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Secondary
Structure

Explore the α-helices, β-sheets, and turns that give proteins their local three-dimensional shape, stabilized by hydrogen bonds along the polypeptide backbone.

Biochemistry

What Is Secondary Structure?

Proteins are built from amino acid chains that fold into defined shapes. Secondary structure refers to the local, regular folding patterns formed by the polypeptide backbone: not the overall 3D shape (tertiary), but the repeating motifs within it.

The two most common secondary structures are the α-helix and the β-sheet, both stabilized by hydrogen bonds between backbone \( \text{N-H} \) and \( \text{C=O} \) groups.

The Role of Hydrogen Bonds

\( \text{C=O} \)
\( \text{N-H} \)
 H-bond Cα Cα N C N

Every peptide bond contains a \( \text{C=O} \) (carbonyl) and an \( \text{N-H} \) (amide). The hydrogen bond forms between these groups on different residues, not between side chains. This backbone-to-backbone bonding is the driving force behind secondary structure.

Why Does It Matter?

Stability
Regular H-bonding patterns satisfy all backbone donors and acceptors, minimizing free energy.
Function
Helices form transmembrane channels; sheets create rigid structural scaffolds like silk fibroin.
Disease
Misfolded secondary structures cause prion diseases and amyloid plaques in Alzheimer's disease.

The α-Helix

H-bond (i → i+4) 1 turn 5.4 Å pitch α-Helix (right-handed) Key Parameters Residues/turn: 3.6 Rise/residue: 1.5 Å Pitch: 5.4 Å H-bond: i → i+4 Handedness: Right-handed
\( \phi, \psi \) angles:
 -57°, -47°

The β-Sheet

N C N C N C H-bonds Antiparallel Strands run in opposite directions
\( \phi, \psi \) angles:
 -139°, +135° Rise/residue: 3.5 Å

In a β-sheet, two or more β-strands lie side by side. Hydrogen bonds form between strands (not within one strand). Strands can be parallel (same direction) or antiparallel (opposite directions).

Parallel vs Antiparallel β-Sheets

Parallel
Adjacent strands run in the same N→C direction. H-bonds are angled and slightly weaker. φ, ψ: \( -119°, +113° \).
Antiparallel
Adjacent strands run in opposite directions. H-bonds are straight and stronger. φ, ψ: \( -139°, +135° \). More common.

Turns and Loops

i i+1 i+2 i+3 β-Turn (Type I) H-bond: C=O(i) → N-H(i+3)

Turns are tight, 3-4 residue segments that reverse the chain direction. The most common is the β-turn, which connects adjacent antiparallel β-strands. A hydrogen bond from residue i to residue i+3 stabilizes the turn.

The Ramachandran Plot

\( \phi \) (phi)
\( \psi \) (psi)
 -180° 0° +180° -180° 0° +180° α-helix β-sheet L-handed helix -57°,-47° -139°,+135°

α-Helix vs β-Sheet

α-Helix β-Sheet
Shape Coiled rod Flat, pleated sheet
H-bond pattern Within one strand (i → i+4) Between adjacent strands
\( \phi, \psi \) angles \( -57°, -47° \) \( -139°, +135° \) (anti.)
Rise per residue 1.5 Å 3.5 Å (more extended)
R-group orientation Radiate outward from helix Alternate above/below sheet
Example protein Keratin (hair), myoglobin Silk fibroin, immunoglobulin

How to Identify Secondary Structures

1
Check the H-bond partners. Are H-bonds within one chain segment (helix) or between separate strands (sheet)?
2
Look at \( \phi \) and \( \psi \) angles. Plot them on a Ramachandran diagram: α-helix clusters near \( (-57°, -47°) \), β-sheet near \( (-139°, +135°) \).
3
Check strand directionality. If β-sheet, determine if strands are parallel (same N→C) or antiparallel (opposite).
4
Identify connecting elements. Look for β-turns (3-4 residues, Gly/Pro enriched) or longer loops between regular structures.
5
Consider amino acid propensities. Ala, Leu, Met favor helices. Val, Ile, Thr favor sheets. Gly, Pro break helices.

Example: H-Bonds in an α-Helix

Problem: A polypeptide segment forms a perfect α-helix with 20 residues. How many intra-helical hydrogen bonds are present?

1
Recall the H-bond rule: each bond links residue \( i \) to residue \( i+4 \).
2
First H-bond: residue 1 (C=O) → residue 5 (N-H).
3
Last H-bond: residue 16 (C=O) → residue 20 (N-H).
4
Count: donors range from residue 1 to 16, so there are \( 20 - 4 = 16 \) hydrogen bonds.

Example: β-Sheet Topology

Problem: A protein domain contains four β-strands arranged as strands 1↓ 2↑ 3↓ 4↑. What type of β-sheet is this, and how many inter-strand H-bond networks exist?

1↓ 2↑ 3↓ 4↑ All antiparallel

Example: Amino Acid Preferences

Problem: A researcher synthesizes a peptide: Ala-Ala-Leu-Ala-Gly-Pro-Val-Ile-Val-Thr. Predict which residues are likely in helical vs sheet regions.

Helix Formers
Ala, Leu, Met, Glu, Lys have high helix propensity. Residues 1-4 (Ala-Ala-Leu-Ala) likely form an α-helix.
Helix Breakers
Gly (too flexible) and Pro (too rigid) disrupt helices. Residues 5-6 (Gly-Pro) likely form a turn.
Sheet Formers
Val, Ile, Thr have branched β-carbons favoring extended conformations. Residues 7-10 likely form a β-strand.

Common Mistakes

✕ Confusing H-bond partners
Secondary structure H-bonds involve backbone N-H and C=O only. Side chain H-bonds contribute to tertiary structure, not secondary.
✕ Mixing up i→i+4 vs i→i+3
α-helices use i→i+4. The 3₁₀ helix uses i→i+3 and the π-helix uses i→i+5. Don't swap these.
✕ Forgetting sheet directionality
Saying "β-sheet" without specifying parallel or antiparallel is incomplete. They have different H-bond geometries and φ,ψ angles.

Summary Comparison

α-Helix β-Sheet β-Turn
H-bond type Intra-chain (i→i+4) Inter-strand i→i+3
Residues involved 3.6 per turn 2+ strands 3-4 residues
\( \phi, \psi \) \( -57°, -47° \) \( -139°, +135° \) Variable
Key residues Ala, Leu, Met Val, Ile, Thr Gly, Pro
Shape Coiled rod Flat, pleated Tight bend
Example Keratin, myoglobin Silk, antibodies Connects β-strands

Question 1 of 5

In an α-helix, hydrogen bonds form between the C=O of residue \( i \) and the N-H of which residue?

Question 2 of 5

Which of the following best describes hydrogen bonding in a β-sheet?

Question 3 of 5

Which amino acids are most commonly found in β-turns?

Question 4 of 5

An α-helix contains 30 residues. Approximately how many complete turns does the helix make?

Question 5 of 5

On a Ramachandran plot, the β-sheet region is located where the backbone dihedral angles \( \phi \) and \( \psi \) are approximately: