BIOCH 200: Slide 6 Biological Membranes (Lipids, Structure + Transport)

What are lipids? Characteristics?

Compounds that are mostly:

• Nonpolar

• Hydrophobic

What are 4 types/ examples of lipids?

1. Fatty Acids

2. Triacylglycerol (Contains F.A)

3. Membrane lipids (Contains F.A.)

4. Cholesterol

What makes up fatty acids?

Long chain Hydrocarbons (nonpolar) + Carboxylic acid (polar)

• General formula = CH3(CH2)nCOO-

What is the charge of Fatty Acids @ pH 7, and thus what is the pka

Negative

• pka = less than 7

Are Fatty acids Amphipathic?

Yes

• there are polar + Non polar portions

Sat vs. Unsat. F.A.

Sat = No double bonds

Unsat = Double bonds

What conformation cis/ trans are most naturally occurring doubles in F.A.’s in?

CIS/Z

How do double bonds in CIS conformation affect the F.A?

LOWERS boiling point

• introduces kinks

What us the shorthand notation for F.A.’s?

Where do you start numbering the carbons on a F.A?

Carbonyl Carbon = 1

What are the shorthand notations for these?

a) 18:0

b) 18:1 delta 9

c) 18:3 Delta 9,12,15

What 2 characteristics of F.A’s increase the M.P.?

• Longer Chain = high M.P.

• LESS double bonds = High M.P.

Of the 2 characteristics: Length vs. Unsaturation (# DB) which has a greater effect on the melting point?

UNSATURATION = greater effect

TRUE or FALSE: the better F.A can pack together the higher the melting point?

TRUE

What is the ranking of the 4 possible combinations of Length and Unsaturation from highest melting point to lowest?

1. Long sat(low # BD)

2. Short sat.(Low # DB)

3. Long Unsat.

4. Short Unsat.

Do Cis or Trans F.A. pack better

TRANS: adopt diff (more linear) shape than cis

What is one Method of storing F.A.s"?

Triacylglycerol (TAG)

• 3 acyl chains (from FA) attached to a glycerol

Are TAG’s amphipathic?

No

are TAG hydrophobic or hydrophilic?

HIGHLY hydrophobic

By what type of link do acyl chains attach to the glycerol? (TAG)

Ester-linked

Will the acyl chains on TAG’s always be the exact same?

No, Mixed TAGs are most common

What are the 3 Membrane lipids

1. Glycerophospholipids

2. Sphingolipids

3. Cholesterol

Of the 3 membrane lipids, which contain fatty Acids?

Glycerophospholipids + sphingolipids

• cholesterol = not contain F.A. as part of membrane (just associate with membranes)

Are membrane lipids amphipathic?

YES

What is the general structure of glycerophospholipids? How are they the same or diff from TAGs

Same: Glycerol w acyl chains attached covalently (Non-amphipathic)

Diff: ONLY 2 acyl chains

• Additional PHOSPHATE HEAD (what makes it amphipathic)

General Cholesterol structure

Rigid structure (rings)

Is cholesterol strongly or weakly amphipathic?

VERY Weakly

• majority = hydrophobic (27 Carbons

• 1 OH group

What % of the mammalian plasma membrane is made up of cholesterol? (don’t know how important)

~ 30%

Does cholesterol form membranes on its own?

NO

Micelles + bilayers? What environment do they form and why do they form

• In water

• They form because the arrangement is favorable

  • polar head = solvated by H2O + Hydrophobic head is hidden from H2O

What forms micelles and what forms bilayers?

FA = Micelles

Membrane lipids = Bilayers

What are Liposomes?

Spherical vesicles of membrane Bilayers

What is the environment on the inside of a liposome? What is the Environment between the 2 leaflets?

• Inside = Aqueous compartment

• Between = hydrophobic (lipid soluble)

Do TAGs form bilayers/micelles? Why?

