Proteins are biological polymers constructed from amino acids joined together to form peptides. These peptide subunits may bond with other peptides to form more complex structures. Multiple types of chemical bonds hold proteins together and bind them to other molecules. Take a closer look at the chemical bonds responsible for protein structure.
The primary structure of a protein consists of amino acids chained to each other. Amino acids are joined by peptide bonds. A peptide bond is a type of covalent bond between the carboxyl group of one amino acid and the amino group of another amino acid. Amino acids themselves are made of atoms joined together by covalent bonds.
The secondary structure describes the three-dimensional folding or coiling of a chain of amino acids (e.g., beta-pleated sheet, alpha helix). This three-dimensional shape is held in place by hydrogen bonds. A hydrogen bond is a dipole-dipole interaction between a hydrogen atom and an electronegative atom, such as nitrogen or oxygen. A single polypeptide chain may contain multiple alpha-helix and beta-pleated sheet regions.
Each alpha-helix is stabilized by hydrogen bonding between the amine and carbonyl groups on the same polypeptide chain. The beta-pleated sheet is stabilized by hydrogen bonds between the amine groups of one polypeptide chain and carbonyl groups on a second adjacent chain.
Hydrogen Bonds, Ionic Bonds, Disulfide Bridges
While secondary structure describes the shape of chains of amino acids in space, tertiary structure is the overall shape assumed by the entire molecule, which may contain regions of both sheets and coils. If a protein consists of one polypeptide chain, a tertiary structure is the highest level of structure. Hydrogen bonding affects the tertiary structure of a protein. Also, the R-group of each amino acid may be either hydrophobic or hydrophilic.
Hydrophobic and Hydrophilic Interactions
Some proteins are made of subunits in which protein molecules bond together to form a larger unit. An example of such a protein is hemoglobin. Quaternary structure describes how the subunits fit together to form the larger molecule.