Alpha Helix
Definition and meaning of Alpha Helix in chemistry.
The alpha helix is a common, localized secondary structure in proteins characterized by a right-handed coiled or spiral shape. This highly stable structure is held together tightly by a continuous network of hydrogen bonds running parallel to the central axis of the coil.
In more detail
The alpha helix is one of the most fundamental structural motifs in biochemistry, acting as a crucial building block for the three-dimensional architecture of proteins. As a long chain of amino acids is synthesized in a cell, it does not remain a floppy, linear string.
Instead, specific segments of the chain quickly fold into organized, repetitive patterns known as secondary structures. The alpha helix resembles a tightly wound spring or a spiral staircase, with the molecular backbone curling gracefully to create a highly rigid and structurally sound microscopic cylinder. The incredible stability of the alpha helix comes entirely from a vast network of internal hydrogen bonds.
Within the coiled backbone, the slightly negative oxygen atom of one amino acid reaches down and forms a strong hydrogen bond with the slightly positive hydrogen atom of an amino acid situated exactly four places further down the chain. Because every single amino acid in the helix participates in this repetitive bonding pattern, the entire coil is locked firmly into place.
The bulky side chains of the amino acids are physically forced to point outward, away from the crowded center of the helix, like bristles on a round hairbrush. The precise sequence of amino acids heavily influences whether an alpha helix will successfully form. Some amino acids, like alanine and leucine, have side chains that pack together beautifully, actively promoting the formation of the helix.
Conversely, the amino acid proline has a rigid, awkward ring structure that physically cannot bend into the required spiral shape, causing it to act as a known "helix breaker" that terminates the coil. Alpha helices are prominently featured in structural proteins like keratin, which forms human hair, and they frequently act as the anchoring segments that hold transmembrane proteins securely within cell membranes.
Key facts
| Field | Biochemistry |
|---|---|
| Structure Level | Secondary protein structure |
| Shape | Right-handed coil or spiral |
| Stabilizing Force | Hydrogen bonds between backbone atoms |
| Side Chain Orientation | Pointing outward from the central axis |
| Helix Breaker | The amino acid proline |
The structural protein keratin, which gives human hair and fingernails their rigid strength, is primarily composed of long, intertwined alpha helix structures.
Frequently asked questions
What holds an alpha helix in its spiral shape?
It is held together by a network of hydrogen bonds forming between the amino acids in the main backbone of the protein chain.
Why do the side chains point outward on an alpha helix?
The center of the spiral is too tightly packed to fit the bulky side chains, so they are physically forced to point outward into the surrounding environment.
Why is proline called a helix breaker?
Proline has a rigid ring structure that cannot bend enough to fit into the tight spiral of the alpha helix, causing the coil structure to stop or kink.