|MadSci Network: Biochemistry|
1. Lipids do not in general regulate transport proteins. In a few specific cases they may but this is an extremely hard question to answer. The lipid environment may affect transport, but this is probably a bulk property of lipid not an effect of a specific lipid. That is, if a transport protein is in a membrane composed largely of phosphatidylcholine (the major phospholipid in many mammalian membranes) it would have one set of properties. If the same protein were in a membrane composed largely of phosphatidylethanolamine (the major phospholipid in most bacteria), it would have slightly different properties. It would transport the same substance(s) but with a different rate and affinity.
2. Now many proteins involved in cell signaling, as opposed to transport, are "lipidated". They have a single (sometimes 2) covalent lipids attached near one terminus of the protein. This lipid is usually a fatty acid (myristate, palmitate). The fatty acid dissolves in the membrane and hold the protein next to the membrane. Thus, the protein is not truly an integral membrane protein. The lipid thus hold the protein near other proteins that may be involved in signaling so that it can interact better. That's the first function of the lipid. It's second function is to allow regulation. The lipid can be clipped off by a specific enzyme. This will free the protein from the membrane and turn it into a truly soluble protein. It may still be able to interact with the signalling pathway that it normally interacts with, but since it is no longer bound to the membrane, it will diffuse away from the membrane fairly rapidly. Therefore, with time, the concentration of the protein next to the membrane and thus available for signalling approaches zero thus preventing any significant amount of signalling. There are also pathways which will put the lipid back on the protein so that it can attach to the membrane again. Other signalling proteins have the lipid attached but are folded so that the lipid is 'hidden' from the aqueous environment. When this protein interacts with another protein, the first will change conformation slightly, freeing the lipid and thus allowing attachment to the membrane.
3. Try the following sites for more help. http://www.biology.arizona.edu/cell_bio/tutorials/membranes/main.html
4. The following URL is a molecular model of a lipid bilayer. You will need RASMOL to see it displayed. This is a simple (free) add-on to your browser. Get RASMOL at http://www.umass.edu/microbio/rasmol/
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