Higher-order structures
المؤلف:
Peter Atkins، Julio de Paula
المصدر:
ATKINS PHYSICAL CHEMISTRY
الجزء والصفحة:
ص678-679
2025-12-18
40
Higher-order structures
Covalent and non-covalent interactions may cause polypeptide chains with well-defined secondary structures to fold into tertiary structures. Subunits with well-defined tertiary structures may interact further to form quaternary structures. Although we do not know all the rules that govern protein folding, a few general conclusions may be drawn from X-ray diffraction studies of water-soluble natural proteins and synthetic polypeptides. In an aqueous environment, the chains fold in such a way as to place nonpolar R groups in the interior (which is often not very accessible to solvent) and charged R groups on the surface (in direct contact with the polar solvent). A wide variety of structures can result from these broad rules. Among them, a four-helix bundle (Fig. 19.32), which is found in proteins such as cytochrome b562 (an electron transport protein), forms when each helix has a nonpolar region along its length. The four nonpolar regions pack together to form a nonpolar interior.
Similarly, interconnected β-sheets may interact to form a b-barrel (Fig. 19.33), the interior of which is populated by nonpolar R groups and which has an exterior rich in charged residues. The retinol-binding protein of blood plasma, which is responsible for transporting vitamin A, is an example of a β-barrel structure. Factors that promote the folding of proteins include covalent disulfide (-S-S-) links, Coulombic interactions between ions (which depend on the degree of protonation of groups and therefore on the pH), hydrogen bonding, van der Waals interactions, and hydrophobic interactions (Section 18.4g). The clustering of nonpolar, hydro phobic, amino acids into the interior of a protein is driven primarily by hydrophobic interactions.
Fig. 19.32 A four-helix bundle forms from the interactions between nonpolar aminoacids on the surfaces of each helix, with the polar aminoacids exposed to the aqueous environment of the solvent.
Fig. 19.33 Eight anti-parallel β-sheets, each represented by a purple arrow and linked by short random coils fold together as a β-barrel. Nonpolar aminoacids are in the interior of the barrel.
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