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Disulfide bridges are strong stabilizers of tertiary structure (Figure 33). hydrophobic and van der Waals interactions CH CH CH 3 3 H 3 H 3 C C polypeptide backbone HC proline kink CH2 S S CH2 disulfide bridge CH 2 OH CH2 OH Figure 33 Hydrophobic interactions, hydrogen bonds, ionic bonds, and disulfide bridges help to hold polypeptides in tertiary structure. quaternary structure two or more polypeptide subunits forming a functional protein 46 Chapter 1 O C CH2 CH 2 CH O 2 CH 2 NH + – 3 O C C H2 ionic bond hydrogen bond In some cases, two or more polypeptide subunits must come together to form a functional protein.

If a protein produced in this environment is placed in different environmental conditions, it may unravel. A change in the three-dimensional shape of a protein caused by changes in temperature, pH, ionic concentration, or other environmental factors is called denaturation. 1 (a) dead flattened cells of a shaft of hair one hair cell (b) microfibril (three chains coiled into one strand) coiled keratin polypeptide chain keratin microfibril SH HS S S SH S S SH HS HS SH HS SH HS S S S S SH HS S S S new bridges form SHHS S SH HS SH SH SH HS HS S S bridges broken hair wrapped around curlers SH S S SH HS SH S S HS HS (c) hair’s cuticle disulfide bridges between two keratin chains A denatured protein cannot carry out its biological functions.

Making Connections Conduct library and/or Internet research to answer questions 2 to 5. com 2. What was the first protein to be fully sequenced? Who conducted the experiment and when was it accomplished? What substance was used to identify the amino-terminal amino acid? What problems were encountered in the process? 3. What “wet” methods could be used to identify the PTH derivative produced in the Edman process? 4. Identify another method used to sequence a protein. Describe the method. 5. A number of organizations on the Internet offer protein sequencing and protein synthesis services.