Proteins are molecules made of nitrogen, carbon, oxygen and hydrogen and sometimes sulfur. These molecules are found in animal tissue such as meat (including fish), eggs, cheese and legumes, and many other plant foods.
Proteins are large molecules assembled from smaller units called amino acids and serve many important roles in living things. Amino acids are linked by covalent bonds called peptide bonds. Proteins can serve as enzymes, hormones as well as the building blocks of complex physical features. Proteins comprise teeth, bone, muscle, tendon, cartilage, skin – most of what makes up an organism.
As enzymes, proteins facilitate chemical reactions that would not otherwise happen in the short time, or lower temperatures required by living things. As hormones (such as the previously mentioned appetite inhibitor leptin) they are protein messengers that deliver information around the body.