Question

biochem question

1. Distinguish between β-barrel and α-helical structures used for membrane transport

Answer 1

Membrane proteins can be alpha - helices or beta - strands.

Proteins can span the membrane with alpha helices. Membrane - spanning alpha - helices are the most common structural motif in membrane proteins. An examination of the primary structure reveals that most amino acids in the membrane protein are nonpolar and very few are charged. One of the first salpha - proteins found was the bacteriorhodopsin. It uses light energy to transport protons from inside the cell to outside generating a proton gradient used to form ATP. The seven alpha - helices are closely packed and arranged perpendicular to the plane of the cell and they span 45A in width.

Membrane proteins can also be made out of beta strands.

Beta Strands form channel proteins. They are less common than alpha - helices. Channel proteins are formed by beta arrangement of beta strands. Each strand is hydrogen bonded to its neighbor in an anti-parallel arrangement, forming a single beta sheet. The beta sheet then curls up to form a hollow cylinder that forms a channel in the membrane. An example is Porin. The outside surface is non-polar and interacts with the hydrocarbon core of the membrane, while the inside channel is hydrophillic and filled with water. The arrangement of polar and non-polar is accomplished by the alternation of hydrophobic and hydrophillic amino acids along with each beta strand.