Lipids and Membrane Proteins

Question 1

Constitution of membranes

Answer 1

Lipids, proteins, carbohydrates (part of lipids and proteins present)

Question 2

Function of proteins and lipids in membranes

Answer 2

Proteins in membrane mediate most functions of membrane

Lipids help coordinate protein functions

Question 3

Properties of membranes

Answer 3

Non-covalent assemblies
Asymmetric
Fluid
Electrically charged (voltage helps drive movement)

Question 4

Fatty acid

Answer 4

Long hydrocarbon chain, saturated or unsaturated, with a carboxyl group at the end

Question 5

Numbering of fatty acids

Answer 5

Numbering begins at carboxyl terminus
C2 is alpha, C3 is beta,… C-end is omega
Position of double bonds is indicated as delta

Question 6

Length of fatty acids in nature

Answer 6

Most are 16-18 carbons in length

Range is between 14-24

Question 7

Melting point and length of fatty acid

Answer 7

Longer fatty acid: higher melting point (more tightly packed: more Van der Waals forces)

Question 8

Phosphoglycerides

Answer 8

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

Question 9

Sphingosine

Answer 9

Can also act as a backbone for a phospholipid

Has room for 1 fatty acid chain to be added on

Question 10

Glycolipids

Answer 10

Sugar-containing lipids
Derived from sphingosine
Sugar replaces phosphate on backbone
Sugars are always extracellular (added by enzymes in Golgi to outside of cell)

Question 11

Cholesterol

Answer 11

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

Question 12

Micelles

Answer 12

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

Question 13

Formation of membranes by phospholipids

Answer 13

Phospholipids self-form membranes spontaneously

Membrane wraps around itself to keep hydrophobic tails away from water

Question 14

Lipid bilayer as a barrier

Answer 14

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

Question 15

Membrane protein functions

Answer 15

1. Sending and receiving signals
2. Physically connecting to extracellular environment
3. Transporting molecules and protons/electrons across membrane

Question 16

Ways membrane proteins associate with the membrane

Answer 16

1. Integral membrane spanning domain (hydrophobic portion)
2. Binding to another membrane protein
3. Covalent modification with a fatty acid
4. Interaction with polar head group of membrane fatty acids

Question 17

Hardest membrane proteins to crystallize

Answer 17

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

Question 18

Most common method of membrane spanning

Answer 18

Proteins have alpha-helices with many hydrophobic amino acids that can span through membrane

Question 19

Method of membrane spanning used in creating pores

Answer 19

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

Question 20

Predicting whether a protein could span the membrane

Answer 20

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

Question 21

Membrane-spanning proteins that cannot be predicted using free energy transfer in water calculation method

Answer 21

Porins are missed using free energy transfer in water calculation: the alternating charges of their amino acids cancel each other out

Question 22

Fluorescent recovery after photobleaching (FRAP)

Answer 22

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

Question 23

Math behind FRAP

Answer 23

S=(4Dt)^0.5
S= distance traveled
D= diffusion coefficient
t= time
Larger D value- easier to travel through membrane

Question 24

Movement of phospholipids in membrane

Answer 24

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

Question 25

Membrane fluidity and temperature

Answer 25

More movement of membrane phospholipids at higher temperatures

Question 26

Fatty acid composition and membrane fluidity

Answer 26

More double bonds in fatty acids- more membrane fluidity