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Lesson 5 of 7 / Biomolecules: structure explains function

Four levels of protein structure

Which bonds hold each level together?

I can explain primary, secondary, tertiary and quaternary structure and the interactions that maintain shape.

  • I can explain protein structure levels and stabilising interactions.Syllabus

    Syllabus 8876, Core 1(k). Explain protein structure levels and stabilising interactions.

About 6 min

Make a guess. You are not marked.

A protein has only one polypeptide chain. Does it have quaternary structure?

  1. It does if the chain folds very tightly.
  2. Every protein has all four levels.
  3. Quaternary structure needs two or more chains.
Show the answer

Quaternary structure needs two or more chains.

Quaternary structure is how separate polypeptide subunits fit together. One chain stops at tertiary structure.

The key idea

Sequence constrains folding. Hydrogen bonds stabilise local structures, while interactions among side chains help create the overall three-dimensional shape.

Explore the idea

Move between levels of structure

A particular sequence

Peptide bonds join amino acids in a particular order. Different colours represent different R groups.

Explanation

Primary structure is the amino-acid sequence held together by peptide bonds. Secondary structure is regular local folding, such as an alpha-helix or beta-pleated sheet, stabilised by hydrogen bonds between peptide-backbone groups, not simply between R groups.

Tertiary structure is the overall three-dimensional shape of one polypeptide. Hydrogen bonds, ionic bonds between oppositely charged groups, disulfide bonds between suitable sulfur-containing side chains, and hydrophobic interactions contribute to its stability.

Hydrophobic side chains tend to cluster away from water, while many polar or charged side chains face the aqueous surroundings in a soluble protein. This is a general pattern, not a rule that every residue in every membrane protein follows. Disulfide bonds are covalent; hydrogen bonds are weaker individually.

Quaternary structure is the association of two or more polypeptide subunits in a functional protein. A protein with one polypeptide can have tertiary structure without quaternary structure. Multiple secondary structures can occur within one folded chain.

Step by step
  1. 1

    Count chains

    One folded chain has tertiary structure; an assembly of several chains has quaternary structure.

  2. 2

    Name the level

    Sequence, local regular folding, whole-chain shape and multi-chain assembly are different levels.

  3. 3

    Name an appropriate interaction

    Peptide bonds maintain sequence; hydrogen bonds stabilise helices and sheets; side-chain interactions contribute to tertiary shape.

Worked example

A two-chain protein

A protein contains two folded polypeptides held together. Describe two levels of structure it must have.

One way to explain it

Each polypeptide has a primary amino-acid sequence and a tertiary three-dimensional shape. Their association also gives the protein quaternary structure.

Why this answer works
  • Treat a subunit and the whole protein separately.
  • Do not call the multi-chain association secondary structure.

Watch out for this

A student says: "Every protein has quaternary structure." What is wrong with this?

Show the answer

Quaternary structure requires more than one polypeptide subunit. A functional single-chain protein lacks this level.

Check your understanding

Which statement about an alpha-helix is correct?

  1. It is the overall three-dimensional shape of one entire polypeptide.
  2. Hydrogen bonds between backbone groups stabilise its local coiling.
  3. Hydrogen bonds between R groups define the helical backbone.
Show the answer

Hydrogen bonds between backbone groups stabilise its local coiling.

The peptide backbone remains intact while adopting this regular arrangement.

Quick recall

Card 1 of 3. Answer in your head, then check.

Which bonds hold each level together?

You can return to this lesson any time.