Alpha-helix
A fundamental protein secondary structure characterized by a right-handed spiral conformation where amino acids coil around a central axis, stabilized by hydrogen bonds.
The alpha-helix represents one of the most elegant and essential structural elements in protein folding, first predicted by Linus Pauling in 1951 through careful analysis of peptide bond geometry and hydrogen bonding patterns.
Structure and Properties
The alpha-helix exhibits specific geometric characteristics:
- 3.6 amino acids per turn
- A pitch of 5.4 Å (0.54 nm) per turn
- Hydrogen bonds form between the C=O group of residue n and the N-H group of residue n+4
- Right-handed spiral configuration (most common in nature)
Stabilizing Forces
Several factors contribute to alpha-helix stability:
- Hydrogen bonding networks along the backbone
- Van der Waals forces between side chains
- Hydrophobic interactions in the protein core
Biological Significance
Alpha-helices play crucial roles in:
- Protein structure determination
- Membrane proteins where they often span lipid bilayers
- DNA binding proteins particularly in transcription factors
- Coiled-coil formation in structural proteins
Variations and Special Cases
Several specialized forms exist:
- 310-helix (tighter spiral)
- π-helix (wider spiral)
- Transmembrane helix specialized for membrane environments
Detection and Analysis
Modern techniques for studying alpha-helices include:
- X-ray crystallography for atomic-level resolution
- Circular dichroism for quick assessment of helical content
- NMR spectroscopy for dynamic structural analysis
Disease Relevance
Disruption of alpha-helical structures can lead to various protein misfolding diseases, including:
- Alzheimer's disease
- Huntington's disease
- Various protein aggregation disorders
The alpha-helix stands as a testament to nature's ability to create stable, functional structures from simple building blocks, demonstrating how local interactions can give rise to complex biological machinery through protein folding mechanisms.