Proteins are polymers made of amino acid monomers linked by peptide bonds. They perform essential structural and functional roles in the human body such as giving structure, regulating processes, providing protection, and catalyzing chemical reactions as enzymes. Proteins are made of combinations of 20 different amino acids that contain carbon, hydrogen, oxygen, and nitrogen and consist of polypeptide chains formed from joining amino acids through dehydration synthesis reactions. They have a variety of important physiological roles including acting as enzymes, antibodies, transporters, and components of the cytoskeleton.