S

1.4 proteins

Describe the general structure of an amino acid

  • COOH = carboxyl group

  • R = variable side chain/ group

  • H2N =amine group

The 20 amino acids that are common in all organisms differ only in the side group (R)

How do amino acids join together

  • condensation reaction

  • Removing a water molecule

  • Between carboxyl group of one and amine group of another

  • Forming a peptide bond

Dipeptide = 2 amino acids joined together

Polypeptide = many amino acids joined together

A functional protein = may contain one or more polypeptides

The primary structure of a protein

  • a sequence of amino acids in a polypeptide chain, joined by peptide bonds

The secondary structure of a protein

  • folding (repeating patterns) of polypeptide chain (e.g alpha helix/ beta pleated sheets

  • Due to hydrogen bonding between amino acids

  • Between NH group of one amino acid and C=O group

The tertiary structure of a protein

  • 3D folding of polypeptide chain

  • Due to interactions between amino acid R groups (dependent on sequence of amino acids)

  • Forming hydrogen bonds, ionic bonds and disulfide bridges

The quaternary structure of a protein

  • more than one polypeptide chain

  • Formed by interactions between polypeptides (hydrogen bonds, ionic bonds and disulfide bridges)

The test for proteins

  1. Add biuret reagent (sodium hydroxide + copper sulphate)

  2. Positive result= purple/ lilac, indicating the presence of peptide bonds