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Most proteins are fixed in the membrane by alpha helices. But some use beta barrels.

Wikipedia describes beta barrels as used for porins, preprotein translocases, and lipocalins. To me, a coiled coil alpha helix structure could surely perform the same functions and given the vast number of proteins using alpha helices there might be some out there.

So my question is twofold:

  1. Are these functions exclusive to beta barrels?

  2. If so, what about alpha helices makes them unsuitable for those functions?

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As you can see on the wikipedia page you linked yourself that beta barrels have far larger diameters (in some cases up to 40 angstroms as stated in this paper) depending on the numbers of beta sheets that make up the barrel, compared to the diameter of an alpha helix (that is fixed) of 6 angstroms. (source http://www.chem.uwec.edu/Modeling/GGProteinStructures/Pages/alpha_helix/helix.html). Alpha helices are good to anchor proteins into the membrane, or forming small holes that is suitable for compounds like ions, while with beta barrels larger compounds such as sugars, or various drugs can be transported.

There's a good list of proteins with different functions for both types on wikipedia

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