Proteins

• Proteins are very complex and diverse.

  Some have structural functions, and some have metabolic functions.

  Proteins are essential for growth and repair.

  They are obtained through diet through foods such as dairy, meat, eggs and nuts.
• Proteins are made up of amino acids.

  Amino acids are the monomer units which combine to make up complex protein structures.

  A chain of amino acids is called a polypeptide.
• Each amino acid in a protein has a central carbon atom to which four chemical groups are attached:

  • Single hydrogen
  • Amino group (NH3)
  • R group (which varies dependent on the type of amino acid)
  • Carboxyl group (COOH)
• Amino acids join to each other as a result of condensation reactions, forming peptide bonds.

  Dipeptide – Two amino acids joined together.

  Polypeptide – Many amino acids joined together.

  Peptide bonds can be hydrolysed (broken down) using water.
• Primary Structure

  The sequence of amino acids in the polypeptide chain of a protein

  Determines its overall shape and its chemical properties which is crucial to how/if it works.
• Secondary Structure

  Dependent on whether the original polypeptide chain is folded or coiled.

  Folds/coils are kept in place by weak hydrogen bonds.
• Tertiary Structure

  Dependent on the secondary structure being twisted and folded to create more complex shapes.
• The bonds which maintain the tertiary structure are:

  Disulphide bridges - Strong covalent bonds between sulphur atoms that are close together.

  Ionic bonds - Formed between carboxyl and amino acid groups. Fairly strong.

  Hydrogen bonds - Numerous but weak.
• Quaternary structure

  Only found in large protein molecules with multiple polypeptide chains.

  Examples:

  Haemoglobin – Globular protein, metabolic function.

  Collagen – Fibrous protein, structural function.
• Biruet Test

  Place a few drops of your chosen solution in a test tube at room temperature.

  Add a few drops of biuret solution.

  A protein is present if the solution turns from blue to purple.