PP: Protein Structure

Proteins have complex structures that determine the many functions proteins will perform in the body, and these structures are a result of the endless combinations of the 20 biological amino acids. There are four levels of protein structure. The primary structure of a protein is simply its amino acid sequence. Covalent peptide bonds between the amino and carboxyl groups of amino acids form, building a polypeptide chain. The secondary structure is the structure of the polypeptide backbone, excluding the R groups of the amino acids. It involves hydrogen bonds that stabilize alpha-helices and beta-sheets formed. The tertiary structure factors in the chemistry of the R groups, finalizing the overall structure of the 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 from the non-covalent electrostatic interactions. Finally, the quaternary structure is only relevant to proteins that are composed of multiple polypeptides. It involves the various electrostatic interactions between the different subunits within the overall protein.