BIOCHEM 2280 TOPIC 6

LIPIDS AND BIOLOGICAL MEMBRANES

Objectives:

1. Lipid classes 5. Functions of membrane proteins

2. Roles of lipids 6. Transport across membranes
3. Bilayer properties 7. How proteins associate with bilayers
4. Bilayer fluidity 8. Difficulties in studying membrane proteins

Four main classes of biomolecules

 Our body doesn’t absorb energy from nucleic acids

 1 calorie: amount of energy needed to raise temperature of 1 kg of water by 1 degree Celsius
o 1 dietary Calorie = 1000 scientific calories

Types of Lipids
1. Fatty acids  energy and structure

 A lipid is a biological molecule that has limited to little solubility in water

 Fatty acids are a subset of lipids
o Used for structural purposes
o Called acid because it has a carboxylic group at the end
 Typical fatty acids will not have branches in their hydrocarbon chains
 Saturated:
o Has as many hydrogen atoms around the carbon as possible
o There are no double bonds
o There is a double bond for the oxygen
 Unsaturated:
 o Has double bond so you sacrifice 2 hydrogens
o Trans or cis carbon-carbon double bonds
o Cis double bond gets a little kinky
o Trans is straight

 When organisms are putting double bonds in fatty acids, they put it on the 9 th carbon down from
the carboxylic acids
o When these bonds are put in, living organisms put in cis
 Prefer cis over trans at this particular carbon
 You can have more than one double bond (polyunsaturated)
o There is a group (carbon) that exists between the 2 double bonds
o Bond that is not conjugated
o Each individual cis double bond will add a kink in the chain

 (16:0) 16 refers to the number of carbon atoms and 0 refers to the number of double bonds
 Different systems of telling you where the double bond is located
2. Triaclyglycerols  energy storage

 Second class are triacylglycerols

 3 fatty acids of different lengths and different saturations and react with glycerol
o 3 carbons and each carbon has hydroxyl group
o Great site for activity and there are enzymes that will take the fatty acids and attach it to
the glycerol
o O and H will be lost in condensation reaction
 Forming ester groups
 A form of energy storage
o When you have fatty acids that you don’t want to use right away, you store them as this
molecule

3. Glycerophospholipids  membranes

 Triacylglycerol and then remove one of the fatty acids and attach to phosphate group
 This one has 2 fatty acids joined to it and a phosphate group
 Can have different lengths of chains and different saturations
 Group that is added can be hydrophilic
o Amphipathic: hydrophobic and hydrophilic
 Can have different polar groups that are attached to the phosphate group (hydrophilic addition)
 Glycerophospholipid is based on glycerol
 o Would have 3 OH groups and one OH is used in ester linkage

4. Sphingolipids  membranes

 Used in membranes and has amphipathic property

 Not based on glycerol
 Based on sphingosine
 Has an amine group
 Has an OH in the bottom but that is not involved in making the molecule any more different
 An acyl group is defined by the ester linkage
 The double bond is trans
o This is not a fatty acid so trans makes sense
 You can add a fatty acid to the amide linkage
 Sphingosine is a single molecule but you can have different ceramides
 Glycosphingolipids: head group contains a sugar
5. Steroids  hormones (cholesterol  membranes)
a. Cyclopentanoperhydrophenanthrene ring system

6. Other lipids
Amphipathic lipids

 Hydrophilic head group and hydrophobic or non-polar tail group

 Amphipathic is 2 natures
o If you put these lipids in water, the hydrophobic parts will cluster together and the
hydrophilic parts will be okay interacting with the water
Amphipathic lipids associate in a bilayer

 Then you get the formation of the lipid bilayer

o hydrophobic parts are protected
 3D sphere has water within it and there is a bilayer around it
o sealed compartment formed spontaneously by phospholipid bilayer
 lipids moving in 2 dimensions (dimensions of the plane of the bilayer)
o they stay within the plane but they can freely move
Lipids can diffuse within the bilayer

