Lipids and Membranes Flashcards
What are the cellular functions of lipids?
- Essential components of membranes
- Energy storage
- Signaling events
What are the properties of lipids?
- Not true polymers
- Soluble in organic solvents and detergents
- Mostly amphipathic
- Huge variety of structures
What are fatty acids?
- Carboxylic acid followed by long hydrocarbon chains
- 14-40 carbon long in plants and animals
- C16 and C18 are the most common
- Can have saturated or unsaturated bonds (affects physical properties)
Stearic acid
18 carbons, 0 double bonds (fully saturated)
Oleic acid
18 carbons, 1 double bond (one degree unsaturated)
Linoleic acid
18 carbons, 2 double bonds (2 degrees unsaturated)
alpha-Linoleic acid
18 carbons, 3 double bonds (3 degrees unsaturated)
Palmitic acid
16 carbons, 0 double bonds (fully saturated)
What are triacylglycerols?
- Aka triglcerides
- Basis for storage fat in animal tissues
- Glycerol backbone
- 3 fatty acids connected to backbone by ester linkages
- Nonpolar
What are glycerophospholipids?
- Aka phospholipids
- Major component of biological membranes
- Glycerol-3-phosphate backbone
- 2 fatty acids (C1 and C2) connected by ester linkages
- Head group connected to phosphate group is polar
- Amphipathic (hydrophobic tails and hydrophilic head)
- C1 fatty acid is fully saturated C16-C18
- C2 fatty acid is unsaturated C14-C20
Phosphatidic acid
H head
Phosphatidylethanolamine
Ethanolamine head (CH2-CH2-NH3+)
Phosphatidylserine
Serine head CH2-CH(NH3+)COO-
Phosphatidylcholine
Choline head CH2-CH2-N(CH3)3+
Phosphatidylinositol
Inositol head group
What are sphingolipids?
- Have a sphingosine backbone
- Fatty acid at C2 attached by amide linkage
- Trans double bond in C3
- Important for membranes (axons)
- Made of ceramides (sphingosine with fatty acid residue)
What are sphingomyelins?
- Importatnt in plasma membrane
1a. Wrap axon sheath to insulate electrical signals/create myelin sheath - Cerminde + phosphocholine or phosphoethanolamine group
What are cerebrosides?
A type of sphingomyelin made of ceramide and 1 sugar residue
What are gangliosides?
- Most complex sphingolipid made of ceramide and multiple carbohydrate groups
- Important in brain lipids
- Receptors for certain toxins
What are steroids and sterols?
- Lipids mostly found in eukaryotes
- Made of 4 fused rings
- Most common is cholesterol
- Sterol is steroid with hydroxy group on C3
- Sterols are weakly amphipathic
- Steroids are precursors for many hormones
- Steroids are a major part of animal plasma membrane (fluidity)
What are isoorenoids?
- 5 carbon unit lipids
ex. ubiquinone/coenzyme Q in mitochondrial membrane, plant pigments, vitamins A, K and E
What are the 3 ways lipids assemble in water?
- Micelles (single membrane spheres)
- Bilayers (enclosed double membrane)
- Liposomes or vesicles (closed bilayer with solvent in center)
What are biological membranes?
- Bilayer for phospholipids that encloses cells and organelles
- Formed by hydrophobic effect
- Fluid mosaic (2D fluid)
3a. Lateral diffusion common, transverse diffusion rare (needs help)
3b. Constant movement
3c. Transition temperature - Made of different combinations of lipids
4a. Lipids are asymmetrically distributed in each leaflet of a bilayer
Why is PI an important phospholipd?
-Different PI (modified at specific locations with extra phosphate groups) identify different types of membranes
What can membrane-associated proteins do?
- Catalyze reactions (enzymes)
- Transport molecules (channels, pores, pumps)
- traffic molecules (cargoes, coats, adaptors)
- Relay signals (G proteins and receptors)
What are the 3 types of membrane proteins?
- Transmembrane (integral)
- Peripheral
- Lipid-linked
What are the properties of transmembrane proteins?
- Tightly assocaited with bilayers
- Require detergents for removal
3.Most fully transverse membrane
3a. single or multiple alpha helices
3b. Beta barrel structures
3c. Need minimum of 25 amino acids to span 35 angstrom bilayer - usually amphiphillic
- Can be glycosylated
5a. Glycoproteins are formed via a glycosidic bond
5b. Can be O linked (Ser and Thr)
5c. Can be N linked (As, beta linkage)
5d. Very hydrophilic (found in surface loops and turns)
5e. Common in secreted and transmembrane proteins
ex. GPCr
What are the properties of peripheral proteins?
- Loosely associated with bilayer by electrostatic interactions or H bondings
- Can be dissociated from membrane using extreme pH or high salt concentration solution
- Can bind phosphoinostitide head groups
ex. G protein
What are the properties of lipid-linked proteins?
- Protein is covalently attached to a lipid
a. through prenylation (bind isoprenoid/prenyl group to protein at C-X-X-Y/Y is aliphatic)
b. fatty acid linkage (myritstic acid or palmitic acid)
—Fatty acids can insert themselves in bilayer
What is myristolation? (lipid-linked protein)
Amide linkage to N-terminal Gly in protein
What is palmitoylation? (Lipid-linked protein)
Thioester linkage of Cys residue in protein
What are the functions of a membrane?
- Structure and support
- Separate cellular processes
- Sequester chemical reactions
- Provide platform or staging area for cellular processes
All membranes are connected by trafficking
What is membrane trafficking?
- Movement of protein and lipids between different organelles
- often occurs in small membrane-bound vesicles
- Protein orientation is preserved (Er lumen and Golgi cisternae= outside cell)
What are coat proteins?
- Proteins that surround vesicles
- Large multi-subunit protein complexes (4-9 proteins)
- Mediate trafficking pathways
- Bind PI head group of membrane, transmembrane protein cargo, and other proteins needed to build vesicle
What is the APII/clathrin complex?
- AP2 are adaptor proteins that bind clathrin scaffold to vesicle membrane, accessory proteins, and cargo proteins
- Beta 2 subunit binds clathrin, sigma 2 and N mu 2 subunits binds cargo)
- AP2is a heterotetramer with flexible and unstructured regions
- AP2 binds PIP2 head groups = peripheral protein
- Binds short/linear amino acid motifs in transmembrane protein cargo C terminus (YxxO, O=bulky, hydrophobic residue) with C-mu2 subunit
4a. C-mu2 subunit creates Beta sheet banana and motif binds deep hydrophobic grooves on coat protein surface (Y and O plug into grooves) - AP2 exists in 2 conformations: open and closed
5a. Closed in cytoplasm (cargo binding site is deep in protein/blocked”
5b. AP2 binds PIP2 in plasma membrane and undergo conformation change to open state
