does anyone know why g-proteins has a shape of a helix??
Judge by yourself.
G-proteins are stuck in membranes, thus they have a structure that allows them to be most stable.
It's just the most chemically stable conformation of the protein. In a way, the question is asked a bit backward. The important question isn't why a g-coupled protein would form a helix, but why a helical structure would be advantageous to the function of a g-coupled protein. The simplified answer to that question is that it allows the hydrophobic region of the protein to interact with the hydrophobic cell membrane.
Yes, the hydrophilic amino-acids can turn inwards of the helix in a beta-sheet such shealding is difficult.
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