Alpha-helical

Alpha-Helical Structure

The alpha-helical structure is a common motif in the secondary structure of proteins and is a right-handed coiled or spiral conformation. It is a fundamental component of the protein structure and plays a crucial role in the function and stability of many proteins.

Structure

The alpha-helix is characterized by its helical shape, which is stabilized by hydrogen bonds. These bonds form between the carbonyl oxygen of one amino acid and the amide hydrogen of another amino acid that is four residues earlier in the sequence. This pattern of bonding results in a helical turn every 3.6 amino acids, creating a tightly packed structure.

Dimensions

• Pitch: The alpha-helix has a pitch of 5.4 Å, which is the vertical distance between turns of the helix.
• Diameter: The diameter of the helix is approximately 10 Å.
• Residues per turn: There are 3.6 amino acids per turn of the helix.

Formation

The formation of an alpha-helix is influenced by the sequence of amino acids in the polypeptide chain. Certain amino acids, such as alanine, leucine, and glutamate, are more likely to be found in alpha-helices due to their favorable interactions and structural properties. Conversely, amino acids like proline and glycine are less common in alpha-helices because they introduce kinks or flexibility that disrupt the helical structure.

Function

Alpha-helices are involved in a variety of functions within proteins:

• Structural support: They provide rigidity and stability to the protein structure.
• Transmembrane domains: Many transmembrane proteins have alpha-helical regions that span the lipid bilayer, facilitating communication and transport across the membrane.
• DNA binding: Alpha-helices can fit into the major groove of DNA, allowing proteins to interact with specific DNA sequences.

Examples

Several well-known proteins contain alpha-helical structures:

• Hemoglobin: Contains alpha-helices that contribute to its ability to bind and transport oxygen.
• Myoglobin: An oxygen-binding protein with a high content of alpha-helical structure.
• Keratin: A fibrous protein with a coiled-coil structure formed by alpha-helices, providing strength to hair and nails.

Discovery

The alpha-helical structure was first proposed by Linus Pauling, Robert Corey, and Herman Branson in 1951. Their work laid the foundation for understanding protein secondary structure and was a significant milestone in the field of structural biology.

See Also

• Beta sheet
• Protein folding
• 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 Science.