|MadSci Network: Cell Biology|
The answer is in two parts: the first describes some aspects of what we know about how membrane proteins interact with lipid membranes generally, the second describe questions that would be appropriate to ask, but for which we have no current answers. First, the basic purpose of the membrane (for all proteins) is 3-fold. 1) It provides a hydrophobic (water-hating) environment for the parts of a protein that are likewise hydrophobic and would not be very soluble in water. 2) It acts as a capacitor. That is, because it is a non-aqueous and electrically non-conducting layer between two aqueous electrically conducting layers, it acts as an insulator. If charge/energy builds up on one side of the membrane, it prevents that energy from dissipating and diffusing away into an (infinite) aqueous solution. This is essentially the definition of a capacitor. It is a system in which energy can be stored on one side of a barrier. Then, proteins within that insulating barrier can allow some/all of that energy to flow through the membrane to the other side. BUT, the point is that the proteins control that flow. 3) The membrane provides vectorial space. That is, it divides the universe into distinct sections or environments. It provides sidedness. Thus, when proteins go into the membrane they could, in theory, be oriented either of two directions, 180 degrees apart. HOWEVER, the machinery of the cell and the properties of the membrane make sure that proteins insert into the membrane in one and only one orientation, so that ALL molecules of a given protein face the same way. (If, in the laboratory, we simply mix lipid vesicles with protein, the protein can insert into the vesicle and can function, but it inserts in both possible orientations because the machinery of the cell and the vectorial properties of the membrane are absent.) Now for the other part. Here's what we know and don't know about individual proteins, and specifically what we know and don't know about the synthesis of the F1F0-ATPase a.k.a. ATP synthase. This enzyme makes that large, large majority of ATP in most cells and is the SOLE membrane enzyme that makes ATP. It is present in all three kingdoms of life. In the Bacteria and Archaea, it is present in the cell membrane. In eukaryotic cells (yeast and plants to humans), it is present in the inner membrane of the mitochondrion or chloroplast (which is the equivalent membrane to the bacterial membrane and is logical since mitochondria and chloroplast used to be symbiotic bacteria that had invaded and taken up residence in another cell). It is probably the single most important enzyme in an organism, certainly in animals. We know that ATP synthase is vectorially inserted with the F1 part (fragment one, soluble in aqueous solutions) on the inside of the respective membranes. The F0 part (fragment 0 (zero) inserted into the membrane. This vectorial insertion is absolutely required. Outside the membrane or mitochondrion, the pH is acid (more protons) relative to the inside (less protons). This establishes a chemical and potential energy gradient across the membrane directed inwardly (i.e., the protons flow out to in). The passage of protons through the membrane from out to in allows the use of the energy they contain (and are dissipating by traveling down their energy gradient) to be captured by the F1F0-ATPase/ATP synthase. That energy is then used by the ATP synthase to attach a phosphate to ADP, making ATP. So the membrane MUST be impermeant to protons (some membranes are slightly permeable). That's the primary property it must have. There are no specific lipids required, only that they can form a bilayer. Some lipids work better than others, but most everything works at least a bit as long as it is sealed against proton. So, that's a long-winded explanation of what we know (not much) and what we don't know (a lot). Probably the ATP synthase interacts with some lipids to help maintain its structure, but we have no details of what might be happening. Almost certainly in some cells the stoichiometry of the F0 part (number/ratio of the different proteins making up F0) varies BECAUSE in different cell types the lipid content/identity varies somewhat. Membrane structure/lipid identity will differ depending on what temperature(s) an organism lives at. Indeed, in the Kingdom Archaea, these prokaryotes have a very different set of membrane lipids, sometimes a very thick, rigid membrane, composed of lipids of very different structure from that of Bacteria or Eukaryotes. Yet, they also use F1F0-ATPase is just the same manner, the proteins adapting to the different lipids and the lipids adapting to the protein. We clearly do not have a sufficient understanding yet of membrane structure and how it influences proteins inserted in it. In some cells, sodium is used instead of proton to make ATP. This is rare and is only used in some (but not all bacteria) that live in an alkaline (low proton, e.g., pH 10) environment.
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