Proteins have a very distinctive and intricate structure composed of four different levels of organization. The first of these is referred to as the primary structure, which is the linear sequence of the amino acids that compose a polypeptide chain. The secondary structure refers to localized regions of the polypeptide chain folded into certain shapes. The shapes arise from the various bonds that link amino acids together. These bonds include hydrogen bonds, electrostatic forces, and van der Waal’s forces. The most recognizable examples of secondary protein structures are alpha helixes and pleated sheets, either of which can be found in different protein complexes. Tertiary structure is the result of folding the secondary structure. Occasionally, proteins will possess a quaternary structure in which the protein is composed of two or more separately folded proteins.
The shape of a protein is crucial because its shape will determine its function within a cell. The proteins’ shape is determined by its primary amino acid sequence and the cellular conditions that affect the folding/bonding that allow the protein to reach higher level structure. The amino acid sequence determines which R groups are present in certain positions, and therefore where the protein can bind to other cell constituents. Currently, the exact mechanics of how primary structure is folded and altered into higher- level forms is not completely understood. Despite this lack of knowledge, knowing the primary amino acid sequence of a particular protein is enough to help one predict the functions of certain regions that can be represented. Specific amino acid sequences or folded proteins can be linked to certain functions known as domains, which a protein can possess multiples of.