Proteins are classified by four levels of structure. Primary structure of a protein is the sequence of amino acids which make up the protein. The protein's secondary structure involves two structures within the protein. These are referred to 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 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. Protein structure tells us a lot about the protein and much can be theorized about its possible function and properties by analyzing the protein structure.
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