α-Helix

Several different polypeptide helices are found in nature, but the α-helix is the most common. It is a rigid, right-handed spiral structure, consisting of a tightly packed, coiled polypeptide backbone core, with the side chains of the component L-amino acids extending outward from the central axis to avoid interfering sterically with each other (Fig. 1).

A very diverse group of proteins contains α-helices. For example, the keratins are a family of closely related, rigid, fibrous proteins whose structure is nearly entirely α-helical. They are a major component of tissues such as hair and skin. In contrast to keratin, myoglobin, whose structure is also highly α-helical, is a globular, flexible molecule  found in muscles.

Figure 1. Structure of an α-helix.

1. Hydrogen bonds: An α-helix is stabilized by extensive hydrogen bonding between the peptide bond carbonyl oxygens and amide hydrogens that are part of the polypeptide backbone . The hydrogen bonds extend up and are parallel to the spiral from the carbonyl oxygen of one peptide bond to the –NH group of a peptide linkage four residues ahead in the polypeptide. This insures that all but the first and last peptide bond components are linked to each other through intrachain hydrogen bonds. Hydrogen bonds are individually weak, but they collectively serve to stabilize the helix.
2. Amino acids per turn: Each turn of an α-helix contains 3.6 amino acids. Thus, amino acids spaced three or four residues apart in the primary sequence are spatially close together when folded in the α-helix.

3. Amino acids that disrupt an α-helix: The R group of an amino acid determines its propensity to be in an α-helix. Proline disrupts an α-helix because its rigid secondary amino group is not geometrically compatible with the right-handed spiral of the α-helix. Instead, it inserts a kink in the chain, which interferes with the smooth, helical structure. Glycine is also a “helix breaker” because its R group (a hydrogen) confers high flexibility. Additionally, amino acids with charged or bulky R groups (such as glutamate and tryptophan, respectively) and those with a branch at the β-carbon, the first carbon in the R group (for example, valine), have low α-helix propensity.