PROTEINS 37
The tertiary structure of proteins arises from the interactions of the various
R groups along the polypeptide chain. Some of the forces responsible for the
tertiary structures include van der Waals forces, ionic bonds, hydrophobic
bonds, and hydrogen bonds. Usually, hydrophilic R groups arrange themselves
on the exterior of the tertiary structure and interact with the aqueous environ-
ment, whereas hydrophobic R groups usually orient themselves on the interior
of the protein ’ s tertiary structure and exclude water. Most proteins are defi ned
as being amphipathic — that is, they contain both nonpolar and polar side
chains and environments. The nonpolar groups or areas will align themselves
with nonpolar or hydrophobic molecules, while the polar groups or areas will
align themselves with polar or hydrophilic molecules.
One ionic bond that often helps establish tertiary structure is a disulfi de
bond between two cysteine side chain groups — for instance, in the enzyme
lysozyme as shown in Figure 2.10. Lysozyme is not a metalloprotein, such as
will be studied in this text, but it is a small enzyme and is illustrative of some
secondary and tertiary structures found in the more complex molecules
described in the following chapters. Lysozyme protects biological species from
Figure 2.10 Secondary and tertiary structure of the enzyme lysozyme, PDB: 2C8O.
Visualized using Cambridge Soft Chem3D Ultra 10.0 with notations in ChemDraw
Ultra 10.0. ChemDraw Ultra, version 10.0. (Printed with permission of CambridgeSoft
Corporation)
N-terminal
endC-terminal
endcys30cys115cys127cys6cys94 cys76cys64α-helix 1 α-helix 3 cys80antiparallelβ-pleated sheetα-helix 5S - S disulfide bondindicates direction of α-helixLysozyme visualized from PDB: 2C8O