NO

• Non-amphiphilic + Structure does not support (3 acyl chains makes a triangle shape, cannot line up side by side)

Why can glycerophospholipids form bilayers?

Amphiphilic + structure = good

• only 2 acyl chains = can line up side by side

What makes up the majority of biological membranes?

Bilayers

What kind of interactions are used to assemble Bilayers?

NON-COVALENT

• van der waal

Are bilayers perfectly even like shown in texts books

NO

• Head = different sizes

• Tail = diff length

How does cholesterol associate with the membrane

Non-polar portion = inside the membrane

OH - associates with polar head group of other lipids

What is the transition temperature of a lipid bilayer?

AKA the MELTING temp

• Temp at which it transitions from ordered to a more fluid state

  • dependent on acyl chain

What is the name of the phase of the bilayer below the transition phase? What about above?

• Ordered gel

• Disordered Liquid

Difference in transition temp between artificial and biological membranes

Artificial = NARROW transition temp

• homogenous preparation

Biological = NOT

What causes the biological membrane transition temp to be not as sharp

Mixture of compounds (diff lipids/proteins)

How does the lipid composition of a biological membrane adapt to differences in temp?

Decreasing temp = More UNSAT. + SHORT FA

Increasing = opposite

In humans FA composition in skin cells diff than internal organ cells b/c internal organs are @ higher temp. Which diff in composition is most likely to be observed

B

What does CHOLESTEROL do to the Fluidity of the lipid membrane?

Increases the Effective/usable TEMP RANGE for the membrane

How does cholesterol Increase the Usable range of membrane in low temps

Prevents close packing of acyl chains = more fluid

How does cholesterol Increase the Usable range of membrane in High temps

(ring structure = rigid) Decrease motion/disorder of acyl chains + INCREASING VAN DER WAAL interactions = more rigid

What 2 types of movement can a lipid undergo in a bilayer?

1. Lateral diffusion (side to side within same leaflet)

2. Transverse diffusion (flip-flop from one leaflet to the other)

Which type of movement in bilateral membranes is more common.

Lateral diffusion (fast + free)

• little enegry barrier

Why is transverse diffusion so unfavorable + slow

Need to transport POLAR head through the hydrophobic tails to get to the other side = significant energy barrier

When a bacterium such as E.coli is shifted to a cooler environment, how does it compensate? How is it bacterium

C

• Bacterium don’t synth cholesterol

How can transverse diffusion rates across a membrane be increased?

Flipases

What are Flipases?

Enzymes(membrane proteins) that allow for specific transportation

What are 3 types of membrane proteins?

1. Integral/ transmembrane

2. Peripheral

3. Lipid-linked

How do each of the three membrane proteins associate with the membrane?

1. Integral = Hydrophobic

2. Peripheral = Electrostatic

3. Lipid linked = Lipid prosthetic group (hydrophobic)

What type of membrane protein must it be when it comes to transporting things across/through a membrane

Integral/Transmembrane

What kind of amino acid side chains must be on the surface of the portion of the integral membrane protein that is in contact with he acyl chains?

Hydrophobic/ Non polar

Which types of AA side chains are associated with each color of this integral membrane protein? and what do they each interact with?

Red = Polar charged —> interact with solvent

Yellow = Polar uncharged —> interact with polar head + solvent

White = Non-polar —> Interact with lipid tails

What are the 2 most common structures in PROTEINS that can cross a lipid bilayer?

Alpha-helices + Beta-sheets/barrels

True or false: amino acids that form a transmembrane alpha-helix = generally hydrophobic. Also ~ 20 AA can cross membrane as alpha-helix

True

Can cholesterol by itself form a bilayer? Why?

No

• not amphiphilic enough

• geometry does not allow

True or false: Membranes are dynamic, covalent, complex assembly, containing of lipids and proteins only

FALSE:

• membranes are assembled using NON-COVALENT BONDS

• carbohydrates are also present

True or false: Lipids move laterally but cannot easily undergo transverse diffusion. The opposite is true for proteins.

