A protein is an organic compound that is made of one or more chains of amino acids and that is a principal component of all cells. Proteins play a variety of roles in the body, such as providing structure, catalyzing metabolic reactions, replicating DNA, responding to stimuli, and transporting molecules from one location to another. They are also responsible for many of the functions associated with life and are required for the structure, function, and regulation of the body's tissues and organs.