AQA A-Level Chemistry Paper 2, 2022: Question 4

9 marks · Medium difficulty · State/Explain/Describe

Proteins and Amino Acid Interactions This question focuses on the structure and properties of proteins and amino acids. It requires identifying the protein structure level depicted, drawing the structure of the –Cys–Ser– section of a protein, and naming the by-product of amino acid condensation. It also examines the strength of interactions between cysteine R groups versus serine and aspartic acid R groups and deducing the type of interaction between lysine and aspartic acid R groups in a protein.

Practise this question

Question

AQA A-Level Chemistry Paper 2, 2022: Question 4
Question text

04 Proteins are polymers made from amino acids.

Part of the structure of a protein is shown.

–Cys–Ser–Asp–Phe–

Each amino acid in the protein is shown using the first three letters of its name.

04.1 Identify the type of protein structure shown.

[1 mark]

Tick ( ) one box.

Primary

Secondary

Tertiary

04.2 Draw a structure for the –Cys–Ser– section of the protein.

Use the Data Booklet to help you answer this question.

[2 marks]

04.3 Name the other substance formed when two amino acids react together to form part

of a protein chain.

[1 mark]

The general structure of an amino acid is shown.

R represents a group that varies between different amino acids.

R groups can interact and contribute to protein structure.

04.4 Explain why the strength of the interaction between two cysteine R groups differs from

the strength of the interaction between a serine R group and an aspartic acid R group.

Use the Data Booklet to help you answer this question.

[4 marks]

04.5 Deduce the type of interaction that occurs between a lysine R group and an

aspartic acid R group.

[1 mark]

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 2, 2022: Question 4

Question Answers Additional Comments/Guidelines Mark

Primary 1

04.1

(AO1)

M1 for correct peptide link (Allow -CONH- as a M1

minimum)

M2 for the correct amino acid R groups

Dipeptide can only score M1

04.2 OR

M2

(2 x AO2)

Trailing bonds not needed

Water Allow H2O 1

04.3

(AO1)

Two Cys R groups form a disulfide bridge/link stated or described Could score via a correct diagram showing M1

minimum -S-S-

20 Ser and Asp R groups form Hydrogen bonds Allow H bonds M2

04.4 Disulfide bridges are stronger than Hydrogen bonds Interactions between cys R groups are stronger M3

Because disulfide bridges are covalent bonds (while Hydrogen bonds Because covalent bonds are stronger (than H M4

aren’t) bonds) (2 x AO1,

2 x AO3)

Ionic (bond) 1

04.5

(AO3)

How to answer it

Guide to Protein Structure and Interactions

Part (a): Protein Structure Type

Question: Identify the type of protein structure shown.

Correct Answer: Primary

Most students correctly identified this as a primary structure.

Part (b): Drawing a Protein Section

Question: Draw the structure for the –Cys–Ser– section of the protein.

Correct Structure: See Mark Scheme

Common Errors:

  • Drawing a dipeptide instead of the specific protein section.
  • Failing to include the peptide link (-CONH-).
Why it Happens: Students often overlook the difference between a dipeptide and a protein section. This highlights the importance of carefully reading the question and referring to the structure of amino acids.

Part (c): Reaction Byproduct

Question: Name the other substance formed when two amino acids react together to form a protein chain.

Correct Answer: Water (H2O)

This was well answered by most students.

Part (d): R Group Interactions

Question: Explain why the strength of the interaction between two cysteine R groups differs from the strength of the interaction between a serine R group and an aspartic acid R group.

Correct Answer:

  • Cysteine R groups form disulfide bridges, which are covalent bonds and therefore stronger.
  • Serine and aspartic acid R groups form hydrogen bonds, which are weaker.

Common Errors:

  • Not identifying the disulfide bridges as covalent bonds.
  • Failing to explain why disulfide bridges are stronger than hydrogen bonds.
Why it Happens: Students often know the types of bonds but fail to relate their strength to the chemical nature (covalent vs hydrogen bonding).

Part (e): Lysine and Aspartic Acid Interaction

Question: Deduce the type of interaction that occurs between a lysine R group and an aspartic acid R group.

Correct Answer: Ionic bond

Common Errors:

  • Stating hydrogen bonding instead of ionic bonding.
  • Misunderstanding the charges on the R groups of lysine (positively charged) and aspartic acid (negatively charged).
Why it Happens: Students may not associate charged amino acid side chains with ionic interactions. It’s important to connect R group chemistry to bonding types.
Tip: Remember that ionic bonds form between positively and negatively charged groups in proteins.

Final Notes

Understanding protein structures and interactions requires attention to detail, especially regarding bond types and their relative strengths. Practising drawing peptide chains and identifying specific interactions will help solidify these concepts.

Topics

Organic Chemistry · 3.3.13 Amino Acids, Proteins and DNA

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 2, 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.