Human beings and animals obtain proteins from foods. Foods high in protein include cheese, eggs, fish, meat, and milk. The structure of proteins. All proteins contain carbon, hydrogen, nitrogen, and oxygen. Some proteins also contain iron, phosphorus, and sulfur.
Proteins are large, complex molecules made up of smaller units called amino acids.
The amino acids are linked together into long chains called polypeptides. A few polypeptides are straight, but most are bent into complex three-dimensional shapes. A protein consists of one or more polypeptide chains. Twenty common amino acids are assembled into the thousands of different proteins required by the human body. To assemble the proteins it needs, the body must have a sufficient supply of all these amino acids.