FALSE: BOTH lipids AND proteins move laterally but cannot easily undergo transverse diffusion

What may limit the movement of proteins in a lipid membrane?

Cytoskeleton

Where are carbohydrates attached on a membrane?

To the extracellular surface of some proteins and lipids

What molecules do not require proteins to cross a lipid bilayer? (diffusion)

Small non-polar

• gases

• hydrophobic molecules

• small polar molecules

What substances require a proteins transporter to move across a membrane

• Large polar molecules

  • glucose

• Charged molecules

  • amino aci+ ions

What 3 things does the rate of simple unmediated diffusion depend on?

1. Size

2. Concentration gradient

3. Lipid Solubility

What are the 2 major types of TRANSPORT?

Active + Passive

What are the Delta G for each type of transport?

Passive = NEGATIVE

Active = POSITIVE

• E = needed

For transport to occur what must l delta G be?

It must be NEGATIVE (less than 0)

What do transport proteins do in terms of transport?

They reduce the activation energy barrier

What are the 3 types of things that enable transport across membranes?

1. Porins

2. Ion channels

3. Transport proteins

How do Porins and Ion channels differ from transport proteins?

Porins and Ion channels do not undergo CONFORMATIONAL CHANGE while transport proteins DO

What type of transport do porins + ion channels provide?

PASSIVE only

Porins vs. Ion channels. Selectivity?

Porins = NON-selective

Ion channels = HIGHLY selective

What determines the selectivity of an ion channel?

The geometry of the pore + the side chains in it

Porins vs. Ion channels. Subunits?

ALL porins = HOMOTRIMERS (3 subunits)

Ions = many subunits (no set #)

How many pores does a porin have?

3

What are porins made up of? Alpha-helix or beta-sheet or beta-barrel?

3 Beta barrels that are identical

True or false: Transporter/ carrier proteins have membrane spanning pores?

FALSE

Are transporter/carrier proteins selective?

Yes

Are transporter/carrier proteins active or passive?

Can be either

How do transporter/carrier proteins work?

Conformational change = alternates opening from one side to the other

What is the shape of the curve in regards the graph of passive transport using a transporter proteins?

Hyperbolic

• upper limit to the speed of transport

What are the 3 classifications for transporter proteins?

1. Uniport

2. Symport

3. Antiport

What are the 2 types of active transport?

1. Primary

2. Secondary

Primary vs. Secondary active transport?

For both, solutes on their own = not favorable

• Primary = uses ATP or other rxn as source of E

• Secondary = uses ION gradient as source of E

As a result, net delta G = favorable (negative)

Transport protein vs transporter protein

Transport = channels, pores, Active + passive transporter/carrier proteins

Transporter = Active and passive transporters

Which transport proteins have an upper/Max V / is saturable with substrate?

Transporter proteins

• Passive

• Active

  • primary + secondary

What type of transport protein is an Na+/K+ ATPase

Primary Active transport

• It uses ATP + H2O to transport the 2 ions AGAINST their [ ] gradients

How many Na+ and K+ transported by Na+/K+ ATPase and in what direction?

3 Na+ OUT

2 K+ IN

True or false: the [ ] gradient cause by the ATPase are used as the source of E for various Secondary active transport

TRUE

• high [Na] out

• High [K+] In

**Overview of how Na+/K+ ATPase works?

2431

• Na+ binds, then ATP, (2) Phosphorylation + conformational change, Na+ ion dissociates, (4) K+ binds, (3)Phosphate hydrolyzed, (1) conformational change = K+ dissociate inside

Na/K ATPase = Electrogenic antiport. What is an electrogenic antiport?

Antiport that creates a charge gradient

What type of transporter is the Na+/Glucose transporter?

Secondary Active transport SYMPORTER

How does the Na+/Glucose symporter work?

Glucose piggybacks on the favorable [ ] gradient of Na+