Question

Consider an ion channel membrane protein:

•Even amino acids with non-polar R groups have unavoidable polar regions—where?

•Given this, why do you suppose most transmembrane domains exist as α-helices or β-sheets (what does this allow?)

•Describe the types and locations of amino acid R groups that would be found at various surfaces of this ion channel. How would this contribute to the pore’s function as an ion channel?

Answer 1

In some of these transmembrane proteins, the polypeptide chain crosses the bilayer as a single α helix (single-pass proteins). In others, including those responsible for the transmembrane transport of ions and other small water-soluble molecules, the polypeptide chain crosses the bilayer multiple times—either as a series of α helices or as a β sheet in the form of a closed barrel (multipass proteins). Other membrane-associated proteins do not span the bilayer but instead are attached to either side of the membrane. Many of these are bound by noncovalent interactions with transmembrane proteins, but others are bound via covalently attached lipid groups.