To appreciate the many vital functions of proteins, we must understand their structure. One key difference from carbohydrate and fat, which contain only carbon, hydrogen, and oxygen atoms, is that proteins contain nitrogen atoms. These nitrogen atoms give the name amino (“nitrogen containing”) to the amino acids of which protein is made. Another key difference is that in contrast to the carbohydrates—whose repeating units, glucose molecules, are identical—the amino acids in a strand of protein are different from one another.
All amino acids have the same, simple chemical backbone, with an amine group (the nitrogen-containing part) at one end and an acid group at the other end. The differences among the various amino acids are due to the varying structures of the chemical side chains that are attached to the backbone. Twenty amino acids with 20 different side chains make up most of the proteins of living tissue.
The side chains vary in complexity from a single hydrogen atom, like that on glycine, to a complex ring structure, like that on phenylalanine. Not only do these structures differ in composition, size, and shape, but they also differ in electrical charge. Some are negative, some are positive, and some have no charge. These side chains help to determine the shapes and behaviors of the larger protein molecules that the amino acids make up.