Protein Structural Organization
Primary Structure
Proteins and polypeptides possess up to four levels of structural organization that contribute to their final three-dimensional shape and overall function in the cell. Each amino acid contains one of functional groups that provide unique chemical characteristics.
The primary structure is the linear sequence of amino acids joined together by peptide bonds.
Secondary Structure
As the primary structure is established, the amino acid chain orientates itself in space to form the secondary structure, which is held in place by hydrogen bonding between the peptide bonds. Common secondary structures include:
- \n\alpha\n-helix: A right-handed spiral similar to a spiral staircase.
- \n\beta\n-pleated sheet: A structure similar to a handheld fan.
Tertiary Structure
The tertiary structure is the final three-dimensional shape of a polypeptide. This shape is maintained by interactions between the R groups in the amino acids, including:
- Hydrogen bonds
- Ionic bonds
- Covalent bonds (such as disulfide linkages formed between R groups containing sulfur)
Quaternary Structure
While some proteins consist of a single polypeptide chain, others are composed of multiple chains, each possessing its own primary, secondary, and tertiary structure. The spatial arrangement of these individual polypeptide chains forms the quaternary structure. Complex proteins, such as hemoglobin and most enzymes, exhibit quaternary structure.