Protein structure and folding !

A protein is a chain of amino acids that folds into a precise shape; if it misfolds, it stops working or piles up and damages tissues.

Updated 2026-11-01

Key facts

Primary: the sequence joined by peptide bonds (planar, almost always trans). Secondary: α-helix and β-sheet, held by backbone hydrogen bonds.
Tertiary: the 3D fold of one chain (hydrophobic core, disulfide bridges, ionic bonds). Quaternary: several chains together, such as haemoglobin α₂β₂.
Sequence determines the fold (Anfinsen), but in the cell chaperones (HSP70, HSP60 chaperonins) and enzymes such as protein disulfide isomerase help.
Misfolded proteins are tagged with ubiquitin and destroyed in the proteasome; if they escape, they form aggregates such as amyloid (cross-β fibrils).
Denaturation (heat, extreme pH, urea, detergents) destroys secondary to quaternary structure but not peptide bonds.

The peptide bond has partial double-bond character: it does not rotate, and the chain bends only at the φ and ψ angles of the α carbon.
α-helix: 3.6 residues per turn; each residue's C=O hydrogen-bonds to the N–H four residues ahead. Proline interrupts it.
β-sheet: extended strands side by side (parallel or antiparallel), linked by hydrogen bonds between strands; β-turns often contain glycine and proline.
In water, hydrophobic side chains hide in the core and polar ones stay on the surface: this is the main driving force of folding.
Domains are folded units that recur between proteins (for example immunoglobulin-like domains or the EGF-like domains of fibrillin).
In the endoplasmic reticulum, proteins that fail to fold switch on the unfolded protein response (UPR); if it persists, the cell undergoes apoptosis.
Conformational diseases: α1-antitrypsin Z polymerises in the hepatocyte; the prion converts the normal protein into its β form; unstable transthyretin forms amyloid.

Amyloidosis, prion diseases, Alzheimer disease (β-amyloid and tau) and Parkinson disease (α-synuclein) share one problem: misfolded proteins that aggregate.
“Stabiliser” drugs such as tafamidis hold transthyretin in its native shape and slow amyloid formation.
In cystic fibrosis, the commonest CFTR variant (F508del) misfolds and is degraded before it reaches the membrane.

Think of the answer, then tap the question to see it.

What kind of bond holds the α-helix and the β-sheet together?

Hydrogen bonds between the C=O and N–H groups of the peptide backbone, not between side chains.

Which levels of structure are lost when a protein is denatured by heat?

Secondary, tertiary and quaternary; the peptide bonds (primary structure) stay intact.

Which amino acid interrupts an α-helix, and why?

Proline: its amino group sits in a ring, so it has no N–H for the hydrogen bond, and its rigid angle does not fit the helix.

What structure do all amyloid fibrils share?

The cross-β sheet: β-strands running perpendicular to the fibril axis; that is why they stain with Congo red.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: What are proteins and what do they do?: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); PDB-101: Molecule of the Month: Myoglobin (RCSB Protein Data Bank (PDB-101), 2026); Amyloid fiber formation (Reactome, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.