quaternary structure
/ KWAT-er-nair-ee /
Some proteins work alone, as a single folded chain. But many are teams: two, four, or more separately folded chains that come together and clasp into one larger machine. That assembly of multiple chains into a working complex is the protein's quaternary structure — the fourth and highest level of protein architecture.
Each individual folded chain in the assembly is called a subunit. The subunits are held together not by peptide bonds but by the same weak forces that hold a single chain's fold: complementary shapes pressing together, hydrophobic patches matching up, hydrogen bonds, and salt bridges across the interface. The subunits can be identical copies (a homo-assembly, like four of the same chain) or different (a hetero-assembly). Quaternary structure exists only for multi-chain proteins; a protein made of one chain stops at tertiary structure.
Why bother assembling several chains? Teamwork buys abilities a single chain cannot have. The classic case is hemoglobin, four subunits that talk to each other so that grabbing one oxygen molecule makes the others grab theirs more eagerly — cooperative behavior that no lone chain could show. Assemblies also let cells build big structures from small, reusable parts, repair them by swapping one bad subunit, and switch a machine on or off by changing how the pieces fit. So quaternary structure is not a mere afterthought; it is often where some of biology's cleverest behavior lives.
Hemoglobin is four subunits — two alpha and two beta chains — clasped together. Antibodies are four chains (two heavy, two light) in a Y. Neither could do its job as a lone chain.
Multiple folded chains assembled into one complex — and able to do what no single chain can.
Quaternary structure applies only to proteins built from more than one chain; the subunits are held by noncovalent contacts (sometimes plus disulfide bonds between chains), never by peptide bonds.