26. The length of an α-helical section of a polypeptide chain of 20 residues spanning hydrophobic region of membrane would be
(1) 30 A
(2) 54 A
(3) 5.4 A
(4) 3.6 A

Determining the Length of an α-Helical Section in a Membrane-Spanning Polypeptide

Introduction

The α-helix is a fundamental secondary structure in proteins, playing a crucial role in membrane-spanning domains of transmembrane proteins. Understanding its length helps in predicting the structural organization of proteins within biological membranes. This article explains how to calculate the length of an α-helix composed of 20 residues spanning a hydrophobic region of a membrane.

Key Characteristics of an α-Helix

• Right-handed helical structure stabilized by hydrogen bonding.
• 3.6 residues per turn of the helix.
• Each residue contributes 1.5 Å (angstroms) to the helical length.
• Common in transmembrane proteins, particularly in hydrophobic regions.

Calculation of the α-Helical Length

Given:

• Number of residues = 20
• Rise per residue = 1.5 Å

Total length = 20 × 1.5 Å = 30 Å

Correct Answer: (1) 30 Å

Role of α-Helices in Membrane-Spanning Proteins

1. Transmembrane Domains

• Many integral membrane proteins contain α-helical transmembrane segments.
• These helices span the hydrophobic lipid bilayer, allowing protein anchoring and function.
• Typically, 20–25 hydrophobic residues are required to span the 30 Å hydrophobic core of a membrane.

2. Stability and Function

• Hydrogen bonding within the helix stabilizes its structure.
• Hydrophobic side chains interact with the lipid bilayer, anchoring the helix.
• Some transmembrane α-helices form channels or receptors, facilitating molecular transport and signaling.

Experimental Techniques to Study α-Helical Structures

1. X-ray Crystallography – Provides high-resolution structures of membrane proteins.
2. Circular Dichroism (CD) Spectroscopy – Determines secondary structures in solution.
3. Cryo-EM (Cryogenic Electron Microscopy) – Analyzes membrane proteins in their native state.

Conclusion

The length of an α-helical section spanning a hydrophobic membrane region can be calculated based on the rise per residue (1.5 Å). A 20-residue α-helix extends 30 Å, which matches the thickness of the hydrophobic core of a biological membrane. Understanding these structural properties is essential in membrane protein research and drug design.