Alpha Helix

Overview

The alpha helix (a-helix) is one of the two principal secondary structure elements in peptides and proteins, alongside the beta sheet. First proposed by Linus Pauling and Robert Corey in 1951, the alpha helix is a right-handed coiled conformation in which the polypeptide backbone winds around a central axis, with amino acid side chains projecting outward from the helix surface.

The alpha helix is stabilized by a regular pattern of intramolecular hydrogen bonds between backbone atoms: specifically, the carbonyl oxygen (C=O) of each amino acid residue forms a hydrogen bond with the amide hydrogen (N-H) of the residue four positions ahead in the sequence (i to i+4). This repetitive hydrogen bonding pattern gives the alpha helix its characteristic geometry and remarkable stability.

Detailed Explanation

Structural Parameters

The alpha helix has precise geometric properties:

• Residues per turn: 3.6 amino acids
• Rise per residue: 1.5 angstroms (0.15 nm) along the helix axis
• Pitch: 5.4 angstroms (0.54 nm) — the distance for one complete turn
• Hydrogen bond pattern: i to i+4 (each C=O bonds to the N-H four residues ahead)
• Handedness: Right-handed (the helix spirals clockwise when viewed from the N-terminus)
• Phi/Psi angles: Approximately -57 degrees / -47 degrees

Amino Acid Preferences

Not all amino acids have equal propensity to form alpha helices:

Helix-promoting residues — Alanine, leucine, methionine, glutamate, and lysine have high helical propensity due to favorable backbone geometry and minimal steric strain.

Helix-breaking residues — Proline is the strongest helix breaker because its cyclic side chain constrains the backbone phi angle, preventing the geometry required for helical hydrogen bonding. Glycine, with no side chain, has too much conformational freedom and destabilizes the helix through entropic effects.

Amphipathic helices — When hydrophobic and hydrophilic residues alternate in a pattern matching the 3.6-residue periodicity (approximately every 3-4 residues), the resulting helix has one hydrophobic face and one hydrophilic face. These amphipathic helices are common at membrane-water interfaces and in peptide-receptor interactions.

Helix Capping

The first and last few residues of an alpha helix (the N-cap and C-cap regions) have unsatisfied hydrogen bond donors or acceptors, respectively. Specific amino acids at these positions (asparagine at the N-cap, glycine at the C-cap) can stabilize the helix termini through compensatory hydrogen bonding — a phenomenon called helix capping.

Relevance to Peptide Research

Alpha-helical structure is directly relevant to peptide function and design:

Receptor Interaction — Many peptide hormones adopt alpha-helical conformations when binding to their receptors. The helical structure presents side chains in a defined spatial arrangement that matches the receptor binding pocket. GLP-1, parathyroid hormone (PTH), and calcitonin gene-related peptide (CGRP) all contain functionally important alpha-helical regions.

Stapled Peptides — A significant area of peptide research involves "stapling" — introducing hydrocarbon or other cross-links between residues on the same face of an alpha helix to lock the peptide into its helical conformation. Stapled peptides demonstrate improved protease resistance, enhanced cell membrane permeability, and increased binding affinity compared to their unstapled counterparts.

Antimicrobial Peptides — Many antimicrobial peptides adopt amphipathic alpha-helical conformations that enable them to insert into and disrupt microbial membranes. The degree of helicity, the angle of the hydrophobic face, and the overall hydrophobic moment are critical design parameters.

Cell-Penetrating Peptides — Alpha-helical cell-penetrating peptides can traverse cell membranes through direct translocation or endocytic mechanisms, offering a delivery vehicle for intracellular cargo.

Examples

GLP-1(7-36) contains an alpha-helical region spanning approximately residues 13-31 that is essential for receptor binding. Structural studies show this helix inserts into a groove on the GLP-1 receptor extracellular domain, with specific side chains on the helix making critical contacts. Modifications that disrupt this helical region substantially reduce receptor affinity and biological activity.

Melittin, the principal active component of bee venom (26 amino acids), adopts an amphipathic alpha-helical conformation in membrane environments. This helix enables melittin to insert into lipid bilayers and form pores, demonstrating how helical structure directly mediates biological function.

Related Terms

The alpha helix is one of two major secondary structures, complementing the beta sheet. It is formed by amino acids connected through peptide bonds and can be further stabilized by disulfide bonds. Helical conformation influences receptor affinity and binding selectivity.