Alpha helices

Alpha Helices

Alpha helices are a common structural motif in proteins, characterized by a right-handed coiled or spiral conformation. They are a fundamental element of the secondary structure of proteins, alongside beta sheets and random coils.

Structure

The alpha helix is stabilized by hydrogen bonds between the backbone amide hydrogen of one amino acid and the carbonyl oxygen of another amino acid four residues earlier. This pattern of bonding results in a helical structure with 3.6 amino acids per turn and a pitch of 5.4 Ångströms.

Geometry

• Rise per residue: 1.5 Å
• Residues per turn: 3.6
• Helix pitch: 5.4 Å

The side chains of the amino acids in an alpha helix extend outward from the helical axis, allowing for interactions with other parts of the protein or with the surrounding environment.

Formation

Alpha helices form spontaneously in proteins due to the favorable energetics of the hydrogen bonding pattern and the intrinsic properties of the amino acids involved. Certain amino acids, such as alanine, are more likely to be found in alpha helices, while others, like proline, are known as helix breakers due to their structural constraints.

Function

Alpha helices play a crucial role in the function of many proteins. They can form the structural framework of fibrous proteins such as keratin and collagen, and they are also involved in the formation of transmembrane proteins, where they span the lipid bilayer.

Examples

• Hemoglobin: Contains several alpha helices that are critical for its function in oxygen transport.
• Myoglobin: Composed primarily of alpha helices, which help in binding and storing oxygen in muscle tissues.

Discovery

The alpha helix was first proposed by Linus Pauling, Robert Corey, and Herman Branson in 1951. Their model was based on X-ray diffraction data and chemical principles, and it has since been confirmed by numerous studies.

See Also

• Protein structure
• Beta sheet
• Secondary structure

References

• Pauling, L., Corey, R. B., & Branson, H. R. (1951). The structure of proteins: Two hydrogen-bonded helical configurations of the polypeptide chain. Proceedings of the National Academy of Sciences, 37(4), 205-211.
• Branden, C., & Tooze, J. (1999). Introduction to Protein Structure. Garland Publishing.

External Links

• [Alpha Helix on Proteopedia](https://proteopedia.org/wiki/index.php/Alpha_helix)