How do alpha helices form and why are they the predominent structure found in transmembrane domains?

Answer 1

Generally speaking, a helix is a very simple structural motif which helps the monomers (amino acids) be in a conformation with minimal energy penalties due to steric effects.

The secondary structure of proteins depends on the amino acid sequence (primary structure) and the environment.

The protein backbone has lots of peptides bonds and their NH and CO groups need to make as many hydrogen bonds as possible. In water you can have some H-bonds forming with water.
However in a hydrophobic environment, such as the lipid bilayer, you can't have such H-bonds; all H-bonds have to come from peptide bonds and the system has to be such that no groups are left "facing" the lipids, without having an interacting partner.

How could a peptide sequence cross the lipid bilayer?

First of all it would require amino acids with hydrophobic side-chais interacting with the lipids-otherwise the energy penalty is huge.
Still the backbone needs to form H-bonds. Since the groups can interact only with each other (not with the lipids) they have 2 options: either form an a-helix where hydrogen bonds form between the groups of amino acids which are very close in the primary sequence (2-3 positions away) or form a b-sheet barel; if a barel is not formed then the NH & CO groups at the edge of the sheet would not be satisfied.

The latter requires many more amino acids. Thus, a-helix, is the predominant form because it requires the smallest number of amino acids to transverse the membrane without an energy penalty.

Sorry for the bad answer but it's too late...

Answer 2

Alpha helices form because of a precise arrangement of amino acids allowing them to create a spiral for any given distance. They are often found in structural parts of proteins and heavily in proteins involved with cellular structure. Think of them as similar to the beams in a house which give it strength. In transmembrane domains, they give the long, straight structure necessary to pass across a membrane.

Answer 3

Alpha-helices generally form a helical structure with about 3.6 amino acids per spiral. They're great for the plasma membrane since they generally have non-polar residues in the middle section which are able to interact with the non-polar fatty acid tails. Their column-like structure works well with passing substances through the membrane in either active or passive transport.