Molecular, Cellular and Developmental Neuroscience

January 10, 2008 9-10:50 am

Membranes and Membrane Proteins

Lecturer: Professor Eileen M. LaferContact Info.: 415B, 567-3764, Lafer@biochem.uthscsa.edu

Recommended Reading: Stryer Edition 5: Chapters 12 and 13, pp. 319-369

or Stryer Edition 6: Chapters 12 and 13, pp. 326-372

BIOLOGICAL MEMBRANES

1. The boundaries of cells are formed by biological membranes.

2. The boundaries of organelles are also formed by biological membranes.

3. Membranes define inside and outside of a cell or organelle.

4. Membranes confer cells and organelles with selective permeability.

CHEMICAL SYNAPSE

MACROMOLECULAR CONSTITUENTS OF MEMBRANES

1. LIPIDS:cholesterol

sphingolipids: sphingomyelin (SP); gangliosides glyceryl phospholipids: phosphatidylcholine (PC),

phosphatidlyethanolamine (PE), phosphatidylglycerol (PG),

phosphatidlyserine (PS), phosphatidlyinositol (PI), cardiolipin (CL).

2. PROTEIN: integral and peripheral.

3. CARBOHYDRATE: in the form of glycoprotein and glycolipid, never free.

BIOLOGICAL

LIPIDS ARE

AMPHIPATHIC

Hydrophobic Tail

Hydrophilic Head

HOW DO AMPHIPATHIC MOLECULES ARRANGE

THEMSELVES IN AQUEOUS SOLUTIONS?

1. Micelles:

limited structures

microscopic dimensions:

<20 nm in diameter

MemSnD1.avi

MOVIE: MICELLE FORMATION

2. Lipid Bilayers:

bimolecular sheet

macroscopic dimensions:1 mm = 106 nm

MOVIE: BILAYER FORMATION

MemSnD2.avi

Both of these arrangements allow the hyrdrophobic regions to be shielded from the aqueous environment, while

the hydrophilic regions are in contact with the aqueous environment.

Which arrangement is favored by biological lipids?

BILAYERS

The two fatty acyl chains of a phospholipid or glycolipid are too bulky

too fit in the interior of a micelle.

LIPID BILAYERS FORM BY SELF-ASSEMBLY

1. The structure of a bimolecular sheet is inherent in the structure of the constituent lipid molecules.

2. The growth of lipid bilayers from phospholipids is a rapid and spontaneous process in aqueous solution.

LIPID BILAYERS ARE COOPERATIVE STRUCTURES

1. They are held together by many reinforcing non-covalent interactions, which makes them extensive.

2. They close on themselves so there are no edges with hydrocarbon chains exposed to water, which favors compartmentalization.

3. They are self-sealing because a hole is energetically unfavorable.

CHEMICAL FORCES THAT STABILIZE LIPID BILAYERS

1. Hydrophobic interactions are the primary force. These occur between the extensive hydrophobic lipid tails that are stacked in the sheet.

2. van der Waals attractive forces between the hydrocarbon tails favor their close packing.

3. Electrostatic interactions lead to hydrogen bond formation between the polar head groups and water molecules in the solution.

THEREFORE THE SAME CHEMICAL FORCES THAT STABILIZE

PROTEIN STRUCTURES STABILIZE LIPID BILAYERS

MEMBRANES THAT PERFORM DIFFERENT FUNCTIONS CONTAIN DIFFERENT SETS OF PROTEINS

A. Plasma membrane of an erythrocyte.

B. Photoreceptor membrane of a retinal rod cell.

C. Sarcoplasmic reticulum membrane of a muscle cell.

PROTEINS ASSOCIATE WITH THE

LIPID BILAYER IN MANY WAYS

1. A,B,C: Integral membrane proteins-Interact extensively with the bilayer. -Require a detergent or organic solvent to solubilize.

2. D,E: Peripheral membrane proteins-Loosely associate with the membrane,

either by interacting with integral membrane proteins or with the polar head groups of the lipids.

-Can be solubilized by mild conditions such as high ionic strength.

PROTEINS CAN SPAN THE MEMBRANE WITH ALPHA HELICES

Structure of bacteriorhodopsin.MOST COMMON STRUCTURAL MOTIF

-helices are composed of hydrophobic amino acids.

Cytoplasmic loops and extracellular loops are composed of hydrophilic amino acids.

A CHANNEL PROTEIN CAN BE FORMED BY BETA SHEETS

Structure of bacterial porin.

Hydrophobic amino acids are found on the outside of the pore.

Hydrophilic amino acids line the aqueous central pore.

INTEGRAL MEMBRANE PROTEINS DO NOT HAVE TO SPAN THE

ENTIRE LIPID BILAYER

Prostaglandin H2 synthase-1(one monomer of dimeric enzyme is shown)

PROTEIN DIMERIZATION LEADS TO THE FORMATION OF A HYDROPHOBIC

CHANNEL IN THE MEMBRANE

PERIPHERAL MEMBRANE PROTEINS CAN ASSOCIATE WITH MEMBRANES THROUGH COVALENTLY ATTACHED HYDROPHOBIC

GROUPS

FLUID MOSAIC MODEL ALLOWS LATERAL MOVEMENT BUT NOT

ROTATION THROUGH THE MEMBRANE

LIPID MOVEMENT IN MEMBRANES

LIPIDS UNDERGO A PHASE TRANSITION WHICH FACILITATES THE LATERAL DIFFUSION

OF PROTEINS

MOVIE: LIPID DYNAMICS

MemSnD3.avi

MOVIE: FLUID MOSAIC MODEL

MemSnD4.avi

MEMBRANE FLUIDITY IS CONTROLLED BY FATTY ACID

COMPOSITION AND CHOLESTEROL CONTENT

DIFFUSION ACROSS A MEMBRANE

The rate of diffusion of any molecule across amembrane is proportional to BOTH the molecule's diffusion coefficient and its

concentration gradient.