 Lipids can move and there are different ways that they can move
o the main way is through lateral diffusion
 The lipids can move around laterally and also have bending motions (can also rotate)
o can also rotate
o lipids do these movements but they do not go up and down and they do not readily move
from one half of the bilayer to the other half
 requires hydrophobic zone getting exposed to hydrophilic zone
 Because the lipids are not stuck in place and the membrane itself has some waviness, membranes
can have some flexible motion to them
o this is called fluidity
o the fluidity relates to the diffusion  the higher the diffusion rate, the greater the fluidity
 if the lipids can move around quickly it is a more fluid membrane
 if the lipids are more sluggish, the less fluid the membrane
 Higher the temperature the faster the lipids move so more fluid
 Generally speaking, the longer the chains, the less fluid the membrane will be
o There is interactions between the chains when they are moving around each other
o So the longer the chains, the more interactions you can have
o As lipids are moving past each other, they do rub up against each other
o the longer the chains are the more attraction between the lipids and can slow them down
(less fluidity)
 If you have more degrees of unsaturation, it is going to improve the fluidity
o If there are no C=C they will pack maximally well so it is hard to move around
Impact on cholesterol on membrane fluidity

 Cholesterol is quite hydrophobic

o Has tiny OH group that gives it polar area but most of it is hydrophobic
 Cholesterol will stick in the membrane and it has different effects in the membrane depending on
what the temperature is
o under high temperature, if you add more cholesterol, it will occupy spaces between the
lipids and make it harder to move around
 increases interactions so makes the membrane less fluid
o However, if the temperature is low, the lipids almost freeze where they will want to pack
together and pack a lot (become much less mobile)
o at the lower temperatures, if you add cholesterol, it can hinder the packing
o at low temperatures it makes the membrane more fluid when packing is needed
 at high temperatures, cholesterol makes the membrane less fluid and at low temperatures,
cholesterol makes the membrane more fluid
 Bacterium goes from high temperature to low temperature. What adjustment would bacterium
make to maintain the same level of membrane fluidity?
o Produce lipids with hydrocarbon tails that have more double bonds
o Produce lipids with longer hydrocarbon tails
o Decrease the amount of glycolipids in the membrane
Bilayers are asymmetrical

 A typical membrane will not have the same composition on one side compared to the other
o Top tends to have different chemical groups as the head groups compared to the bottom
parts of the bilayer
 Asymmetrical means that the head groups on the lipids are going to be different depending on
which side of the membrane you are on
 The hexagons are sugars
o Glycolipids tend to be on the outside of the cell
 You do not typically see glycolipids facing the side of the cytosol
 The transfer of a lipid from one side of the bilayer to the other doesn’t tend to happen but it can
happen
o Given enough time, even if you start with an asymmetrical membrane, the driving force
is towards equilibrium which is favourable
o There are enzymes that are meant to maintain the asymmetry of the bilayer by movement
of lipids from one side to another
 Lipids do not tend to move easily, but when they do, there are enzymes that put
them back
 Cholesterol is symmetrical
o Equal number on either side of the membrane
o OH group of cholesterol is not enough to prevent it from easily equilibrating back and
forth
 OH group is not enough to keep it on one side so the cholesterol is going to be
moving all around
Composition of membranes, by mass
 There are more lipid molecules than proteins, but protein is larger and weighs more
o There is action at the membrane due to the proteins

Functions of membrane proteins

  Membranes are not just passive barriers that separate the inside and outside of the cell
 Transporters and channels
o If you have polar molecules, there are not moving across the membrane very readily
 So we need form of transporter that is going to be able to move things inside and
outside
 Anchors
o Structural proteins that are used in things like cytoskeleton to provide anchor points or
structure to the cell
 Receptors
o Sensors of the cell
o Have a site on the outside that is going to detect something in particular which will come
in bind
 Results in a signal that gets sent inside
 Enzymes
o Protein catalyst that will speed up a chemical reaction
o Can be associated with the membrane

Facilitated transport

 Facilitated transport is anytime you are using a protein to assist moving a molecule across a
membrane
o usually need help because they are polar
 some molecules are polar, such as water, and can go across the membrane without help
o non-polar molecules can go across the membrane without help
 But if you have a very charged molecule, it is not going to be able to go across the membrane so
you need to facilitate that to happen
o 2 proteins that do this: transporter or channel
  Transporters
o Specific binding site and then a molecule binds, it changes its shape, and the transporter
will expose the molecule to the other side
 By alternating back and forth between these two conformations, you can move
things from one side to the other
 Channels
o A channel is a pathway that is either open or closed for an (ion) to pass through
 When it is open, certain things can go through it
 Channel discriminates based on charge or the size
 Channel is highly specific
o conformational change between active or inactive (open or close)
 A lot of molecules will go across the membrane without inputted energy
o This is called passive transport

