The shape of a protein is significant. The shape of a protein dictates the job that that protein can do. Loss or change of shape will hinder the ability of the protein to do the job it was created for. Proteins form a specific shape by folding in a way that allows bonds to form between specific amino acids.
Amino acids bound together by peptide bonds make up a polypeptide. This is considered the primary structure of the protein. The secondary structure is created by hydrogen bonding across the amino acid chain to create two types of structures: alpha-helices and beta-sheets. The tertiary structure is driven by interactions between amino acids in the protein sequence and their affinity for water. Some amino acids are hydrophilic, meaning they are attracted to water; others are hydrophobic, meaning they repel water. The protein will fold so amino acids that repel water are internal to the structure, and proteins with an affinity for water are on the protein surface. Remember, protein formation occurs in the cell's cytoplasm, which is mainly composed of water. The tertiary structure ultimately determines the 3-dimensional shape of the protein. A quaternary structure is created when multiple polypeptides interact to form a complex for a single purpose. This occurs frequently in the cell; one example of a protein with a quaternary structure is a receptor on the cell surface with two identical subunits.
