1. Introduction
The α-helix is one of the two main secondary structures of proteins (along with the β-sheet). Described by Linus Pauling and Robert Corey in 1951, it is present in about 30% of residues in globular proteins and is the dominant structure in fibrous proteins such as keratin and myosin.
2. Structure of the α-Helix
The α-helix is a right-handed helix with 3.6 residues per turn and a pitch of 0.54 nm. Each amino acid is offset by 100° from the previous one. The peptide backbone forms the helix axis, with side chains pointing outward. Hydrogen bonds form between the C=O group of residue i and the N-H group of residue i+4, stabilizing the structure.
3. Helix Stabilization
The stability of the α-helix depends on the nature of the amino acids. Proline is a "helix breaker" because its pyrrolidine ring prevents formation of the required hydrogen bond. Same-sign charged amino acids repel each other and destabilize the helix. Glycine, too flexible, favors other conformations. Amphipathic helices have a hydrophobic face and a hydrophilic face, making them useful for membrane interactions.
4. Biological Functions
- Keratin: supercoiled α-helices forming hair, nails and horns
- Myosin: helical tail enabling assembly into thick filaments
- Hemoglobin: 8 α-helices per subunit framing the heme group
- Ion channels: transmembrane helices forming the pore
- Transcription factors: DNA-recognition helices (helix-turn-helix motif)
5. Conclusion
The α-helix is an optimal geometric solution for organizing a polypeptide chain: it maximizes intramolecular hydrogen bonds while allowing great functional variety depending on the nature of side chains. Its ubiquitous presence in proteins reflects the thermodynamic robustness of this structure.