α-helices
Summary
An α-helix is a secondary structure in which the polypeptide backbone coils into a right-handed spiral held by hydrogen bonds between backbone C=O and N–H groups four residues apart. Testable point: the R groups point outward, and proline tends to disrupt the helix.
Detail
Meaning: An α-helix is a secondary structure in which a stretch of polypeptide coils into a right-handed spiral. Each backbone C=O forms a hydrogen bond with the backbone N–H four residues farther along, giving about 3.6 residues per turn. The hydrogen bonds run roughly parallel to the helix axis, and the R groups point outward from the coil.
On the MCAT: Passages often describe a mutation and ask for its effect. Replacing a residue in the middle of a helix with proline tends to break it, because proline's ring restricts the backbone angle and its nitrogen has no hydrogen to donate. Glycine is very flexible and also tends to destabilize helices. Heat, urea, or extreme pH disrupts the hydrogen bonds and unfolds the helix. Helices are abundant in keratin, myoglobin, and the membrane-spanning segments of membrane proteins, where the outward-facing side chains are hydrophobic. Questions may also ask you to tell a helix from a β-pleated sheet.
Common confusion: Students credit side-chain interactions for the helix. Secondary structure comes from backbone hydrogen bonds; side-chain interactions belong to tertiary structure. Also, helix hydrogen bonds form within one chain segment, while β-sheet bonds link neighboring strands.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.