Different cellular processes are the result of proteins interacting with protein receptors within or on a cell. These interactions are much like those of a lock and key. The protein receptor is the lock, and the protein is the key. If the protein shape is altered, it either cannot be used by the cell, or could have a different effect on the cell.
Scientists have identified four levels of protein structure. Changes to the most basic level can affect the structure of subsequent levels. This is of great importance because the structure of a protein dictates its function.
Primary Structure
A polypeptide is a chain of amino acids bound together. The sequence, or order, of the amino acid is the primary structure of a polypeptide. The average number of amino acids in a polypeptide is 300, although the number can vary significantly.
Secondary Structure
Different atoms attract and repel each other. This property is partly responsible for the structure of a molecule, like an amino acid. When amino acids bind together to form a polypeptide, attractions and repulsions cause the combined molecules to form a certain shape. A polypeptide can take the shape of a helix or a pleated sheet, like that of an accordion. This is known as the secondary structure.
Tertiary Structure
A polypeptide does not only take the shape of a simple, elongated helix or pleated sheet. Folds form in the polypeptide, much like coiling a ribbon. The secondary structure folds back and forth to form the tertiary structure.
Quaternary Structure
Multiple tertiary-structured polypeptides form a quaternary structure. The quaternary structure consists of two or more polypeptides that combine together to form a protein.
Factors Affecting Protein Structure
- Disulfide bridges are formed when sulfhydryl groups from different amino acids bind to form a connection between the two amino acids. A sulfhydryl group consists of a sulfur atom combined with a hydrogen atom (–SH).
- Hydrogen bonding is the attraction of a hydrogen atom to an electronegative atom such as nitrogen (N), oxygen (O), or fluorine (F). By itself, a hydrogen bond is quite weak; but when many hydrogen bonds exist, such as with the structure of DNA, collective hydrogen bonding can be quite strong.
- Hydrophobic effect is the repulsion of nonpolar molecules from water, which is a polar substance. The nonpolar chains of a polypeptide move into the center of the structure away from the water and towards other nonpolar chains. Like is attracted to like, so nonpolar substances attract each other, whereas polar substances attract each other. A nonpolar substance and a polar substance will repel each other.
- Polar interactions are the attraction of negative side chains to positive side chains. Ionic bonding is a polar interaction. An ion is a positively- or negatively-charged atom or molecule. Ions are attracted to other ions of the opposite charge so the binding of the ions can result in a net charge of zero.
- Van der Waals forces are the weak attractions and repulsions of atoms. If an atom is within a certain distance of another atom, they will experience an attraction. If the atoms are too close, there will be a repulsion.
There are four structural levels of a protein. While the first level is merely the amino acid sequence of a protein, the subsequent structural levels are determined by chemical and physical laws affecting shape. These forces include hydrogen bonding and polar interactions. The structure of a protein determines its shape, and its shape dictates its interactions with cells that are responsible for maintaining life.
Reference
Brooker, Robert J., Widmaier, Eric P., Graham, Linda E., and Stiling, Peter D. Biology. New York: McGraw-Hill, 2008.
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