1. The diffusion coefficient (D) is mainly a function of the lipid solubility of the molecule.

Hydrophilic molecules (water soluble, e.g. sugars, charged ions) diffuse more slowly.

Hydrophobic molecules (e.g. steroids, fatty acids) diffuse more rapidly.

2. The concentration gradient (Cside1/Cside2) is

the difference in concentration across the membrane.

Diffusion always occurs from a region ofhigher concentration to a region of lower concentration.

For any given molecule, the greater the concentration difference the greater the rate of diffusion.

OUT IN

10 mM 1 mM

5 mM 5 mM

1 mM 10 mM

no diffusion

THE DIRECTION OF DIFFUSION FOLLOWS THE CONCENTRATION

GRADIENT

The overall rate of diffusion is determined by multiplying the diffusion coefficient and the

magnitude of the concentration gradient:

Rate ~ D x (Cside1/Cside2)

DIFFUSION RATES

SMALL LIPOPHILIC MOLECULES DIFFUSE ACROSS MEMBRANES BY

SIMPLE DIFFUSIONSIMPLE DIFFUSION

1. The small molecule sheds its solvation shell of water.

2. Then it dissolves in the hydrocarbon core of the membrane.

3. Then it diffuses through the core to the other side of the membrane along its concentration gradient.

4. Then it is resolvated by water.

MOVIE: LARGE AND POLAR MOLECULES DO NOT READILY DIFFUSE ACROSS MEMBRANES

BY SIMPLE DIFFUSION

MemIT1.avi

LARGE AND POLAR MOLECULES ARE TRANSPORTED ACROSS

MEMBRANES BY PROTEINACEOUS

MEMBRANE MEMBRANE TRANSLOCATION TRANSLOCATION

SYSTEMSSYSTEMS

1. Passive Transport (also called facilitated diffusion):

The transport goes in the same direction as the concentration gradient. This does not require an input of energy.

2. Active Transport:

The transport goes in the opposite direction as the concentration gradient. This requires an input of energy.

SODIUM-POTASSIUM PUMP

Actively exchanges sodium and potassiumagainst their concentration gradients utilizing

the energy of ATP hydrolysis.

Establishes the concentration gradientsof sodium and potassium essential for

synaptic transmission.

MOVIE:

Movie 12-10.avi

ACETYLCHOLINE RECEPTOR Passively transports sodium and potassium ions along their concentration gradients in response to neuronal signals. Example of a ligand-gated ion channel.

MOVIE: Movement through

an ion channel.

MemIT4.avi

CLASSIFICATION OF MEMBRANE TRANSLOCATION SYSTEMS:

TYPE CLASS EXAMPLE

Channel (translocates ~107 ions per second)

Passive Transport Passive Transport 1. Voltage Gated Na+ channel 2. Ligand Gated AChR 3. cAMP Regulated Cl- channel 4. Other Pressure Sensitive

Transporter (translocates ~102-103 molecules per second)

Passive TransportPassive Transport Glucose transporter

Active Transport Active Transport 1. 1o-ATPase Na+/K+ Pump 2. 1o-redox coupled Respiratory Chain

Linked 3. ATP-binding Multidrug resistance cassette protein transporter 4. 2o Na+-dependent

glucose transport

ACTIVE TRANSPORT MECHANISMS

Utilizes the downhill flow of one gradient to power the formation of another

gradient.

Utilizes the energy of ATPhydrolysis to transport a

molecule against its concentration gradient.

1o 2o

MEMBRANE TRANSLOCATION SYSTEMS DIFFER IN THE NUMBER AND DIRECTIONALITY OF THE

SOLUTES TRANSPORTED

This classification is independent of whether the transport is active or passive.

EXAMPLE: Uniporter

Primary Active Transport-Transports

Ca++ against its concentration gradient utilizing the energy of

ATP hydrolysis

Sarcoplasmic Reticulum Ca++ ATPase

MECHANISM of SR Ca++ ATPase

Na+/K+ Pump

EXAMPLE: SymporterSecondary Active Transport-Na+-Glucose Symporter:Transports glucose against its concentration gradient utilizing the downhill flow of Na+ along its concentration gradient previously set

up by the Na+/K+ pump.

Na+-Glucose Symporter

EXAMPLE: Uniporter

Passive TransportVoltage Gated Ion Channel

K+ Channel: selectively transports K+

PATH THROUGH THE K+ CHANNEL

MECHANISM OF ION SELECTIVITY OF THE K+ CHANNEL

K+ ions interact with the carbonyl groups of the TVGYG sequence in the selectivity filter. Note, Na+ ions are too small to make sufficient productive interactions with the channel.

MECHANISM OF VOLTAGE GATING OF THE K+ CHANNEL

"Ball and Chain Mechanism"

SUMMARY OF TRANSPORT TYPES