Lipids and Membrane Proteins Flashcards
Constitution of membranes
Lipids, proteins, carbohydrates (part of lipids and proteins present)
Function of proteins and lipids in membranes
Proteins in membrane mediate most functions of membrane
Lipids help coordinate protein functions
Properties of membranes
Non-covalent assemblies
Asymmetric
Fluid
Electrically charged (voltage helps drive movement)
Fatty acid
Long hydrocarbon chain, saturated or unsaturated, with a carboxyl group at the end
Numbering of fatty acids
Numbering begins at carboxyl terminus
C2 is alpha, C3 is beta,… C-end is omega
Position of double bonds is indicated as delta
Length of fatty acids in nature
Most are 16-18 carbons in length
Range is between 14-24
Melting point and length of fatty acid
Longer fatty acid: higher melting point (more tightly packed: more Van der Waals forces)
Phosphoglycerides
Glycerol-based phospholipids
3 parts: glycerol, 2 fatty acid tails, phosphate and alcohol
All 3 hydroxyls of glycerol are esterified: two with fatty acids, and one with a phosphate
Sphingosine
Can also act as a backbone for a phospholipid
Has room for 1 fatty acid chain to be added on
Glycolipids
Sugar-containing lipids
Derived from sphingosine
Sugar replaces phosphate on backbone
Sugars are always extracellular (added by enzymes in Golgi to outside of cell)
Cholesterol
Oriented parallel to lipid membranes so that its hydroxyl can interact with the phosphate group
When inserted into membrane, can make membrane either more fluid or more rigid as needed
Acts as temperature buffer in membrane fluidity
Backbone of steroid hormones
Micelles
Circular structure made by fatty acids with hydrocarbon tail facing on inside away from water and carboxyl group facing outside towards water
Aren’t created by phospholipids: extra tails are too bulky
Formation of membranes by phospholipids
Phospholipids self-form membranes spontaneously
Membrane wraps around itself to keep hydrophobic tails away from water
Lipid bilayer as a barrier
Polar molecules have a large p value (permeability) and cannot pass through membrane without assistance
Small polar molecules like water can pass through membrane
Molecules with a very large p value stay in bilayer and don’t leave
Membrane protein functions
- Sending and receiving signals
- Physically connecting to extracellular environment
- Transporting molecules and protons/electrons across membrane
Ways membrane proteins associate with the membrane
- Integral membrane spanning domain (hydrophobic portion)
- Binding to another membrane protein
- Covalent modification with a fatty acid
- Interaction with polar head group of membrane fatty acids
Hardest membrane proteins to crystallize
Integral membrane proteins are the hardest to crystallize: it’s hard to tease them apart from their environment
Solution to problem: crystallize with something else, such as lipid
Most common method of membrane spanning
Proteins have alpha-helices with many hydrophobic amino acids that can span through membrane
Method of membrane spanning used in creating pores
Proteins have beta-sheets with R groups that alternate from one side of the backbone to the other: polar groups face the inside of the pore, while non-polar groups face the outside
Predicting whether a protein could span the membrane
Roughly 20 amino acids are needed to cross the membrane: take 20 amino acids at a time, add up their values of free energy transfer in water, and plot results
Spike in graph shows that greater # of amino acids in chain are hydrophobic- transmembrane domain
Membrane-spanning proteins that cannot be predicted using free energy transfer in water calculation method
Porins are missed using free energy transfer in water calculation: the alternating charges of their amino acids cancel each other out
Fluorescent recovery after photobleaching (FRAP)
Method of testing membrane fluidity
Stain proteins with fluorescent dye -> bleach to create hole of fluorescent molecules that no longer fluoresce -> measure time it takes for hole to get filled in
Math behind FRAP
S=(4Dt)^0.5 S= distance traveled D= diffusion coefficient t= time Larger D value- easier to travel through membrane
Movement of phospholipids in membrane
Lateral movement (switch places with adjacent phospholipid) or flip flop (switch places with opposite phospholipid)
Much more lateral movement than flip flop
Flip flop is catalyzed by flippase
Membrane fluidity and temperature
More movement of membrane phospholipids at higher temperatures
Fatty acid composition and membrane fluidity
More double bonds in fatty acids- more membrane fluidity
