Biochem Ch 11: Membrane Structure and Dynamics

What is one KEY function of membranes?

Separate the cell from the environment, but not be completely impermeable

Do prokaryotes contain membrane bound organelles?

No! Only eukaryotes do

All membranes on different organelles have slightly different…

Functions and composition

Lipids molecules can spontaneously…

Form membranes, grow, and split! Though, these processes are usually regulated by proteins

What are the 3 components of a membrane?

1. Lipids (especially phospholipids)

2. Sterols

3. Proteins

Has you go up to higher organisms starting from prokaryotes, how does the membrane composition change?

You get a higher percentage of sterols by weight, and less protein by weight

Membranes are __ tuned for the specific role they must play

tuned

Amphipathic def

Have a polar end and a non-polar end

Membrane lipids are ___

amphipathic

What is the driving force pushing membrane lipids together?

The hydrophobic effect

The polar heads of membrane lipids are always…

sticking out into aqueous solution

What are the 3 major structures that can form when put into water?

1. Micelle

2. Bilayer

3. Liposomes

What does the formation of those three different structures depend on?

The type/shape of a lipid, and the concentration

Describe micelles

A sphere of lipids, with the hydrophobic tails in the middle and hydrophilic heads sticking out.

Individual lipids are wedge-shaped (ex: fatty acids, only one tail).

Aggregation is concentration dependent- at a certain conc, you can’t get the micelle any bigger, so you just get more of them.

Describe a membrane bilayer.

Two leaflets of lipid monolayers, as one leaflet isn’t stable.

Cylindrical lipids (lipids with two tails).

Hydrophobic fatty acid tails packed inside.

Describe a Liposome.

Can be used for transport.

Small bilayers fold in on themselves and form spherical vesicles that fuse readily with cell membranes or with each other.

Central aqueous cavity.

Sometimes used to try and transport drugs into cells, but it’s not fully successful.

T or F: membranes are very rigid

False! Things can move within the membrane.

Which type of lipid diffusion in a bilayer is very fast?

Lateral diffusion.

The hydrophobic interactions are maintained if lipids are shifted, as are the H-bonds between the polar heads and water.

Which type of lipid diffusion in a bilayer is very slow?

Transverse diffusion (“flip flop”).

Barely occurs.

Energetic penalty dehydration of the polar head group by moving it into the hydrophobic intermembrane space.

What are the 3 main enzymes that can catalyze transverse movement?

1. Flippase

2. Floppase

3. Scramblase

Describe flippases.

• P-Type ATPase

• Moves phosphoamino lipid from the outer leaflet to the inner leaflet.

• Requires ATP.

• Can move the lipid against its conc. gradient

Describe floppases.

• ABC Transporter

• Moves phospholipids from the inner to outer leaflet

• Can use ATP because it can move against the conc. gradient

Describe a scramblase.

• Can move lipids in either direction, but need to move down the conc. gradient.

• Doesn’t require ATP.

What is the generally accepted model for membranes?

Fluid mosaic model

How can membrane fusion be mediated?

Proteins can orient the membranes correctly so that fusion is more likely

What are some examples of membranes needing to be flexible/ fusable?

• Sperm/egg fusion

• Separation of plasma membrane during cell division

• Endo/exo cytosis

Describe neurotransmitter release

• Controlled by SNARE proteins.

• Chemical signals (neurotransmitters) in vesicles released into the synapse

• SNARE proteins help orient the membranes and make fusion quicker

1. T-SNAREs on target membrane (in this case, the plasma membrane)

2. V-SNAREs on vesicle

3. Q-SNAREs are regulatory proteins that are calcium induced

4. When theV and T SNAREs come in close proximity, they" “zip up” and pulls the membranes in, bringing them close together, bending them, and induces tension to make it more likely that they’ll fuse.

5. You first get hemifusion.

6. Then you get full fusion, and release of neurotransmitters into the plasma membrane

About __% of all proteins in humans are membrane proteins!

