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3.1.4 Proteins and enzymes

AQA A-level Biology 3.1.4 practice on Proteins and enzymes, with free MCQs, clues and worked explanations drawn from the Biological molecules section of specification 7402.

Specification route
3.1.4
Question bank
26 questions
Course stage
AS / Year 12

Sample questions

QUESTION 1 · 3.1.4 · LEVEL 2

Which level of protein structure is determined strictly by the sequence of amino acids in the polypeptide chain?

  • Primary structure
  • Secondary structure
  • Tertiary structure
  • Quaternary structure
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This structure is dictated directly by the genetic code.

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Answer: Primary structure

The primary structure is the sequence of amino acids in the polypeptide chain, joined by peptide bonds. This sequence ultimately determines the subsequent folding into secondary, tertiary, and quaternary structures.

QUESTION 2 · 3.1.4 · LEVEL 4

A mutation changes a single amino acid in an enzyme's active site. Why might this prevent the enzyme from functioning?

  • It alters the specific tertiary structure, preventing the formation of an enzyme-substrate complex.
  • It breaks the peptide bonds in the primary structure, degrading the protein.
  • It increases the activation energy to a level that cannot be reached by the cell.
  • It converts the enzyme into a competitive inhibitor.
Show clue

Enzyme function depends heavily on the specific 3D shape of its active site.

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Answer: It alters the specific tertiary structure, preventing the formation of an enzyme-substrate complex.

The properties of an enzyme relate to the tertiary structure of its active site. Changing an amino acid alters the R-groups present, changing hydrogen, ionic, or disulfide bonds, which alters the tertiary structure and prevents complementary substrate binding.

QUESTION 3 · 3.1.4 · LEVEL 4

How does a non-competitive inhibitor affect an enzyme-controlled reaction?

  • It binds to a site other than the active site, altering the enzyme's tertiary structure so the substrate can no longer bind.
  • It binds directly to the active site, physically blocking the complementary substrate from entering.
  • It permanently denatures the enzyme by breaking disulfide bridges within the primary structure.
  • It binds to the substrate molecule, preventing it from fitting into the enzyme's active site.
Show clue

Think about the meaning of 'non-competitive'. It doesn't compete for the same physical space as the substrate.

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Answer: It binds to a site other than the active site, altering the enzyme's tertiary structure so the substrate can no longer bind.

Non-competitive inhibitors bind to the enzyme away from the active site (allosteric site). This binding alters the overall tertiary structure of the enzyme, changing the shape of the active site so it is no longer complementary to the substrate.

QUESTION 4 · 3.1.4 · LEVEL 5

If the substrate concentration is continuously increased in an enzyme-catalysed reaction containing a fixed concentration of a competitive inhibitor, what will happen to the rate of reaction?

  • The rate will eventually reach the same maximum velocity ($V_{max}$) as the uninhibited reaction.
  • The rate will reach a lower maximum velocity ($V_{max}$) than the uninhibited reaction.
  • The rate will continuously increase without ever reaching a maximum velocity.
  • The rate will immediately drop to zero as the active sites become saturated.
Show clue

Competitive inhibitors compete for the active site. If you flood the system with substrate, the substrate is more likely to 'win'.

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Answer: The rate will eventually reach the same maximum velocity ($V_{max}$) as the uninhibited reaction.

Competitive inhibitors bind to the active site. Increasing the substrate concentration increases the probability of a substrate molecule colliding with an active site instead of an inhibitor, eventually overcoming the inhibition and reaching the original $V_{max}$.

QUESTION 5 · 3.1.4 · LEVEL 3

Which functional groups are involved in the formation of a peptide bond between two amino acids?

  • The amine group ($NH_{2}$) of one amino acid and the carboxyl group ($COOH$) of another.
  • The R-group of one amino acid and the carboxyl group ($COOH$) of another.
  • The amine group ($NH_{2}$) of one amino acid and the R-group of another.
  • The hydroxyl group ($OH$) of one amino acid and the amine group ($NH_{2}$) of another.
Show clue

The bond forms the 'backbone' of the polypeptide, independent of the variable side chains.

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Answer: The amine group ($NH_{2}$) of one amino acid and the carboxyl group ($COOH$) of another.

A condensation reaction between two amino acids forms a peptide bond. This occurs between the carboxyl group of one amino acid and the amine group of the adjacent amino acid, releasing water.

QUESTION 6 · 3.1.4 · LEVEL 3

Why do most enzymes function extremely slowly at very low temperatures?

  • The enzyme and substrate molecules have very low kinetic energy, resulting in fewer successful collisions.
  • The low temperature permanently denatures the enzyme by breaking disulfide bridges.
  • The substrate molecules change shape and are no longer complementary to the active site.
  • The activation energy of the reaction is significantly increased.
Show clue

Low temperature does not generally break the strong covalent bonds holding the tertiary structure together.

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Answer: The enzyme and substrate molecules have very low kinetic energy, resulting in fewer successful collisions.

The effects of temperature on the rate of enzyme-controlled reactions: At low temperatures, kinetic energy is low. Molecules move slowly, so there are fewer frequent, successful collisions between the enzyme's active site and the substrate.

QUESTION 7 · 3.1.4 · LEVEL 4

A polypeptide chain contains multiple cysteine amino acids. What specific type of bond is likely to form between the R groups of these amino acids to stabilise the tertiary structure?

  • Disulfide bridges
  • Hydrogen bonds
  • Ionic bonds
  • Peptide bonds
Show clue

These are strong covalent bonds formed between sulfur atoms in the R groups.

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Answer: Disulfide bridges

The role of hydrogen bonds, ionic bonds and disulfide bridges in the structure of proteins. Cysteine contains sulfur, allowing for strong covalent disulfide bridges to form, which highly stabilise the tertiary structure.

QUESTION 8 · 3.1.4 · LEVEL 3

Enzymes can catalyse both intracellular and extracellular reactions. Which of the following is an example of an extracellular enzyme action?

  • The hydrolysis of starch into maltose by amylase in the human digestive system.
  • The phosphorylation of glucose during glycolysis in the cytoplasm.
  • The unwinding of DNA by DNA helicase during replication.
  • The joining of amino acids at the ribosome to form a polypeptide.
Show clue

Extracellular means the reaction occurs outside of a living cell.

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Answer: The hydrolysis of starch into maltose by amylase in the human digestive system.

Enzymes catalyse a wide range of intracellular and extracellular reactions. Digestion in the gut lumen is extracellular, while glycolysis, DNA replication, and translation are intracellular.

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