Amino acids and protein structureAQA A-Level Biology: Subtopic test

1. 1
   Alanine is an amino acid whose R group is a methyl group, CH₃. Cysteine is an amino acid whose R group is CH₂SH, which contains sulfur.

   (a)

   Which part of an amino acid is the only part that differs between the twenty amino acids common to all organisms?

   [1 mark]

   • AThe amine group
   • BThe carboxyl group
   • CThe R group
   • DThe central carbon atom

   (b)

   Which groups react to form a peptide bond between two amino acids?

   [1 mark]

   • AThe amine group of one and the carboxyl group of the other
   • BThe carboxyl groups of both
   • CThe amine groups of both
   • DThe R groups of both

   (c)

   Explain why two cysteine molecules in a polypeptide can form a bond between their R groups that two alanine molecules cannot.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Haemoglobin is a protein made from four polypeptide chains: two α chains of 141 amino acids each and two β chains of 146 amino acids each.

   (a)

   How many peptide bonds are there in one haemoglobin molecule?

   [1 mark]

   • A574
   • B571
   • C573
   • D570

   (b)

   Which level of protein structure is the association of the four polypeptide chains into one functional molecule?

   [1 mark]

   • ATertiary structure
   • BQuaternary structure
   • CPrimary structure
   • DSecondary structure

   (c)

   Explain why a change in the sequence of amino acids in a protein can change its function.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Ribonuclease is an enzyme made from a single polypeptide of 124 amino acids, folded into a compact three-dimensional shape. Its tertiary structure is held together by hydrogen bonds, ionic bonds and four disulfide bridges.

   (a)

   Describe the primary, secondary and tertiary structure of ribonuclease.

   [3 marks]

   (b)

   Explain how the three types of bond contribute to the tertiary structure of ribonuclease, and suggest why the enzyme keeps its shape better with disulfide bridges than with hydrogen bonds alone.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A food technician is testing a colourless solution that may contain a protein. She has sodium hydroxide solution and dilute copper(II) sulfate solution. If a protein is present, it is a globular protein with a precise three-dimensional shape that is essential for its function.

   (a)

   Describe how the technician should carry out the biuret test, and explain what the result shows. Explain also why a solution containing only free amino acids would not give a positive result.

   [6 marks]

   (b)

   Explain how the primary structure of a protein leads to its tertiary structure and how a quaternary structure forms, and why the three-dimensional shape matters for the function of the protein.

   [6 marks]

   Total for question 4: 12 marks

End of questions