1. Read the following slides by “Day 29”

2. Recall that cell membranes are lipid bilayers composed of amphipathic lipid molecules.
Polar heads
Nonpolar tails
Polar heads
If you haven’t already done so, go to these two websites to view a realistic 3D version of the lipid bilayer which you can rotate with the mouse:
(Select JSmol button, then select Lipid bilayer from the menu at the right. Use the checkbox to see where water molecules are located)
Fig from Lehninger

3. Biological Membranes Electron micrograph of a cell membrane
Fig 11-1 from Lehninger

4. Composition and Architecture of Membranes
Biological membranes are lipid bilayers, but membranes also contain proteins.
The relative proportions of protein and lipid vary with the type of membrane.
Each kingdom, species, tissue, or cell type, and the organelles of each cell type,
have a characteristic set of membrane lipids. The protein composition of different membranes varies even more widely.
Table 11-1 from Lehninger

5. The hydrophobic effect is the thermodynamic driving
force for the formation and maintenance of the lipid bilayer.
The hydrophobic tails of lipids can not interact with water– water would form an ordered shell around this part of the molecule. Glycerophospholipids and sphingolipids form enclosed bilayers to prevent this entropically unfavorable ordering of water molecules. The bilayer structure allows only the polar heads to be in contact with water (both inside and outside the membrane), and the nonpolar tails are completely sequestered from water.
exposed edges cause the bilayer to collapse into an enclosed membrane
Fig 11-4 from Lehninger

6. Mosaic– a picture made up of many small, diverse elements (usually colored pieces of stone or glass).
Biological membranes are often referred to as “fluid mosaics.”

7. The “fluid mosaic” model for the structure of membranes.
As you can see from this figure, the membrane surface does look like a mosaic. It is called a fluid mosaic because individual lipid and protein molecules constantly move about laterally within one layer of the bilayer. So the mosaic picture is fluid in the sense that it is constantly changing. Note: lateral movement of lipid and protein molecules is possible because interactions between them are noncovalent (weak).
Fig 11-3 from Lehninger

8. Membrane proteins are classified as integral or peripheral.
Peripheral proteins associate with the surface of the membrane through:
-electrostatic interactions between charged R groups and charged
groups on the polar heads of lipids
-hydrogen bonds between polar protein groups and polar lipid heads
-electrostatic interactions or hydrogen bonds with integral proteins
Peripheral membrane proteins sit on the surface of the membrane and interact with it.
Fig 11-3 from Lehninger

9. Membrane proteins are classified as integral or peripheral.
Integral membrane proteins span the lipid bilayer and protrude on either or both sides. The membrane-spanning region of the protein is hydrophobic and may be composed of one or more a-helices or a b-barrel.
a-helical integral membrane proteins have one or more helices that completely span the membrane.
In b-barrel integral membrane proteins, the structure of the protein is like a tube (or barrel), formed from many b-strands lining up side-by-side in a circle.
Integral membrane protein with a total of 7 a-helices that span the membrane.
Integral membrane protein with a b-barrel structure.
Fig from Lehninger
Fig from Lehninger

10. a-helical integral membrane proteins
Review of a-helix structure:
The side chains stick out from the helix and cover most of its outer surface.
purple spheres
represent
side chains
green spheres
represent
side chains
Fig 4-4 from Lehninger
Fig from Biochemistry, 5th ed.,
by Berg, Tymoczko, and Stryer.

11. a-helical integral membrane proteins
Examples are shown below. ~20 amino acids (6-7 turns of the helix) are required in order to span the lipid bilayer.
Membrane-spanning helices are always composed of nonpolar residues since the helix is surrounded by lipids and all the side chains are in contact with the hydrophobic lipid tails. The helix remains in place due to the hydrophobic effect.
Proteases and reagents that react with certain R groups will not react with the membrane-spanning region of an integral protein because it is protected/sequestered by the membrane lipids. If the cell is intact, the regions of the protein that protrude on the
inside surface of the
membrane will also be
protected from
proteases and other
reagents that are present
outside the cell, since
proteases/reagents cannot
normally cross the
membrane of an intact
cell.
Fig 11-9 from Lehninger

12. b-barrel integral membrane proteins
Review of b-sheet structure:
Recall that the side chains project above and below the plane of a b-sheet, so each surface of the sheet is covered with side chains.
The side chains alternate in projecting above and below the plane of the b-sheet. (1st R-group points up, 2nd R-group points down, 3rd R-group points up, and so on…)
the flat ribbon
represents the
backbone of one b-strand
purple spheres
represent
side chains
edge view of one b-strand

13. Review of b-sheet structure:
The side chains alternate in projecting above and below the plane of the b-sheet.
(1st R-group points up,
2nd R-group points down,
3rd R-group points up,
and so on…)
A five-stranded b-sheet is shown here.

