Lesson 5 of 7 / Biomolecules: structure explains function
Four levels of protein structure
Which bonds hold each level together?
In this lesson: Explain primary, secondary, tertiary and quaternary structure and the interactions that maintain shape.
About 6 min
The key ideaSequence 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
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
Count chains
One folded chain has tertiary structure; an assembly of several chains has quaternary structure.
- 2
Name the level
Sequence, local regular folding, whole-chain shape and multi-chain assembly are different levels.
- 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.
Is this true? "Every protein has quaternary structure."
Quaternary structure requires more than one polypeptide subunit. A functional single-chain protein lacks this level.