|Lecture Notes: 24 October
© R. Paselk 2008
Membrane shows up as a bilayer structure in electron micrographs (text Figure 11-1).
Fluid Mosaic Model: (text Figure 11-3)
This model has as its core element a lipid bilayer (predominantly glycero-phospholipid). This bilayer makes a very effective barrier for the flow of charged and polar species between aqueous compartments. Within the bilayer itself, however, flow occurs readily - it is a two- dimensional liquid with a viscosity similar to olive oil. Thus we see rapid exchange between adjacent phospholipid molecules on a face of the bilayer, but very rare exchange between faces (the polar "head" groups would have to cross the non-polar bilayer interior). A lipid bilayer membrane thus separates the interior of the cell from the outside.
- Note differences in lipid composition between inside and outside, as seen in the rbc membrane (text Figure 11-5).
Of course a cell also needs to communicate with the outside world - doors and windows are needed. Such communication occurs largely through proteins acting as pores, gates, and shuttles.
- Note the different types of proteins, peripheral vs. integral (text Figure 11-6).
- Note that integral proteins "float" in the bilayer. They have unconstrained movement in the two-dimensions of the sheet. Changes in protein conformation can also cause them to "sink" into the hydrophobic interior of the bilayer etc.
- All known integral proteins have at least one surface exposure.
- The intrabilayer secondary elements have mostly hydrophobic aa residues in contact with the bilayer hydrocarbon interior.
- Six categories of lipid proteins are named as Types I-VI, all but one based on membrane spanning -helixes. (text Figure 11-8)
[See below for -barrel-based integral membrane proteins.)
- Bacteriorhodopsin is an example of Type 3, having a single peptide chain with seven membrane spanning -helixes. (text Figure 11-9)
- Note the hydrophobicity of the residues as shown on the hydropathy plot in text Figure 11-11b
- Charged residues are nearly exclusively in aqueous contact, while Tyr & Trp residues tend to occur at the lipid-water interfaces of membrane spanning proteins as seen in text Figure 11-12, where trp is color coded in red, tyr in orange, and charged residues in blue.
- Can also have integral proteins based on -barrel structures. (text Figure 11-13).
- Have 20 or more transmembrane beta-strand segments to make the barrel.
- Note for -strands only need just 7-9 residues to span membrane instead of the 20-25 required for the 6-7 turns of an -helix to span the membrane.
- Note that in both families ("all-alpha" membrane domain & beta-barrel membrane domain) membrane spanning segments adjacent in the sequence are also adjacent in the structure and thus commonly antiparallel.
- This "up-down" topology probably results from constraints in the biosynthetic and insertion processes.
- Peripheral proteins can be on internal or external membrane surface, often attached to lipids of bilayer. (text Figure 11-14)
- Protein movement can be constrained by linkage to protein networks (cytoskeleton) within the cell as is exemplified by red blood cells (RBC's).
Last modified 25 October 2008