ABI 103 Lipid Bilayer and Components

lipids aggregate to form ___ and ___, which form the structural basis for biological membranes

micelles, bilayers

globular aggregates, hydrocarbon groups are out of contact with water; their arrangement eliminates contact between water and hydrophobic tails and permits the solvation of the polar head groups

micelles

single tailed amphiphiles form ___ or ___ micelles

spheroidal, ellipsoidal

___ lipid molecules would expose the hydrophobic core of micelle to water

too few

___ lipid molecules would give micelle an energetically unfavorable hollow center

too many

what could a very large micelle do to be more effective?

flatten out --> resulting in decrease in curvature which could also make more empty spaces

what two molecules have: two hydrocarbon tails, a rectangular cross section, and form large disk like micelles

glycerophospholipids and sphingolipids

formed by suspension of phospholipids, bounded by a single bilayer, uniform in size, serve as models of biological membranes; are absorbed by many cells through fusion with the plasma membrane

liposomes

how can liposomes be purified?

dialysis, gel filtration chromatography, or centrifugation

transfer of lipid molecule across a bilayer; hydrated, polar head group of lipid has to pass through the anhydrous hydrocarbon core of bilayer

transverse diffusion (flip-flop)

lipids are highly mobile in the bilayer plane; lipid bilayer is a two dimensional fluid

lateral diffusion

what allows for constant motion of lipid bilayer's interior?

the ability of the C-C bonds of the lipids tails to rotate

viscosity of the bilayer increases dramatically closer to the ___

lipid head groups

when the lipid bilayer cools below a characteristic transition temp...

it becomes a gel like solid and viscosity decreases

when the lipid bilayer is above the transition temperature...

there are highly mobile lipids (liquid crystal)

transition temp increases with?

1. chain length of fatty acid residues
2. degree of saturation of its fatty acid residues

what does cholesterol do to the membrane fluidity?

it decreases it by the rigid steroid ring system that interferes with the motions of the fatty acid side chains

the lipid-rich myelinated membrane, inner membrane of mitochondria, and plasma membranes all contain what type of proteins?

membrane proteins

what are the functions of membrane proteins?

1. catalyze chemical reaction
2. mediate the flow of nutrients and wastes across membranes
3. facilitate to relate information from the extracellular environment to intracellular components
4. carry out functions in association with lipid bilayer
5. interact with the hydrophobic core and/or the polar surface of the bilayer

associate tightly with membranes through hydrophobic interactions, can be separated from membranes only by treatments with agents that disrupt agents

integral (intrinsic) membrane proteins

what is a detergent that can solubilize membrane proteins?

sodium dodecyl sulfate (SDS)

integral membrane proteins are ___

amphiphiles

completely span the membrane

transmembrane proteins

has 3 domains:


1. 72- residue externally located N-terminal bearing 16 carb chains
2. 19-residue sequence of entirely hydrophobic residues that spans the erythrocyte membrane
3. 40-residue cytoplasmic C-terminal domain that has a high proportion of charged and polar residues

glycophorin A (transmembrane protein)

in order for a polypeptide chain to penetrate or span the lipid bilayer...

1. it must have hydrophobic side chains
2. it must shield its polar backbone groups

how do transmembrane proteins shield the polar backbone

formation of secondary structure

transmembrane B barrels of known structures consist of __ to __ strands, which have to be ___ to permit the B sheet to close on itseld

a helices, B sheets

247-residue homotrimeric protein, light driven proton pump, binds retinal covalently, consists largely of a bundle of seven \~25-residue a helical rods

bacteriorhodopsin

___ are the dominant forces stabilizing the 3D structures of water-soluble globular proteins

hydrophobic effects

transmembrane B barrels of known structures consist of __ to __ strands, which have to be ___ to permit the B sheet to close on itself

8,22,even

have B barrels; channel-forming proteins in the outer membrane of gram-negative bacteria; permits the entry of small polar solutes; also occur in eukaryotes in the outer membranes of mitochondria and chloroplasts

porins

what are the 3 varieties of lipid-linked proteins?

1. prenylated proteins
2. fatty acylated proteins
3. glycosylphosphatidylinositol-linked proteins

have covalently attached lipids built from isoprene units; most common isoprenoid groups are C15-farnesyl and C20-geranyl reisdues

prenylated proteins

biologically rare saturated C14 fatty acid; appended to a protein via an amide linkage to the a-amino group of an N-terminal Gly residue

myristic acid (lipid linked protein)

is stable; fatty acyl group remains attached to the protein throughout its lifetime

myristoylation acid (lipid linked protein)

saturated C16 fatty acid palmitic acid is joined in thioester linkage to a specific Cys residue

palmitoylation

palmitoylated proteins occur almost exclusively on the ___ and also participate in ___

cytoplasmic face of the plasma membrane, transmembrane signaling

how can the palmitoyl group be removed?

through action of palmitoyl thioesterases

what regulates the association of the protein with the membrane and thereby modulate the signaling processes?

reversible palmitoylation

where are GPI-linked proteins located?

exterior surface of the plasma membrane

what does the GPI core consist of?

phosphatidylinositol glycosidically linked to a linear tetra saccharide composed of 3 mannose residues and one glucosaminyl residue

at the GPI core, the mannose at the ___ end forms a ___ bond with a ___ residue that is amide-linked to the protein's C-terminal carboxyl group

nonreducing, phosphodiester, phosphoethanilamine

can be disassociated from membranes by relatively mild procedures; do not bind lipids; when purified they behave like water-soluble proteins; associate with membranes by binding at their surfaces through electrostatic and hydrogen bonding interactions

peripheral proteins

a peripheral membrane protein that is associated with the outer surface of the inner mitochondrial membrane

cytochrome c