Protein

About 120,000 different types of protein molecules have yielded up their structures to science. That sounds like a lot, but it isn't. The techniques, such as X-ray crystallography and nuclear magnetic resonance (NMR), which are used to elucidate such structures do not work on all proteins. Some types are difficult to produce or purify in the volumes required. Others do not seem to crystallize at all, a prerequisite for probing them with X-rays. As a consequence, the structures that have been determined include representatives of less than a third of the 16,000 known protein families. Researchers can build reasonable computer models for around another third because the structures of these resemble ones already known. For the remainder, however, there is nothing to go on. In addition to this lack of information about protein families, there is a lack of information about those from the species of most interest to researchers: Homo sapiens. Only a quarter of known protein structures are human. The majority of the rest comes from bacteria. This paucity is a problem, for in proteins form and function are intimately related. A protein is a chain of smaller molecules, called amino acids, that are often hundreds or thousands of links long. By a process not well understood, this chain folds up, after it has been made, into a specific and complex three-dimensional shape. That shape determines what the protein does: acting as a channel, say, to admit a chemical into a cell; or as an enzyme to accelerate a chemical reaction; or as a receptor, to receive chemical signals and pass them on to a cell's molecular machinery.