How it works
A protein structure diagram with all four levels: primary, secondary, tertiary and quaternary. The first drawing names the levels, the second the chain and its folds, the third the bonds that hold a folded protein.
Read the full guide
Read the four levels in order
Primary structure is the order of amino acids in a chain, joined by peptide bonds. Secondary structure is the local folding of that chain into alpha helices and beta sheets, held by hydrogen bonds. Tertiary structure is the full three-dimensional shape of one chain. Quaternary structure is two or more chains, called subunits, joined as one protein.
What holds the fold
Side chains hold the tertiary shape: nonpolar ones pack together inside, charged ones attract each other in ionic bonds, and two sulfur atoms can join in a disulfide bond.
- A protein made of one chain has no quaternary structure: it stops at tertiary.
Questions and answers
What are the four levels of protein structure?
Primary, secondary, tertiary and quaternary. They are the sequence of amino acids, its local folds into helices and sheets, the overall shape of one chain, and the arrangement of several chains in one protein.
What is the difference between an alpha helix and a beta sheet?
Both are secondary structure held by hydrogen bonds along the backbone. In an alpha helix the chain winds into a coil. In a beta sheet, stretches of chain lie side by side.
What bonds hold the tertiary structure of a protein?
Interactions between side chains: hydrophobic interactions, ionic bonds, hydrogen bonds and disulfide bonds. Only the disulfide bond is a covalent bond.
Can I practice here for “drag the labels onto the diagram to identify the structures of proteins.”?
Yes, for practice. That sentence is the instruction of a labeling exercise. The drawings here are our own and not a copy of any course figure, so your figure may show other examples.