14. b-barrel integral membrane proteins
b-barrel structure:
Imagine a many-stranded b-sheet wrapping around to form a barrel or tube shape. If this happens, the side chains will cover the inner and outer surfaces of the barrel.
Side view of barrel
Top view of barrel

15. b-barrel integral membrane proteins
Fig 11-13,
Lehninger 4th ed.
A b-strand formed from ~7-9 residues is required in order to span the bilayer.
The outside surface of the barrel is in contact with the hydrophobic tails of the lipids surrounding the barrel. Therefore, the outside surface of the barrel is covered by nonpolar side chains.
The interior surface of the barrel never makes contact with the lipids. Therefore, any type of side chain can be located on the inside surface of the barrel.
Any type of side chain
Nonpolar side chains, in contact with lipids

16. Membrane proteins covalently attached to lipids.
Another type of membrane protein that is considered integral (rather than peripheral) is a protein that is covalently bonded to a membrane lipid. In this case the lipid (but not the protein) spans the bilayer. The specific lipid to which the protein is attached dictates which side of the membrane the protein will be on.
The pink shaded objects represent proteins. They are covalently linked to yellow shaded lipids which are an integral part of the membrane.
Fig from Lehninger

17. Methods for releasing proteins from membranes in order to study these proteins.
Peripheral membrane proteins can be released from the membrane by
treatments that interfere with the electrostatic interactions or hydrogen bonds
between the protein and the membrane surface:
• change in pH
• change in ionic strength (change in salt concentration)
• urea
• carbonate (CO32-)
Integral membrane proteins can be released only by disrupting the
membrane. The hydrophobic surface of the protein, which was originally
buried in the hydrophobic interior of the bilayer, must be “coated” with other
hydrophobic molecules in a micelle-like cluster to protect it from water:
• detergents are used for this purpose (see next slide)
Proteins attached to a membrane lipid can be released by cleaving the covalent bond that attaches the lipid to the protein:
• lipases are used for this purpose; they are enzymes that cleave the
ester or phosphodiester linkage that attaches the lipid to the protein

18. from a membrane in order to study these proteins.
Releasing proteins
from a membrane in order to study these proteins.
Note how the hydrophobic tails of the detergent molecules associate with the hydrophobic region of the integral protein, protecting it from water. The polar heads of the detergent molecules are in contact with water. Below is the structure of a detergent commonly used for this purpose.
use a lipase
Fig 11-7 from Lehninger

19. Membrane Dynamics
Membranes are flexible and fluid (fluid mosaic model), but transient holes don’t open up in the membrane. The extracellular fluid does not come in contact with the intracellular fluid.
Quote from Lehninger:
“One remarkable feature of all biological membranes is their flexibility–
their ability to change shape without losing their integrity and becoming
leaky. The basis for this property is the noncovalent interactions among lipids in the bilayer and the mobility allowed to individual lipids because they are not covalently anchored to one another.”
We will look at the motions that occur within membranes next.

20. The motion and arrangement of the nonpolar lipid tails in the bilayer is dependent on temperature.
At very low temperature:
Very little bond rotation in the nonpolar tails;
Lipid tails are extended and arranged in an orderly fashion;
Lipids do not move laterally within each layer.
At physiological temperatures:
Some rotation of carbon-carbon bonds in tails;
Lipid tails are mostly extended and quite orderly;
But lipids can move laterally within each layer.
At very high temperature:
Constant rotation of carbon-carbon bonds in tails;
Lipid tails are very disordered in their arrangement;
Lipids are constantly moving laterally.
Fig from Lehninger

21. The motion and arrangement of the nonpolar lipid tails in the bilayer is dependent on the types of lipids present in the bilayer.
At physiological temperatures (20-40 oC):
The nonpolar tails of long-chain saturated fatty acids pack together in an orderly arrangement. Their presence in the bilayer makes the bilayer less fluid. (By “fluid” I am referring to the amount of movement occurring, in terms of carbon-carbon bond rotation in the tails and lateral movement of lipids.)
The kinks in the nonpolar tails of unsaturated fatty acids prevent orderly packing of the tails (tails are more disordered). Their presence in the bilayer makes the bilayer more fluid.
Sterols are rigid, planar molecules. Their presence in the membrane interferes with the movement of neighboring nonpolar tails, tending to keep the tails in an orderly arrangement. Sterols make the bilayer less fluid. Cholesterol is a very important, stabilizing component of animal cell membranes!! Similar sterols stabilize plant cell membranes.

22. Sterols, such as cholesterol, are essential components of cell membranes because they stabilize the membrane by decreasing its fluidity.
The figure shows the orientation of cholesterol within one layer of the membrane. Its polar hydroxyl group is adjacent to the polar heads of the membrane lipids, and its nonpolar ring system and carbon tail are adjacent to the nonpolar tails of the membrane lipids.
Fig from Biochemistry Free For All

23. A membrane sphingolipid associating with cholesterol
A membrane sphingolipid associating with cholesterol. Note the orientation of the polar and nonpolar regions on the two neighboring molecules.
Fig from Biochemistry Free For All

24. Many membrane proteins are free to diffuse laterally; others are
anchored to internal structures that prevent free diffusion.
This membrane protein is bonded to a
cytoskeletal
protein, which prevents it from moving very far.
Fig from Lehninger