Proteins

• Proteins are amphoteric- can react as a base and an acid

  A) Amine group

  B) Carboxyl group

  
• The primary structure is the sequence of amino acids joined by peptide bonds.
• Secondary structure refers to the folding of polypeptides into alpha helices or beta pleated sheets, which stabilises them through hydrogen bonding between different parts of the same chain.
• Tertiary structure describes how secondary structures interact with one another to form a three dimensional shape.
• Quaternary structure involves multiple chains coming together to form a larger protein complex.
• In tertiary structures:

  Hydrogen bonds occur between R groups with Delta negative oxygen and delta positive hydrogen.

  High temperatures and altered pH can split these bonds
• Disulfide bonds form between cyestine amino acids beacuse they contain sulfur. Strong covalent bonds
• Globular proteins fold to a more spherical orientation, and therefore form specific binding sites through their tertiary structure shape specificity. These globular proteins therefore typically form enzymes (which are globular and have an active site)
• Fibrous proteins fold to form more long/sheet like structures. Collagen is a triple helix of protein cross-linked to give a high tensile strength. Keratin is another example of a tough protein which folds for strength/structural reasons.
• Testing for proteins (biruet):

  1. Place sample in tube and add equal volume of NaOH (sodium hydroxide).
  2. Add a few drops of dilute copper (II) sulphate and mix gently at room temperature.
  3. Orange to purple colour change indicates that a protein is present in the sample.
• Insuli- gobular protein

  Alpha chain- 21 amino acids

  Beta chain- 30 amino acids

  2 disulfide bridges (residues A7 to B7, and A20 to B19) covalently connect the chains, and alpha contains an internal disulfide bridge (residues A6 to A11).
• Keratin- fibrous protein

  15% cyestine- sulfur bridges, strong

  Helix structure
• Hameaglobin (conjugated protein)- globular

  Made of:

  • 2 alpha chains
  • 2 beta chains

  Each polypeptide has its own haem group which associates to oxygen
• Haem is an inorganic ion

  Fe 2+ and porphyrin
• Catalase- gobular protein

  4 polypeptide chains

  Prostetic group Fe 3+

  Oxoreductase enzyme

  Substrate- H202, breaks it down into H20 and 02
• Collagen- fibrous protein

  3 polypeptide chains

  Every 3rd amino acid is glycine, held together by hydrogen bonds
• Proteins do not contain phosphorus but must contain nitrogen
• Describe how R groups interact to determine the tertiary structure of a protein

  Some R groups atrract each other

  Disulfide bridge may form between S atoms in R groups

  Hydrogen bonds can also form between R groups

  Ionic bonds can form if the R groups are oppositely charged