Biochemistry
The four levels of protein structure
A protein's function is dictated entirely by its shape, and that shape is built up in four describable stages.
The four levels
| Level | What it is | Held together by |
|---|---|---|
| Primary | The linear sequence of amino acids | Peptide (covalent) bonds |
| Secondary | Local folding: α-helices and β-pleated sheets | Hydrogen bonds along the backbone |
| Tertiary | The overall 3D shape of one polypeptide | R-group interactions: hydrophobic, hydrogen, ionic, disulfide bridges |
| Quaternary | Two or more polypeptide subunits assembled together | Same interactions as tertiary, between subunits |
Not every protein has quaternary structure — it exists only in multi-subunit proteins. Haemoglobin, with four subunits, is the standard example; myoglobin, with one, has no quaternary structure at all.
Why primary structure determines everything
The sequence of amino acids dictates where hydrophobic residues sit, where disulfide bridges can form, and where charges attract or repel. Change one amino acid and the folding can change. In sickle cell anaemia, a single substitution — valine for glutamic acid at position 6 of the β-globin chain — changes a charged residue to a hydrophobic one, causing haemoglobin to polymerize and distort the red blood cell.
Bond strength, ranked
Disulfide bridges (covalent) are strongest, followed by ionic bonds, then hydrogen bonds, then hydrophobic interactions. But there are so many weak interactions that collectively they dominate the folded structure.
Denaturation
Heat, pH extremes, heavy metals, and organic solvents disrupt the weak interactions maintaining secondary, tertiary and quaternary structure. The primary structure survives — peptide bonds are covalent and are not broken by denaturation. This is exactly why a cooked egg white turns solid and opaque but is still protein, and still nutritionally complete.
Denaturation in the stomach is a normal, useful part of digestion: stomach acid unfolds dietary protein so that proteases can reach the peptide bonds. See how digestion works.
Frequently asked questions
What are the four levels of protein structure?
Primary is the amino acid sequence held by peptide bonds; secondary is local folding into alpha-helices and beta-sheets held by backbone hydrogen bonds; tertiary is the overall 3D shape of one polypeptide; quaternary is the assembly of multiple subunits.
Does denaturation break the primary structure?
No. Denaturation disrupts the weak interactions maintaining secondary, tertiary and quaternary structure, but peptide bonds are covalent and remain intact, so the amino acid sequence is preserved.
Do all proteins have quaternary structure?
No. Quaternary structure exists only in proteins made of two or more polypeptide subunits. Haemoglobin has four subunits and therefore has quaternary structure; single-chain myoglobin does not.
Keep reading: Enzymes explained · Complete vs. incomplete protein · How digestion works