| Chem 431 |
Biochemistry |
Fall 2007 |
| Lecture Notes: 17 October |
© R. Paselk 2007 |
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Biological Membranes, cont.
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 alpha-helixes. (text Figure 11-8)
- 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 ased on beta-barrel structures. (text Figure 11-13).
- Have 20 or more transmembrane beta-strand segments to make the barrel.
- Note for beta only need just 7-9 residues to span membrane instead of the 20-25 required for the 6-7 turns of 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 19 October 2007
Pathway Diagrams
