Summary
The Polyproline II (PPII) helix is a left-handed, extended secondary structure (n=3 residues/turn, rise ≈ 3.1 Å) defined by backbone dihedral angles of φ ≈ -75° and ψ ≈ +145°. Unlike α-helices or β-sheets, PPII helices lack intrachain hydrogen bonds.
Key Points
- 1Left-handed helix with 3 residues per turn
- 2No intrachain hydrogen bonds
- 3Stabilized by hydration and stereoelectronic effects
- 4Key recognition motif for SH3 domains
The Polyproline II helix represents a distinct and often underappreciated element of protein secondary structure with unique properties and biological functions.
Structural Characteristics
The PPII helix is defined by:
- Left-handed helical twist (opposite to α-helix)
- Extended conformation: 3 residues per turn
- Helical rise: approximately 3.1 Å per residue
- Backbone angles: φ ≈ -75° and ψ ≈ +145°
Stabilization Mechanisms
Unlike α-helices and β-sheets, PPII helices lack intrachain hydrogen bonds. Instead, they are stabilized by:
1. Steric repulsion: Particularly effective in proline-rich sequences where the pyrrolidine ring enforces the PPII geometry
2. Backbone hydration networks: Water molecules form bridges between backbone carbonyls
3. **n → π* stereoelectronic interactions**: Orbital overlap between adjacent peptide bonds
Biological Occurrence
PPII helices appear in several important contexts:
- Collagen triple helix: Individual chains adopt PPII conformations that wind together
- Unfolded/denatured states: PPII is heavily sampled, challenging the "random coil" concept
- Intrinsically Disordered Proteins (IDPs): Transient PPII formation is common
Functional Roles
The extended, exposed nature of PPII helices makes them ideal for:
- SH3 domain recognition: The canonical PxxP motif adopts PPII
- Rigid linkers: Preventing aggregation by maintaining extended conformations
- Signal transduction: Serving as recognition motifs for modular domains