Shape is the job

A protein is a chain of smaller building blocks, folded into a particular shape. The shape decides what it does: a pocket that holds one partner, a channel that lets one thing through, a pair of arms that grips.

Change one building block in the wrong place and the shape can change, and with it the job. Sickle-cell disease comes from a single changed block in haemoglobin.

That is why so much of biology is about knowing shapes.