Lipids and Many Membrane Proteins Diffuse Rapidly in the Plane of the Membrane

I. The Molecular Design of Life
12. Lipids and Cell Membranes
12.6. Lipids and Many Membrane Proteins Diffuse Rapidly in the Plane of the
Membrane
Biological membranes are not rigid, static structures. On the contrary, lipids and many membrane proteins are constantly
in lateral motion, a process called lateral diffusion. The rapid lateral movement of membrane proteins has been
visualized by means of fluorescence microscopy through the use of the technique of fluorescence recovery after
photobleaching (FRAP; Figure 12.29). First, a cell-surface component is specifically labeled with a fluorescent
chromophore. A small region of the cell surface (~3 µm2) is viewed through a fluorescence microscope. The fluorescent
molecules in this region are then destroyed (bleached) by a very intense light pulse from a laser. The fluorescence of this
region is subsequently monitored as a function of time by using a light level sufficiently low to prevent further
bleaching. If the labeled component is mobile, bleached molecules leave and unbleached molecules enter the illuminated
region, which results in an increase in the fluorescence intensity. The rate of recovery of fluorescence depends on the
lateral mobility of the fluorescence-labeled component, which can be expressed in terms of a diffusion coefficient, D.
The average distance s traversed in time t depends on D according to the expression
The diffusion coefficient of lipids in a variety of membranes is about 1 µ m2 s-1. Thus, a phospholipid molecule diffuses
an average distance of 2 µ m in 1 s. This rate means that a lipid molecule can travel from one end of a bacterium to the
other in a second. The magnitude of the observed diffusion coefficient indicates that the viscosity of the membrane is
about 100 times that of water, rather like that of olive oil.
In contrast, proteins vary markedly in their lateral mobility. Some proteins are nearly as mobile as lipids, whereas others
are virtually immobile. For example, the photoreceptor protein rhodopsin (Section 32.3.1), a very mobile protein, has a
diffusion coefficient of 0.4 µm2 s-1. The rapid movement of rhodopsin is essential for fast signaling. At the other
extreme is fibronectin, a peripheral glycoprotein that interacts with the extracellular matrix. For fibronectin, D is less
than 10-4 µm2 s-1. Fibronectin has a very low mobility because it is anchored to actin filaments on the inside of the
plasma membrane through integrin, a transmembrane protein that links the extracellular matrix to the cytoskeleton.
12.6.1. The Fluid Mosaic Model Allows Lateral Movement but Not Rotation Through
the Membrane
On the basis of the dynamic properties of proteins in membranes, S. Jonathan Singer and Garth Nicolson proposed the
concept of a fluid mosaic model for the overall organization of biological membranes in 1972 (Figure 12.30). The
essence of their model is that membranes are two-dimensional solutions of oriented lipids and globular proteins. The
lipid bilayer has a dual role: it is both a solvent for integral membrane proteins and a permeability barrier. Membrane
proteins are free to diffuse laterally in the lipid matrix unless restricted by special interactions.
Although the lateral diffusion of membrane components can be rapid, the spontaneous rotation of lipids from one face of
a membrane to the other is a very slow process. The transition of a molecule from one membrane surface to the other is
called transverse diffusion or flip-flop (Figure 12.31) The flip-flop of phospholipid molecules in phosphatidyl choline
vesicles has been directly measured by electron spin resonance techniques, which show that a phospholipid molecule flipflops once in several hours. Thus, a phospholipid molecule takes about 109 times as long to flip-flop across a membrane
as it takes to diffuse a distance of 50 Å in the lateral direction. The free-energy barriers to flip-flopping are even larger
for protein molecules than for lipids because proteins have more extensive polar regions. In fact, the flip-flop of a protein
molecule has not been observed. Hence, membrane asymmetry can be preserved for long periods.
12.6.2. Membrane Fluidity Is Controlled by Fatty Acid Composition and Cholesterol
Content
Many membrane processes, such as transport or signal transduction, depend on the fluidity of the membrane lipids,
which in turn depends on the properties of fatty acid chains, which can exist in an ordered, rigid state or in a relatively
disordered, fluid state. The transition from the rigid to the fluid state occurs rather abruptly as the temperature is raised
above T m, the melting temperature (Figure 12.32). This transition temperature depends on the length of the fatty acyl
chains and on their degree of unsaturation (Table 12.3). The presence of saturated fatty acyl residues favors the rigid
state because their straight hydrocarbon chains interact very favorably with each other. On the other hand, a cis double
bond produces a bend in the hydrocarbon chain. This bend interferes with a highly ordered packing of fatty acyl chains,
and so T is lowered (Figure 12.33). The length of the fatty acyl chain also affects the transition temperature. Long
m
hydrocarbon chains interact more strongly than do short ones. Specifically, each additional -CH2- group makes a
favorable contribution of about -0.5 kcal mol-1 (-2.1 kJ mol-1) to the free energy of interaction of two adjacent
hydrocarbon chains.
