1.7 Proteins

Describe the structure and function of proteins.

• Proteins comprise linear chains of amino acids connected by the formation of covalent (peptide) bonds that form between a carboxyl group (−COOH) of one amino acid and an amine group (N−H)2 of the next amino acid, resulting in a growing peptide chain.
• Amino acids are composed of a central carbon atom with a hydrogen atom, a carboxyl group, an amine group, and a variable R group covalently bound to it. The R group of an amino acid can be categorized by three possible chemical properties: hydrophobic/nonpolar, hydrophilic/polar, or ionic. The interactions of these R groups determine the structure and function of that region of the protein.
• The specific sequence of amino acids in proteins determines the primary structure of a polypeptide as well as the overall shape of the protein.
  • Exclusion: The molecular structure of amino acids is beyond the scope of the AP Exam.
• Secondary structures of proteins are made through the local folding that forms from interactions between atoms of the polypeptide backbone of the amino acid chain. Hydrogen bonding forms shapes such as alpha-helices and beta-pleated sheets.
• The three-dimensional shape of the tertiary structure of a protein results from the formation of hydrogen bonds, hydrophobic interactions, ionic interactions, or disulfide bridges.
• The quaternary structure arises from interactions between multiple polypeptides. All four levels of a protein structure determine the function of a protein.