Proteins are classified by four levels of structure. The primary structure of a protein is the sequence of amino acids which make up the protein. The protein's secondary structure involves two configurations within the protein. These are referred to as alpha-helices (α helices) and beta-sheets (β-sheets). These are two ways that proteins organize themselves that contribute to their spatial arrangement and three-dimensional shape, which is their tertiary structure. A protein's quaternary structure is the arrangement and number of polypeptide chains within a protein, and therefore only exists if there is more than one polypeptide chain present in the protein. Protein structure is largely determined by intramolecular interactions, as well as the protein's function and where it will be located when it does its job. Proteins generally have a hydrophilic exterior and a hydrophobic interior, due to its environment in the cell containing a lot of water. These interactions help to hold the protein together, otherwise a hydrophobic exterior would denature the protein. By analyzing these aspects of a protein, we can begin to deduce how the protein might function.