Protein's have complex structures because a protein's structure determines its function, and there are many functions for proteins within the body. There are 20 biological amino acids, and there are endless combinations of these amino acids that will result in different proteins. A protein's structure has four levels. The primary structure of a protein is the linear sequence of amino acids. This involves peptide bonds between the amino and carboxyl groups of amino acids, building a polypeptide chain. The secondary structure is the structure of the backbone that is created. It involves hydrogen bonds that stabilize alpha-helices and beta-sheets, which are different secondary forms a polypeptide can take. The secondary structure does not include the R groups of the amino acids; however, the tertiary structure does. The tertiary structure involves all the electrostatic interactions that can occur between amino acids in the protein, including the chemistry of the R groups. This will finalize the final structure of the overall protein, which can be globular or fibrous in form. R groups can be nonpolar, polar and uncharged, positively charged, or negatively charged. Depending on the proximity of these groups, different structures can result. Finally, the quaternary structure is only relevant to proteins that are made up multiple polypeptides, or subunits. It is the interaction between the different subunits within the overall protein. This level of protein structure also involves all types of electrostatic interactions.
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