What defines quaternary structure in proteins?

Quaternary structure is what happens when protein pieces team up to do a bigger job.

Imagine you have four toy blocks, each one can stand alone, but together they make a cool tower. Proteins work the same way. Quaternary structure means that multiple protein parts (called subunits) come together like puzzle pieces to form one big, working unit.

Each subunit is like a small robot, on its own, it can do some tasks, but when they join forces, they become a super-robot. This teamwork lets the protein do jobs that single parts can't handle alone, like helping your muscles move or letting your blood carry oxygen.

Think of your favorite sandwich, the bread is one part, the cheese another, and the meat another. Alone, each is okay, but together they make a delicious meal. Proteins are like that sandwich: when their subunits come together in quaternary structure, they become something much more useful, just like your favorite lunch!