1.4.1 general properties of proteins

• Describe / draw 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 their side group (R)

  
• Describe how amino acids join together ?

  ● Condensation reaction

  ● Removing a water molecule

  ● Between carboxyl / COOH group of one and amine / NH2 group of another

  ● Forming a peptide bond

  
• What are dipeptides and polypeptides?

  ● Dipeptide - 2 amino acids joined together

  ● Polypeptide - many amino acids joined together

  • A functional protein may contain one or more polypeptides
• Describe the primary structure of a protein ?

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

  
• Describe the secondary structure of a protein

  ● Folding (repeating patterns) of polypeptide chain eg. alpha helix / beta pleated sheets

  ● Due to hydrogen bonding between amino acids

  ● Between NH (group of one amino acid) and C=O (group)

  
• Describe 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

  
• Describe the quaternary structure of a protein?

  ● More than one polypeptide chain

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

  
• Describe the test for proteins?

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

  2. Positive result = purple / lilac colour (negative stays blue) → indicates presence of peptide bonds
• Exam insight: common mistakes ❌- (1/6)

  ❌-“Amino acids contain DNA triplets.”

  ✅-A DNA triplet codes for a specific amino acid, but these are completely separate structures
• Exam insight: common mistakes ❌- (2/6)

  ❌-“A dipeptide has a primary structure.

  ✅-A dipeptide is not a protein so doesn’t have a primary structure.
• Exam insight: common mistakes ❌- (3/6)

  ❌-“All hydrogen bonds are between R groups.”

  ✅-In the secondary structure, hydrogen bonds are between NH and C=O groups.
• Exam insight: common mistakes ❌- (4/6)

  ❌-“All proteins have a quaternary structure.”

  ✅-Only proteins with more than one polypeptide chain possess a quaternary structure. Examples include haemoglobin and antibodies.
• Exam insight: common mistakes ❌- (5/6)

  ❌-“Quaternary structure is made of four polypeptides.”

  ✅-Not all quaternary structure proteins are made of four polypeptides, but they do consist of more than one polypeptide.
• Exam insight: common mistakes ❌- (6/6)

  ❌-“Quaternary structure is multiple tertiary structures.”

  ✅-Each polypeptide in the quaternary structure of a protein has its own primary, secondary, and tertiary structure. However, by definition the quaternary structure is more than one polypeptide chain and so this won’t achieve a mark.