A-LevelBiologyNucleic acids and protein synthesis9700/21 · Oct/Nov 2017 · Q413 Marks

Fig. 4.1 shows the structure of the R groups of three amino acids, aspartate (Asp), glutamine (Gln) and isoleucine (Ile).

(a)One of the mRNA codons for the amino acid Gln is CAA. A mutation could change this codon from CAA to UAA. UAA is a STOP codon. State what is meant by a STOP codon.[1]
(b)Many proteins that contain the amino acids shown in Fig. 4.1 are globular. Describe how the three R groups in Fig. 4.1 can contribute to the globular structure of a protein.[4]
(c)An example of a globular protein is the enzyme starch phosphorylase. This enzyme catalyses the conversion of starch to glucose-1-phosphate. Name the type of bond that is broken when starch is converted to glucose-1-phosphate.[1]
(d)An investigation was carried out to find out how the activity of starch phosphorylase varied with temperature, when free in solution and when immobilised in alginate. The results are shown in Fig. 4.2. With reference to Fig. 4.2, compare the results for the enzyme activity when free in solution and when immobilised.[4]
(e)State how the Michaelis–Menten constant (Km) is derived from Vmax for an enzyme.[1]
(f)The Km values for starch phosphorylase were determined when free in solution and when immobilised. The Km values were: • free in solution, Km = 0.34 mmol dm-3 • immobilised, Km = 0.84 mmol dm-3. Suggest an explanation for the difference between the two values.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.

[1] (a) codon that terminates / AW, translation / polypeptide synthesis R stops production of amino acids or does not specify any amino acid or has no complementary, tRNA / anticodon or causes the release of the (completed) polypeptide chain ;
[4] (b) four from Asp and Gln R groups, hydrophilic / AW, and Ile R group, hydrophobic / AW ; ref. to R group interactions and, folding / coiling, of polypeptide chain (to give globular structure); ref. to (charged / polar R groups) forming, ionic / hydrogen, bonds ; (amino acids with), hydrophobic / non-polar, R groups, on the inside / AW ; A hydrophobic amino acids (amino acids with), hydrophilic / charged / polar, groups on outside / AW ; A hydrophilic amino acids detail; e.g. ionic R groups can interact with water, hydrogen bonds form with water ref. to promotes stability / helps to maintain globular structure ; in context of interaction with water/formation of (compact) hydrophobic core
[1] (c) glycosidic;
[4] (d) four from 1 both, increase and decrease / peak; 2 both active, over range of temperature tested / between 20 to 55°C; 3 free enzyme lower, optimum temperature / temperature for maximum activity; ora 4 (free) 37°C v (immobilised optimum) 46°C; 5 free enzyme higher, maximum activity / activity at optimum temperature ; ora 6 (free) 8.2 / 8.3, a.u. v 7.5 a.u. ; 7 between 20°C and 39°C the activity of free is higher ; ora 8 between 39°C and 55°C the activity of free is lower ; ora if mp 7 and 8 not gained, allow 1 mark for free activity higher at lower temperatures and lower at higher temperatures (than immobilised) ora 9 comparative data to support mp 7 or 8 ; A comparison at one temperature (stated or use of numerical data) 10 AVP; e.g. both have same activity at 39°C, similar activity between 20°C and 30°C
[1] (e) substrate concentration that gives half Vmax / AW; A description of how to use a graph to derive (must refer to substrate concentration)
[2] (f) two from look for ora throughout free / low(er) Km, has higher affinity for substrate ; free needs a lower concentration of substrate to reach, Vmax / maximum activity / ½ Vmax; takes lower substrate concentration to saturate active sites in free ; free active site more, likely to be saturated with substrate / accessible ; active site (may be) distorted by immobilising; ref. to some product retained within matrix ; ref. to substrate passing through matrix when immobilised ;