The Levels Of Protein Structure
The various levels of protein structural organization are defined as follows.
Primary Structure
The amino acid sequence is the primary (1°) structure of a protein, such as that shown in Figure 4.7
Secondary Structure
Through hydrogen bonding interactions between adjacent amino acid residues the polypeptide chain can arrange itself into characteristic helical or pleated segments. These segments constitute structural conformities, so-called regular structures that extend along one dimension, like the coils of a spring. Such architectural features of a protein are designated secondary (2°) structures (Figure 4.7). Secondary structures are just one of the higher levels of structure that represent the three dimensional arrangement of the polypeptide in space.
Tertiary Structure
When the polypeptide chains of protein molecules bend and fold in order to assume a more compact three dimensional shape, a tertiary (3°) level of structure is generated (Figure 4.7). It is by virtue of their tertiary structure that proteins adopt a globular shape. A globular conformation gives the lowest surface tovolume ratio, minimizing interaction of the protein with the surrounding environment.
Quaternary Structure
Many proteins consist of two or more interacting polypeptide chains of characteristic tertiary structure, each of which is commonly referred to as a subunit of the protein. Subunit organization constitutes another level in the hierarchy of protein structure, defined as the protein’s quaternary (4°) structure (Figure 4.7). Whereas the primary structure of a protein is determined by the covalently linked amino acid residues in the polypeptide backbone, secondary and higher orders of structure are determined principally by noncovalent forces such as hydrogen bonds and ionic, van der Waals, and hydrophobic interactions.
