Alpha-helices

Alpha-helices

The alpha-helix is a common structural motif in proteins, characterized by a right-handed coiled or spiral conformation. It is a fundamental element of the secondary structure of proteins, alongside the beta-sheet. 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.

Structure

The alpha-helix is a right-handed helix with 3.6 amino acids per turn, and a pitch of 5.4 Ångströms. The helical backbone is tightly packed, with the side chains of the amino acids extending outward from the helix, allowing them to interact with other parts of the protein or with the surrounding environment.

Hydrogen Bonding

The stability of the alpha-helix is primarily due to hydrogen bonds. Each carbonyl oxygen (C=O) in the backbone forms a hydrogen bond with the amide hydrogen (N-H) of the amino acid four residues ahead. This pattern of hydrogen bonding is crucial for maintaining the helical structure.

Amino Acid Propensity

Not all amino acids are equally likely to be found in alpha-helices. Amino acids such as alanine, glutamate, and leucine are often found in helices due to their favorable side chain interactions and backbone flexibility. In contrast, proline is known as a "helix breaker" because its rigid cyclic structure introduces a kink in the helix.

Function

Alpha-helices play a critical role in the function of many proteins. They can form coiled-coils, transmembrane helices, and are involved in protein-protein interactions. The amphipathic nature of some alpha-helices allows them to interact with both hydrophobic and hydrophilic environments, making them ideal for membrane-spanning regions.

Coiled-coils

In coiled-coils, two or more alpha-helices wind around each other to form a stable structure. This motif is common in structural proteins such as keratin and myosin.

Transmembrane Helices

Alpha-helices are often found spanning the lipid bilayer of cell membranes. These transmembrane helices are typically composed of hydrophobic amino acids that interact with the lipid environment.

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 experimental studies.

See Also

• Protein structure
• Beta-sheet
• Secondary structure
• Tertiary 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 Science.