25%

Integral def

Embedded in the lipid bilayer, always membrane associated.

Hard to study/extract because they exist solely in membranes.

Peripheral def.

NOT embedded in the bilayer, interacting with other membrane proteins at the surface of the membrane

Amphitropic def

Found both in the cytosol and membrane, typically a post-translational modification allows them to interact directly with membranes, which allows them to move on and off the membrane.

What is a primary post-translational modification that can happen to amphitropic proteins so that they’ll stick into the membrane.

Attaching a lipid on to one of the protein’s amino acids.

What does GPI stand for.

Glycosyl phosphatidylinositiol

Describe a GPI anchor’s structure

• Phosphate glycosidic linkage at the C-terminus

• Phosphate group attached to 4 mannose sugars, last of which is attached to an n-acetylglucosamine sugar.

• GlcNAc is ether linked to an Inositiol, which is linked to a phosphate group, then the hydrophobic tail which is inserted into the membrane

GPI anchor is so complex but it allows for…

A high degree of regulation

How can you extract integral proteins?

1. Use a strong detergent, like SDS, to solubilize the protein and form micelles around the protein. The hydrophobic tails of the SDS will surround the hydrophobic surface of the protein.

What kind of secondary structure is good for integral proteins?

Helices

How can you tell where a protein will insert into a membrane based on seq?

Hydrophilic region → Hydrophobic region → Hydrophilic region

Hydropathy index

More hydrophobic an amino acid is, higher # on index. Strong - # if hydrophilic.

Can scan the amino acid seq of a protein for regions of high hydrophobicity.

What is a Kyte-Doolittle Hydropathy plot?

Can measure BLOCKS of amino acids rather than individual to reduce noisiness.

What is a common protein motif for integral proteins?

7 transmembrane helices

What type of protein exemplifies the 7 helix motif?

G-coupled protein receptors (GPCRs)

Role of GPCRs

Receive and transduce signals

Describe the general mechanism of action of GPCRs

1. The GPCR receives a signal, and becomes activated via a conformational change.

2. Upon activation, the receptor binds to a G-protein and then releases it to let it go transduce that signal

What’s another integral membrane motif we can get?

B-barrels!

T or F: certain cellular compartments can occur without separation by a membrane

T! They’re called “phase separated cellular compartments”

What’s an example of a phase separated cellular compartment?

Nucleolus

Phase separated cellular compartments are made by…

Self-aggregation of molecules, and typically occur via changes in conc of those molecules.

What kind of molecules can generally diffuse across membranes easily?

Small, non-polar molecules (O₂, N₂, CH₄)

What kind of molecules can not diffuse across a membrane?

Polar or charged groups (mostly hydrophobic things with a small polar region can diffuse across very slowly)

What are examples of uncatalyzed transport?

1. Simple diffusion

2. Ion Channels

Difference between unsaturatable and saturatable kinetics

• Unsaturable: you can increase the conc.infinitely (not really) andthe rate of diffusion will continue to go up

• Saturable: There is a “V_max” to the rate of diffusion that will not depend on increasing the conc anymore

Uncatalyzed transport is an example of ___ kinetics

Unsaturable

Ion channels are selective “___” that often have gates that open and close. Can only transport __ molecules.

“Holes”, small

Transporters have __ kinetics

Saturable

What are the two kinds of transporters?

1. Passive transporters

   1. Active transporters

      1. Primary

      2. Secondary

Describe Passive transporters

ONLY can transport along the electrochemical gradient, doesn’t use energy

Describe active transport

Requires energy, because they can transport against an electrochemical gradient.

Primary active transport

Directly hydrolyze ATP to get the energy to do something

Secondary active transport

Coupling an uphill reaction with a downhill reaction following a conc gradient.

Passage of hydrophilic molecules through the lipid bilayer is very energetically ____

Unfavorable- desolvating the hydrophilic molecule has an extremely high activation energy

How do transporters catalyze transport?

