becomes
less tightly packed, more fluid
Membrane lipids form bilayers
• Amphipathic phospholipids have two
regions with incompatible solubilities
– in aqueous solvent, organize into
thermodynamically favorable form (e.g.,
micelle)
Membrane lipids form bilayers
• Bimolecular layer (bilayer) can also satisfy
thermodynamic requirement of amphipathic
molecule
– only ends or edges of bilayer sheet exposed to
unfavorable environment
– can eliminate by folding sheet back upon itself to form
enclosed vesicle with no edges.
– Closed bilayer is essential property of membrane
• impermeable to most water-soluble molecules
Lipid-soluble materials
• Gases (oxygen, CO2, nitrogen)
– little interaction with solvents, readily
diffuse through hydrophobic regions of
membrane
• Lipid-derived molecules (e.g.,
steroid hormones)
– readily transverse bilayer
• Organic nonelectrolyte molecules
– diffusion dependent upon oil-water
partition coefficients (the greater lipid
solubility, the greater its diffusion rate
across membrane)
Non-lipid-soluble molecules
• Proteins are also amphipathic molecules
– inserted into lipid bilayer
– form channels for movement of ions and small
molecules
– serve as transporters for larger molecules
Non-lipid-soluble molecules
• Side chains determine hydrophobic nature
– 6 strongly hydrophobic side chains, few
weakly hydrophobic, remainder hydrophilic
– amphipathic proteins have hydrophobic
region transversing bilayer and hydrophilic
regions protruding inside and outside of
membrane
• protein content varies with membrane
– enzymes, transport proteins, receptors
Membranes and components are
dynamic structures
• Lipids and proteins in membranes turn over
– different lipids and proteins have individual
turnover rates, may vary widely
– membrane may turn over more rapidly than any
of its constituents
Membranes Are Asymmetric Structures
• Irregular distribution of proteins within membrane
• External location of carbohydrates attached to
membrane proteins
• Regional asymmetries
– villous border of mucosal cells
– gap junctions, tight junctions,
synapses
Membranes Are Asymmetric Structures
• Phospholipid asymmetry
– choline-containing phospholipids located mainly in
outer leaflet
• phosphatidylcholine, sphingomyelin
– aminophospholipids preferentially located in inner layer
• phosphatidylserine, phosphatidylethanolamine
– cholesterol generally present in larger amounts on the
outside
Membranes Are Asymmetric Structures
• Must be limited transverse mobility (flip-flop)
– half-life of asymmetry in synthetic bilayers is several
weeks
– enzymes for phospholipid synthesis are located on
cytoplasmic side of microsomal membranes
• flippases
• phospholipid exchange proteins
Integral and peripheral proteins
• Integral membrane proteins
– interact with phospholipids,
require detergents for
solubilization
– usually globular, amphipathic
– may span bilayer many times
– asymmetrically distributed
across bilayer
• orientation determined during
insertion in bilayer
Integral and peripheral proteins
• Peripheral proteins
– do not interact directly with phospholipids
– do not require detergent for release
– weakly bound to hydrophilic regions of specific
integral proteins
Integral and peripheral proteins
• e.g., ankyrin, bound to integral protein
“band 3” of erythrocyte membrane
– spectrin, a cytoskeletal structure within
erythrocyte, bound to ankyrin
• plays important role in maintenance of biconcave
shape of erythrocyte
Artificial membranes model
membrane function
• Mixtures of one or more phospholipids treated
(e.g., sonication) to form spherical vesicles 
liposomes
– can control lipid content to examine effects of lipid
composition on certain functions
– purified membrane proteins can be incorporated into
these vesicles to access factors required for function
– environment can be controlled and varied (e.g., ion
concentrations)
– can be made to entrap compounds inside (e.g., drugs,
isolated genes) for drug delivery, gene therapy
Fluid mosaic model
• Singer and Nicolson (1972)
– icebergs (membrane proteins) floating in a sea of predominantly
phospholipid molecules
– translational diffusion - integral proteins and phospholipids can
move within the plane of the membrane
Fluid mosaic model
• phase changes (fluidity) of membrane are dependent upon
lipid composition
– hydrophobic chains of fatty acids can be highly ordered  rigid
structure
– with  temperature, side chains undergo transition from ordered
state (gel-like or crystalline phase) to disordered (liquid-like or
fluid) phase
• transition temperature (Tm)
• longer, more saturated fatty acid chains interact more strongly,
cause higher Tm
• unsaturated chains tend to  fluidity,  compactness
Fluid mosaic model
• Cholesterol modifies fluidity of membranes
– At temperatures below Tm it interferes with the
interaction of hydrocarbon tails of fatty acids and
increases fluidity
– At temperatures above Tm it limits disorder because it is
more rigid than tails of fatty acids and cannot move in
membrane to same extent, thus limits fluidity
– At high cholesterol:phospholipid ratios, transition
temperatures are abolished
Fluid mosaic model
• Fluidity significantly affects membrane functions
– As membrane fluidity , so does permeability to water
and other small hydrophilic molecules
– Lateral mobility of integral proteins increases
• If active site of integral protein resides exclusively in
hydrophilic regions, changing fluidity probably has little effect
on activity
• If protein involved in transport, with transport components
span membrane, lipid phase effects may significantly alter
transport rate.
• EXAMPLE: Insulin receptor - As concentration of unsaturated
fatty acids in membrane increased (grow in unsaturated. fatty
acid rich medium), fluidity increases, receptor binds more
insulin
Fluid mosaic model
• Some protein-protein interactions within plane of
membrane can restrict mobility of integral proteins
Asymmetry of proteins and lipids
maintained during membrane assembly
• Fusion of a vesicle with the
plasma membrane preserves
the orientation of any
integral proteins embedded
in the vesicle bilayer
Signal Sequences Target Many Proteins
• Many proteins carry signals that target them to
their destination
• Major sorting decision - synthesis on free or
membrane-bound polyribosomes
– cytosolic branch
• no signal peptide, delivered to
cytosol
• can be directed to mitochondria,
nuclei, peroxisomes by specific
signals
Signal Sequences Target Many Proteins
• rough ER branch (Secretory or
exocytotic pathway)
– contain signal peptide
– many destined for various
membranes (ER, Golgi, lysosomes,
and plasma membrane) and for
secretion
– certain proteins sorted in Golgi for
delivery to lysosomes
– proteins destined for secretion
carried in secretory vesicles
• regulated secretion (secretory vesicles)
• constitutive secretion (transport vesicles)
Signal Hypothesis - Entry into ER
• Blobel and Sabatini - explanation for difference
between free and membrane-bound ribosomes
• All ribosomes have the same structure, distinction
dependent upon protein possessing signal
sequence
Synthesis of secretory proteins
1. N-terminal signal sequence is
synthesized
2. Signal bound by SRP, complex
docks with SRP receptor on ER
membrane
3. Signal sequence binds to
translocon, internal channel opens,
inserted into translocon
4. Polypeptide elongates, signal sequence cleaved
5. ER chaperones prevent faulty folding, carbohydrates added to specific
residues
6. Ribosomes released, recycle
7. C-terminus of protein drawn into ER lumen, translocon gate shuts, protein
assumes final conformation