11.1 The Composition and Architecture of Membranes Flashcards
the lipid bilayer is
stable in water
glycerophospholipids, sphingolipids, and sterols:
-virtually insoluble in water
-spontaneously form microscopic lipid aggregates when mixed with water
hydrophobic interactions
the clustering of hydrophobic molecule surfaces in an aqueous environment to find the lowest-energy environment by reducing the hydrophobic surface area exposed to water
micelles
spherical structures containing amphipathic molecules arranged with hydrophobic regions in the interior and hydrophilic head groups on the exterior
favored when the cross-sectional area of the head group is greater than that of the acyl side chain(s)
wedge-shaped
bilayer
lipid aggregate in which two lipid monolayers (leaflets) form a 2-dimensional sheet
favored when the cross-sectional areas of the head group and acyl side chain(s) are similar
cylindrical
vesicle (liposome)
forms spontaneously when a bilayer sheet folds back on itself to form a hollow sphere
fluid mosaic
pattern formed by individual lipid and protein units in a membrane
-pattern can change while maintaining the membrane permeability
functions of biological membranes
flexible
self-repair
selectively permeable
flexible
permit shape changes that accompany cell growth and movement
self-repair
permit exocytosis, endocytosis, and cell division
selectively permeable
serve as molecular gatekeepers
proteins and enzymes in and on membranes
transporters
receptors
ion channels
adhesion molecules
transporters
move specific organic solutes and inorganic ions across the membrane
receptors
sense extracellular signals and trigger molecular changes in the cell
-movement of a signal, not a molecule!
ion channels
mediate electrical signaling between cells
adhesion molecules
hold neighboring cells together
endomembrane system is dynamic and functionally differentiated
single membrane
double membrane
single membrane surrounds
endoplasmic reticulum (make protein)
golgi apparatus (post translational modification sorting of that protein)
lysosomes
various small vesicles
double membrane surrounds
nucleus
mitochondrion
chloroplasts (in plants)
general sequence of membrane trafficking for an extracellular protein
endoplasmic reticulum
cis Golgi
trans Golgi
secretory vesicle
secretion from the cell
the functional specialization of each membrane type is reflected in
its unique lipid composition
membrane trafficking
process by which membrane lipids and proteins that are synthesized in the ER move to their destination organelles or to the plasma membrane
where do lipids and proteins undergo covalent modifications?
in the Golgi apparatus- dictates the eventual location of the mature protein
changes in lipid composition during membrane trafficking
sphingolipids and cholesterol largely replace phosphatidylcholine
how are plasma membrane lipids distributed
plasma membrane lipids are asymmetrically distributed between the two leaflets of the bilayer
lipid transfer proteins (LTPs)
soluble proteins that contain a hydrophobic lipid-binding pocket to carry a lipid from one membrane to another
can be bispecific
move lipids basically
sometimes ATP dependent
groups of membrane proteins
-receptors for extracellular signals
-transporters to carry specific polar or charged compounds across the plasma membrane or between organelles
-enzymes
posttranslational modification of membrane proteins
glycosylation
attachment of 1+ lipids
glycosylation
attachment of oligosaccharides to proteins
-typically on the outer face of the plasma membrane
attachment of 1+ lipids
serve as hydrophobic anchors or targeting tags
membrane proteins differ in the nature of their association with the membrane bilayer
integral membrane proteins
peripheral membrane proteins
amphitropic proteins
integral membrane proteins
firmly embedded within the lipid bilayer
peripheral membrane proteins
associate with the membrane through electrostatic interactions and hydrogen bonding with hydrophilic domains of integral proteins and charged head groups of membrane lipids
more loose
amphitropic proteins
associate reversibly with membranes
-found in both membranes and the cytosol
monotopic proteins
have small hydrophobic domains that interact with only a single leaflet of the membrane
bitopic proteins
span the bilayer once, extending on either surface
-have a single hydrophobic sequence somewhere in the molecule
polytopic
cross the membrane several times
have multiple hydrophobic sequences of ~20 residues that each cross the membrane when in the alpha-helical conformation
phospholipids lie
-on the protein surface
-at interfaces between monomers of multisubunit proteins, forming a “grease seal”
the topology of an integral membrane protein can often be predicted
from it sequence
integral membrane protein: an alpha-helical sequence of
20-25 residues (each 1.5A) is just long enough to span the thickness (30A) of the lipid bilayer
stabilized by intrachain hydrogen bonding and the hydrophobic effect
hydropathy index
expresses the free-energy change associated with the movement of an amino acid side chain from a hydrophobic environment to water
-ranges from highly exergonic to highly endergonic
hydropathy plots
average hydropathy index plotted against residue number
easy way for questions to be asked about membrane proteins
window
segment of given length
hydropathy index (y-axis)
average hydropathy for a window
residue number (x-axis)
the residue in the middle of the window
beta barrel
structural motif in which 20+ transmembrane segments form beta sheets that line a cylinder
-stabilized by intrachain hydrogen bonds
can’t assume all beta barrels are porins
porins
proteins that allow certain polar solutes to cross the outer membrane of gram-negative bacteria
-have beta barrels lining the transmembrane passage
in beta conformation:
-7 to 9 residues are needed to span a membrane
-alternating side chains project above and below the sheet
*alternating: hydropathy index does not work very well
in beta strands of membrane proteins:
-every second residue in the membrane-spanning segment is hydrophobic and interacts with the lipid bilayer
-aromatic side chains are commonly found at the lipid-protein interface
which amino acid side chains serve as membrane interface anchors
Tyr and Trp
positive-inside rule
positively charged Lys and Arg residues in the extramembrane loop of membrane proteins occur more commonly on the cytoplasmic face
GPI-anchored protein
exclusively on the outer face and are clustered in certain regions
extracellular spot of PM
