Chapter 19 · Biochemistry
19.6Protein Structure
10 min · two checks
Predict
Name the four levels of protein structure.
The idea
- Define primary, secondary, tertiary, and quaternary structure.
- Name the interactions that stabilize a fold.
Primary structure is covalent, the sequence. Secondary structure is local and repetitive. In an α-helix the chain coils and backbone hydrogen bonds run along the coil. In a β-sheet strands lie side by side, hydrogen-bonded. Regions that do neither are loops. Tertiary structure is the full three-dimensional fold of one polypeptide: hydrophobic side chains tuck inward, polar ones face water, disulfide bonds between cysteines can lock pieces, and ionic attractions and hydrogen bonds between side chains add specificity.
Quaternary structure exists only when more than one polypeptide makes the working protein. Hemoglobin has four chains. A single-chain enzyme has tertiary structure and no quaternary structure. You cannot skip levels in a description: “it is a helix” is secondary structure, not a full account of the active site. The active site is usually a pocket created by the tertiary fold, sometimes using residues that were far apart in the sequence.
Keep these
- Primary = sequence. Secondary = helix and sheet. Tertiary = one chain’s fold.
- Quaternary = multiple chains. Not every protein has it.
- Hydrogen bonds, hydrophobic contact, ionic pairs, and disulfides stabilize the fold.