The second structure of Christmas is the membrane protein opsin, which allows us to perceive light.
Proteins that control the information going in and out of our cells are harder to crystallise than run-of-the-mill globular proteins, as they have both water-loving and fat-loving parts and are tricky to mass produce. Opsin, our second structure of Christmas, is one such molecule. It is situated on a special membrane in a specialised cell at the back of our eyes, and senses light.