Biological Membranes

薛雅薇中央大學物理系

Cell Membrane

Two primary building blocks : - Protein - Lipid, or fat . Lipids form a bilayer.

The glycocalyx carbohydrate network on the outside surface: - responsible for cell–cell recognition and adhesion to other cells.

Membrane Structure (I)

1. Supporting and retaining the cytoplasm (inner cell)

2. Being a selective barrier: Get nutrients in and waste products out. non-polar molecules and some small polar molecules can cross. Most polar compounds, e.g. amino acids, must be specifically

transported across the membrane by proteins (Transport).

Functions of cell membranes

4. Recognition

3. Communication (signaling, via receptors)

Information Flow Through the Plasma Membrane via a Membrane Receptor

Structure and function are related.

Structure of Lipid MoleculeLipid molecule contains both hydrophilic

and hydrophobic regions.Polar headgroup : hydrophilic (i.e. water-loving)

Tail (or acyl chain): hydrophobic (i.e. water-fearing)

polar

non-polarH2C

H2C

H2C

H2C

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH3

H2C

H2C

H2C

H2C

H2C

H2C

H2C

O

O = C

H

C

O

H C

HO

CH2

O¯ P = O

O – CH2 – CH2 – N+(CH3)3

CH2

CH2

CH2

CH3

H2C

H2C

H2C

H2C

C = O

CH2

CH2

CH2

CH

CH

60:40 DPPC/Chol bilayers

Lipid motion: - Lateral - Rotational - nodding - …

In liquid crystalline phase

Lipid rafts are postulated to be very important in signal transduction in cells (as signaling platforms, for example).

Consists of specialized domains of different compositions and properties.

These specialized domains (called lipid rafts) are enriched in cholesterol (膽固醇 ) and sphingolipids (high melting lipids)

Size of domain: 0 – 10m

Membrane Structure (II)

It has found that many diseases, such as Alzheimer's, Huntington's, Parkinson's disease, AIDS..etc are related to lipid rafts.

Fluorescence image(1:1 brainPC/brainSPM)+25mol%Chol

Crane & Tamm 2004

Kruijff et al. 2001

AFM images(1:1 DOPC/SPM)+25mol%Chol

Two bilayer thicknesses

Nuclear magnetic resonance (NMR, 核磁共振 )

Differential scanning calorimetry (DSC)Fluorescence microscopyAtomic force microscopy (AFM, 原子力顯微鏡 )

Probing Membrane’s structure & dynamics

Use model membranes

Principles of NMR (Nuclear Magnetic Resonance)

H0 : Static Magnetic Field

H1(t): Alternating Field with frequency

For a bare spin in the presence of H0:

Hzeeman = nH0• I Em = nH0m

For 2H nuclear spin, I = 1

m = 1

0

1

L

= E, resonance occurs (absorption of energy)

L

m = 1

0

1

L

For a spin in the membranes

info: local environments, mol. dynamics

H = H zeeman + HQ

Solid-state NMR spectrometer (固態核磁共振儀 )

- Probing specific site by choosing proper L

- Sample in any form- Non-invasive

H 2 C

H 2 C

H 2 C

H 2 C

C D 2

C D 2

C D 2

C D 2

C D 2

C D 2

C D 2

C D 3

D 2 C

D 2 C

D 2 C

D 2 C

D 2 C

D 2 C

D 2 C

O

O = C

H

C

O

HC

H

O

CH 2

O ¯ P = O

O – CH 2 – CH 2 – N + (CH 3 ) 3

CH 2

CH 2

CH 2

CH 3

H 2C

H 2C

H 2C

H 2C

C = O

CH 2

CH 2

CH 2

CH

CH

Multi-component lipid mixtures, with one lipid labeled

x10

-3

-100 -50 0 50 100

kHz

ld

lo

lo

Characteristic 2H NMR spectra for liquid phases

Liquid-disordered(disordered chain)

Liquid-ordered(highly ordered chain)

Cholesterol-associated

Chemical composition, fraction of each domain, lipid-lipid & lipid-protein interactions, dynamics, mechanical properties …

-100 -50 0 50 100

kHz

17 oC

22 oC

25 oC

Inte

nsi

ty

gel + ld

gel

ld

Pure POPE-d31

Probing phase transition

Lattice melting & chain melting

Thank You