Question

Transmembrane proteins are usually alpha helices because the polar groups in the backbone make hydrogen bonds with each other. If the transmembrane protein were to have 20 polar amino acids crossing through a polar lipid bilayer, how would the structure of the transmembrane protein be?

Answer 1

Transmembrane protein with polar amino acid will have beta barrel form. The β barrel proteins are abundant in the outer membrane of mitochondria, chloroplasts, and many bacteria. Some are pore-forming proteins, generating water-filled channels that allow selected hydrophilic solutes to cross the lipid bilayer of the bacterial outer membrane. The porin barrel is formed from a 16-stranded antiparallel β sheet, which is sufficiently large to roll up into a cylindrical structure. Polar side chains line the aqueous channel on the inside, while nonpolar side chains project from the outside of the barrel to interact with the hydrophobic core of the lipid bilayer. Loops of polypeptide chain often protrude into the lumen of the channel, narrowing it so that only certain solutes can pass. Some porins are therefore highly selective: maltoporin, for example, preferentially allows maltose and maltose oligomers to cross the outer membrane of E. coli