Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 75

Some globular proteins, often called oligomeric proteins, are composed of two or more different polypeptide chains. The quaternary structure of these proteins emphasizes the way in which the individual folded protein chains fit together to form an overall three‐dimensional structure (Figure 2.27). A well‐studied example is hemoglobin, the protein that carries oxygen in red blood cells. This protein has a nearly spherical shape, composed of two identical α‐globin subunits and two identical β‐globin subunits arranged symmetrically. The same noncovalent bonds that stabilize the tertiary structure stabilize the quaternary structure of proteins.

Figure 2.27 Illustrative example of quaternary structure of a protein composed of four polypeptide chains. In this example, there are two types of protein, labeled Protein 1 and Protein 2, each with two chains.

Table 2.6 summarizes the four levels of protein structure and the main atomic interactions.

Table 2.6 Summary of the four levels of protein structure and major atomic interactions that stabilize each level.

Structure	Description	Geometry	Major atomic interactions
Primary	Sequence of amino acid residues in chain backbone	—	Covalent bonding between amino acid residues to form amide (peptide bond)
Secondary	Arrangement of chain backbone of individual polypeptide	Random arrangementRegular helical structure (α‐helix)Regular sheet structure (β‐sheet)	Intrachain hydrogen bonding
Tertiary	Three‐dimensional shape (conformation) of polypeptide chain	GlobularFibrous (elongated)	Interchain and intrachain hydrogen, van der Waals, ionic, and covalent bondinga
Quaternary	Overall three‐dimensional structure of oligomeric proteins	Approximately spherical or irregular	Interchain hydrogen and van der Waals

^a Hydrogen and van der Waals bonding often dominate over ionic and covalent bonding.