Blake_Biochem_16_Membranes I Flashcards
What kind of assemblies do biological membranes form?
non-covalent
Are membranes symmetrical?
no
What does it mean that membranes have a fluid structure?
lipids and protein moleculed diffuse rapidly in the plane of the membrane, but they donot rotate accross the membrane.
What is the polarity of the membrane?
inside is negative
What direction does a phospholipid face when floating on the surface of water?
polar heads are in contact with water
non-polar hydrophobic tails project into the air.
What structures can lipids form in aqueous solution?
Micille (tails in, together in a sphere)
Lipid Bilayer (includses liposome)
Micelle (3)
limited structure
less than 20 nm
formed when soaps and detergents are added to water
Why are lipid bylayers more thermodynamically stable than micelles?
- Fatty acid chains are too bulky to fit in a micelle. There is tighter packing possible in a bilayer model.
- Bimolecular sheets can have diameters as large as 1mm (106 nm)a
Formation of lipid bilayers is a self-assembly process. What are the major driving forces for their assembly?
HYDROPHOBIC INTERACTION
Van der Waals btwn hydrocarbon tails
Electrostatic and H-bonds
[same as proteins]
Lipid bilayers are held together predominantly by____
hydrophobic interactions
Hydrophobic interactions have 3 significant consequences:
- lipid bilayers have an inherent tendency to be extensive
- lipid bilayers tend to close on themselves so there are no edges with exposed hydrocarbon chains, and so they form a compartment
- lipid bilayers are self-sealing because holes are energetically unfavorable
Lipid bilayers are self-sealing because___
a hole is energetically unfavorable
Lipid bilayers have a very ________ permeability for ions and most polar molecules.
LOW
Which has a higher degree of permeability?
Na+
H20
H2O
How does water transvers the membrane?
Aquaporins
and small size and lack of complete charge
How are membrane proteins classified?
By their degree of dissociability
Interal or Peripheral
Integral Membrane Proteins
(3)
- Interact extensively with the hydrocarbon tails of membrane lipids
- Can be released only by agents that compete for these nonpolar interactions, such as organic solvents and detergents
- Span the lipid bilayer
How are protein components visuallized in a laboratory setting?
SDS-polyacrylamide gel electrophoresis
Peripheral membrane proteins
(3)
- bound to membranes primarily by electrostatic and hydrogen bond interactions with the head groups of lipids
- Dissociated from membranes by adding salt or pH changes
- Bound to the surfaces of integral membrane proteins
- either on the cytostolic or extracellular side
- anchored to the lipid bilayer by a covalently attached hydrophobic chain (fatty acid)
How are peripheral membrane proteins bound to the membrane?
- by electrostatic and hydrogen bonds with head groups
- to integral proteins
- linked by hydrophobic chains
Which type of secondary structure of protein more readilly crosses the lipid bylayer?
alpha-helices
What is the most comon motif in membrane proteins?
alpha-helices
Bacteriorhodopsin
- A light-driven proton pump, converts the energy of light into transmembrane proton gradient that is used to synthesize ATP
- Built almost entirely of alpha-helices (with mostly non-polar amino acids) arranged almost perpendicularly to the bilayer plane
(COX)1
Cyclooxygenase
[or (PGHS)1 (prostaglandin H2 synthase)]
- an integral protein that binds to the luminal leaflet of the ER
- A homodimer that consists of primarily alpha-helices
- does NOT span the membrane
How are lipid-linked proteins associated?
covalently
Lipids anchor lipid-linked proteins to the _________ and mediate ____-____ interactions
membrane
protein-protein
Three kinds of Lipid-Linked protein modification
- Palmitoylation of cysteine residues by a thioester bond
- Farnesylation of cysteine residues at the C-terminus
- Glycosylphosphatidylinositol-link to the carboxyl terminus (GPI)
Palmitoylation
Lipid linking to a cysteine residue by a thioester bond
Farnesylation
Lipid-linking of a cysteine residue at the C-terminus
Covalent attachement of a farnesyl (C15) unit to the C-terminal tetrapeptide CAAX in which Cys is followed by 2 aliphatic residues
Glycosylphosphatidylinostitol-link
lipid-linking to the carboxyl terminus
Steps in Farnesylation
- Covalent attachement of a farnesyl (C15) unit to the C-terminal tetrapeptide CAAX in which Cys is folowed by 2 aliphatic residues
- After the farnesyl group is appended to the protein in thioether linkage with the Cys residue, the AAX tripeptide is hydrolytically cleaved away
What is the function of Farnesyl-linked proteins
Anchoring the protein to the membrane and facilitating protein-protein interactions
What is the function of GPI-linked protein?
- Anchors the proteins to the outer leaflet of the plasma membrane
- Many cell-surface hydrolytic enzyme and adhesions are tethered to cells by a GPI unit.
What are the parts of a mitochondrion?
Two membranes: internal and external
Two compartments: Intermembrane and Matrix
What is the significant feature of the mitochondrial inner membrane?
Internal ridges (cristae)
What function takes place in the intermembrane space of mitochondria?
Oxidative phosphorylation
What important functions take place in the mitochondrial matrix?
TCA cycle
Fatty acid oxidation
What is the permiability of the different mitochondrial membranes?
Outer membrane is very permeable to most small molecules and ions
The inner membrane is impermeable to nearly all ions and polar molecules
How do molecules and ions enter the inner and outer membranes of the mitochondria?
Inner membrane: Transporters shuttle metabolites such as ATP, Pyruvate, and citrate
Outer membrane: Mitochondrial porins (30-35kDa pore-forming proteins) known as Voltage-Dependent Anion Channel (VDAC)
