Proteins are made up of amino acids linked together by peptide bonds. There are 20 common amino acids which can be classified as essential or non-essential. Proteins have four levels of structure - primary, secondary, tertiary, and quaternary. The primary structure is the amino acid sequence, secondary structures include alpha helices and beta sheets, tertiary structure is the overall 3D shape from interactions between R groups, and quaternary refers to multiple polypeptide chains interacting.