Proteins

Much of our bodies' dry weight is protein—even our bones are about one-quarter protein. The animals we eat and the microbes that attack us are likewise largely protein. The leather, wool, and silk clothing that we wear are nearly pure protein. The insulin that keeps diabetics alive and the "clot-busting" enzymes that may save heart attack patients are also proteins. Proteins can even be found working at industrial sites-protein enzymes produce not only the high-fructose corn syrup that sweetens most soft drinks, but also fuel-grade ethanol (alcohol) and other gasoline additives.

Within our bodies and those of other living things, proteins serve many functions. They digest foods and turn them into energy; they move our bodies and move molecules about within our cells; they let some substances pass through cell membranes while keeping others out; they turn light into chemical energy, making both vision and photosynthesis possible; they allow cells to detect and react to hormones and toxins in their surroundings; and, as antibodies, they protect our bodies against foreign invaders.

Many of these protein functions are addressed or referred to in other articles in this encyclopedia. Yet there are simply too many proteins—possibly more than 100,000—to even consider mentioning them all. Even trying to discuss every possible type of protein is an exercise in futility. Not only is the number of types enormous, but the types overlap. In producing muscle contraction, for example, the proteins actin and myosin obtain energy by breaking down adenosine triphosphate in an enzyme-like fashion.