Lowers the activation energy of transport by maintaining the solvation of the molecule, rather than stripping it

Can replace H-bonds, make center of transporter very hydrophilic.

Transporters are very ___ to a molecule or class of molecules

Specific

In passive transport, the solute conc inside and outside the cell ___, meaning one can never ___ the other.

Equilibrates, exceed

Uniport def

Transporter that only moves 1 thing at once

Symport vs antiport

• Symport: moves two things in the same direction

• Antiport: moves two things in opposite directions

Glucose permease is a ___ transporter and thus is ___

Passive, saturable

Glucose permease is commonly found in…

RBC

RBCs get most of their energy from…

Glycolysis

The conc of glucose in the blood is kept at a constant ___mM

5

Glucose permease has a ___ cavity than can ___ to allow glucose to pass through

Hydrophilic, flip

Transporters have saturable kinetics because…

They don’t have a tunnel like channels, they have a cavity that must flip

Transporters have __ conformations

2 → outward and inward facing

Equation for transporter kinetics

Thermodynamics of transport equation

At equi, free energy change of transport is __, which means…

0, which means it’s equally likely that the solute will move in ether direction

Free energy change doesn’t tell us ___, it tells us ___

Speed, propensity

Free energy equation when its ions involved

Nearly all eukaryotic cells keep a __ voltage inside their cells

Negative

How do human cells typically maintain lower conc of ions inside the cell than outside?

The sodium-potassium pump, which pumps out 3 sodium ions for every 2 potassium ions brought in

Transport inside out = ___, transport outside in = ___

Positive, negative

Examples of primary active transport

Ca⁺² transport within muscle cells, Sodium/potassium pump

Describe Ca⁺² transport within muscle cells

1. Sarcoplasmic reticulums are basically membrane-bound bags of calcium, that have a pump to allow the muscle cell to pump “out” calcium ions when it needs to contract

2. The pump that does this is the SERCA pump- active transport

3. SERCA has a nucleotide binding domain (binds ATP) and a phosphorylation domain, that when phosphorylated, induces a conformational change.

Describe the sodium/potassium pump

1. For each ATP, 3 sodium exit and 2 potassium enter

2. Active transport

3. Results in a buildup of + charge outside the cell

4. Pump is electrogenic → creates an electrical potential across the membrane

Example of secondary active transport

Lactose transporter

Describe the bacterial lactose transporter

1. Coupled with the movement of protons (via a proton pump). Symport of lactose and protons

2. Pumping of protons builds up a + conc. in the periplasmic space. This results in the transporter’s side chains (Glu and Arg) becoming protonated, and flipping conformations.

3. Moves lactose against its concentration gradient and with it, moves protons with its conc. gradient

What kind of transport is the EmrE Efflux Pump?

Secondary Active Transport

Significance of the EmrE pump?

• Responsible for antibiotic resistance

• Pump antibiotics out of the cell

Ion channels/porins are ___

Unsaturable

Ion channels have __

gates

What are the two regulatory systems for ion channels?

Voltage gated or ligand gated

You just have to open a channel __ and then the molecules will __ flow through

Once, freely

Why is the potassium channel extremely selective?

Because of the orientation of the sidechains in the holes

Aquaporins are channels for ___

Water

Aquaporins can move___ molecules of water per second

10⁹

In eukaryotic cells, what organelles are part of the endomembrane system?

ER, golgi, lysosomes and vesicles, mitochondria, nucleus and chloroplasts

In what organelle are most membrane proteins and lipids synthesized

ER

Liquid ordered state of lipids occurs at temps ___ physiological, and liquid disordered occurs at temps __ physiological

Below, above

What does FRAP stand for and measure

• Fluorescence recovery after photobleaching

• measures the rate of lateral diffusion of lipids in a membrane

Glucose enters erythrocytes via passive transport involving ___

GLUT1