Irritate a male platypus during breeding season, and you may end up trapped by its stumpy hind legs, threatened by a set of sharp spurs that are armed with venom. The painful poison hobbles male competitors and is a handy defense against pesky humans and dogs. It is also a somewhat odd concoction, as might be expected from a mammal that is famous for its egg-laying, duck-billed weirdness. Platypus venom contains a class of molecules that biologists once thought did not occur naturally outside the microscopic world of bacteria.
Those molecules are mirror images of the amino acids that cells normally string together to make all of life's proteins, which are vital to proper functioning. The mirror images are composed of the same atoms that make up the 20 or so standard amino acids in biology's tool kit, and the atoms are attached to one another in the same order. Yet the orientation of the attachments diverges slightly, resulting in structures that differ from classic amino acids in much the way a right hand differs from a left hand. The two forms are not, however, interchangeable in biological reactions. Indeed, classic amino acids are now referred to as left-handed, and their mirror images are said to be right-handed.