Proteins and enzymes: exam questions

Answer: A – Peptide bonds form between the carboxyl group of one amino acid and the amino group of the next, in a condensation reaction that releases water. R groups are not involved in the peptide bond.

• condensation reaction (1)
• between the carboxyl group of one amino acid and the amino group of the other / OH from the carboxyl and H from the amino group removed (1)
• water released / peptide bond formed between C and N (1)

• primary structure is the sequence of amino acids / R groups (1)
• determines where hydrogen bonds / ionic bonds / disulfide bonds can form between R groups (1)
• so the chain folds into a specific three-dimensional shape (1)
• so a change in one amino acid can change the folding / the three-dimensional shape (1)
• hydrogen bonds and ionic bonds (1)
• disulfide bonds (between cysteine R groups) (1)
• hydrophobic interactions between non-polar R groups (1)
• Max 5

• similarity: both have quaternary structure / more than one polypeptide chain / both are proteins (1)
• haemoglobin is globular and collagen is fibrous (1)
• haemoglobin is soluble (hydrophilic R groups outside), collagen is insoluble (1)
• haemoglobin has four chains each with a haem group, collagen has three chains in a triple helix (1)
• haemoglobin transports oxygen, collagen provides strength / support (1)
• Max 4

• active site has a specific shape / three-dimensional structure (1)
• complementary to the substrate / only one substrate fits, forming an enzyme–substrate complex (1)
• active site shape is determined by the tertiary structure / the primary structure (1)
• lowers the activation energy (1)
• active site changes shape slightly (induced fit), putting strain on bonds in the substrate (1)
• Max 3

• change in volume ÷ change in time (1)
• 11.0 ÷ 20 (1)
• 0.55 cm³ s⁻¹ (1)
• correct answer with no working gains 3 marks; do not accept 0.65 cm³ s⁻¹ (first 10 s only)
• substrate concentration has fallen / substrate used up, so fewer collisions and fewer enzyme–substrate complexes formed (1)

• measure volume at regular short intervals (e.g. every 10 seconds) rather than one reading at 2 minutes (1)
• plot volume against time and calculate the initial rate from the gradient of a tangent at time zero (1)
• keep temperature constant / use a water bath for all pH values (1)
• same volume and concentration of enzyme and hydrogen peroxide for each pH (1)
• bring solutions to temperature before mixing (1)
• repeat each pH at least three times and calculate a mean / identify anomalies (1)
• use a gas syringe for accurate volumes (1)
• Max 6

Answer: B – Amylase is secreted and acts outside the cells that make it. The others all work inside cells.