Ex: Potassium Channel

\
 4 subunits come together and form a hole that allows potassium ion to fit perfectly
o Anything bigger than the potassium ion is not going to fit through
o Smaller ions will not go through either
 This protein structure is very rigid and a smaller ion can only make certain
interactions
 A smaller ion in solution would prefer the interactions it makes in water to the
interactions it would have in the channel, energetically speaking
Active vs. passive transport

 Excess of positive charge on one side of the membrane

o There is electrical potential across there
 o If you are positively charged you would cross the membrane and would rather be on the
opposite side
o this becomes driving force and if there is an opening, molecules would easily pass
through
 This is passive transport
 Concentration gradient:
o Molecules will move from an area of high concentration to an area of low concentration
o If one side of the membrane has higher concentration of a certain molecule compared to
the other side, and if there is an opening, the molecule will move through the channel
without any energy input due to driving force of gradient
 Two main ways to form driving force for passive transport:
o Electrical potential and concentration gradient
 If a molecule needs to move against the concentration gradient, energy is required
o This is active transport
 Toxins target ion channels
o toxin will bind to ion channel and prevent ion channel from being functional

Membrane proteins

 Most common proteins are A and D

 Transmembrane
o Goes right across the membrane
o Part of them exposed to one side of the membrane and then they go across and are
exposed to the lipid bilayer and then they poke out to the other side
 Can go across once or multiple times
o Exposed to both aqueous parts and have hydrophobic part in the middle
 Monolayer associated protein
o Part of protein is exposed to hydrophobic part of the membrane but does not poke all the
way through
 Lipid linked
o Lipid is covalently attached to the protein so the protein is tethered to the membrane
  Polypeptide part itself may not be exposed to the inside but the lipid is and the
polypeptide is covalently attached so the whole thing is considered the protein
 integral membrane proteins are embedded into membrane  you have to break
membrane to get it out
 what these 3 have in common is that part of the molecule is in touch with the hydrophobic center
o integral membrane proteins
 if you are trying to get these proteins away from the membrane, you cannot do it
unless you break up the membrane somehow using detergent to cover up the
hydrophobic parts which makes it harder to study integral proteins
 Protein attached
o peripheral membrane proteins
o Associated but only peripherally so not directly embedded in the hydrophobic part
o To transmembrane protein you can associate non-covalently attach peripheral protein
o Is not exposed to the hydrophobic interior so easier to purify

Ex: succinate dehydrogenase

 A protein that functions as a receptor is most likely to be a:
o Transmembrane protein
o Monolayer-associated protein
o Lipid-linked protein
o Peripheral membrane protein
 Not likely to function by itself as a receptor
 You need part of the protein to reach the inside to transmit signal across the membrane when
something is detected on the outside of the membrane
Transmembrane proteins

 Side chains in α-helix stick out from the central helix

o So the side chains are going to interact with hydrophobic interior part of the membrane
o There is going to be a bunch of hydrophobic amino acids in a row to span the membrane
o You need about 20 amino acids to span the membrane
o It does not have to be just a single helix  there can be multiple helices
 Can span the membrane a number of times
  When multiple helices come together the requirement for hydrophobic residues is
not stringent then when there is a single helix

 These proteins will take antibiotics and pump them out

o These will move many different things so they are not specific
 12 transmembrane regions and they cluster together

 β-strands can form cylinder or barrel

 In a β-strand you have one residue with one side chain that faces one way and one side chain that
faces another way
Mobility of integral membrane proteins
  Proteins can also diffuse within the plane of the bilayer
 Sometimes they can’t because they are restricted due to association with other proteins
 Just like lipids proteins tend not to flip from one side to another
Summary:

 Lipids are insoluble in water and come in different classes

 Fatty acids are energy sources but can also combine with glycerol or sphingosine for structural purposes
 Amphipathic lipids form bilayers, on which biological membranes are based
 Bilayers are asymmetrical and vary in fluidity depending on lipid composition
 Classes of membrane proteins include transporters, receptors, and enzymes
 Facilitated transport by transporters or channels can be active or passive
 Proteins may be membrane-integral or membrane-peripheral; most transmembrane proteins cross the
membrane as α-helices
 Membrane proteins can diffuse in the bilayer unless restricted