Bacteria regulate the fluidity of their membranes by varying the number of double bonds and the length of their fatty
acyl chains. For example, the ratio of saturated to unsaturated fatty acyl chains in the E. coli membrane decreases from
1.6 to 1.0 as the growth temperature is lowered from 42°C to 27°C. This decrease in the proportion of saturated residues
prevents the membrane from becoming too rigid at the lower temperature.
In animals, cholesterol is the key regulator of membrane fluidity. Cholesterol contains a bulky steroid nucleus with a
hydroxyl group at one end and a flexible hydrocarbon tail at the other end. Cholesterol inserts into bilayers with its long
axis perpendicular to the plane of the membrane. The hydroxyl group of cholesterol forms a hydrogen bond with a
carbonyl oxygen atom of a phospholipid head group, whereas the hydrocarbon tail of cholesterol is located in the
nonpolar core of the bilayer. The different shape of cholesterol compared with phospholipids disrupts the regular
interactions between fatty acyl chains. In addition, cholesterol appears to form specific complexes with some
phospholipids. Such complexes may concentrate in specific regions within membranes. One result of these interactions is
the moderation of membrane fluidity, making membranes less fluid but at the same time less subject to phase transitions.
12.6.3. All Biological Membranes Are Asymmetric
Membranes are structurally and functionally asymmetric. The outer and inner surfaces of all known biological
membranes have different components and different enzymatic activities. A clear-cut example is the pump that regulates
the concentration of Na+ and K+ ions in cells (Figure 12.34). This transport protein is located in the plasma membrane of
nearly all cells in higher organisms. The Na+-K+ pump is oriented so that it pumps Na+ out of the cell and K+ into it.
Furthermore, ATP must be on the inside of the cell to drive the pump. Ouabain, a specific inhibitor of the pump, is
effective only if it is located outside.
Membrane proteins have a unique orientation because they are synthesized and inserted into the membrane in an
asymmetric manner. This absolute asymmetry is preserved because membrane proteins do not rotate from one side of the
membrane to the other and because membranes are always synthesized by the growth of preexisting membranes. Lipids,
too, are asymmetrically distributed as a consequence of their mode of biosynthesis, but this asymmetry is usually not
absolute, except for glycolipids. In the red-blood-cell membrane, sphingomyelin and phosphatidyl choline are
preferentially located in the outer leaflet of the bilayer, whereas phosphatidyl ethanolamine and phosphatidyl serine are
located mainly in the inner leaflet. Large amounts of cholesterol are present in both leaflets.
I. The Molecular Design of Life
12. Lipids and Cell Membranes
12.6. Lipids and Many Membrane Proteins Diffuse Rapidly in the Plane of the Membrane
Figure 12.29. Fluorescence Recovery After Photobleaching (FRAP) Technique. (A) The cell-surface fluoresces
because of a labeled surface component. (B) The fluorescent molecules of a small part of the surface are bleached by an
intense light pulse. (C) The fluorescence intensity recovers as bleached molecules diffuse out of the region and
unbleached molecules diffuse into it. (D) The rate of recovery depends on the diffusion coefficient.
I. The Molecular Design of Life
12. Lipids and Cell Membranes
12.6. Lipids and Many Membrane Proteins Diffuse Rapidly in the Plane of the Membrane
Figure 12.30. Fluid Mosaic Model. [After S. J. Singer and G. L. Nicolson. Science 175(1972):723.]
I. The Molecular Design of Life
12. Lipids and Cell Membranes
12.6. Lipids and Many Membrane Proteins Diffuse Rapidly in the Plane of the Membrane
Figure 12.31. Lipid Movement in Membranes. Lateral diffusion of lipids is much more rapid than transverse diffusion
(flip-flop).
I. The Molecular Design of Life
12. Lipids and Cell Membranes
12.6. Lipids and Many Membrane Proteins Diffuse Rapidly in the Plane of the Membrane
Figure 12.32. The Phase-Transition, or Melting, Temperature (Tm) for a Phospholipid Membrane. As the
temperature is raised, the phospholipid membrane changes from a packed, ordered state to a more random one.
I. The Molecular Design of Life
12. Lipids and Cell Membranes
12.6. Lipids and Many Membrane Proteins Diffuse Rapidly in the Plane of the Membrane
Table 12.3. The melting temperature of phosphatidyl choline containing different pairs of identical
fatty acid chains
Number of carbons Number of double bonds
Fatty acid
Tm (°C)
Common name Systematic name
22
18
16
14
18
I. The Molecular Design of Life
12. Lipids and Cell Membranes
0
0
0
0
1
Behenate
Stearate
Palmitate
Myristate
Oleate
n-Docosanote
n-Octadecanoate
n-Hexadecanoate
n-Tetradecanoate
cis-∆ 9-Octadecenoate
75
58
41
24
- 22
12.6. Lipids and Many Membrane Proteins Diffuse Rapidly in the Plane of the Membrane
Figure 12.33. Packing of Fatty Acid Chains in a Membrane. The highly ordered packing of fatty acid chains is
disrupted by the presence of cis double bonds. The space-filling models show the packing of (A) three molecules of
stearate (C18, saturated) and (B) a molecule of oleate (C18, unsaturated) between two molecules of stearate.