Proteins are molecules composed of one or more chains of amino acids. Proteins are essential components of muscles, skin, bones and the body as a whole. They are required for the structure, function, and regulation of the body's cells, tissues, and organs. Each protein has unique functions, the function is determined by their 3D strucure and the ability to bind to other molecules (called ligands). Examples of proteins include whole classes of important molecules, such as: enzymes, hormones, and antibodies.