A-LevelBiologyEnzymes9700/21 · May/June 2022 · Q38 Marks

A tyrosine kinase receptor (TKR) is a protein complex found in the cell surface membrane of mammalian cells. TKR has two components involved in the process of cell signalling: • a receptor for the signalling molecule (ligand) • an enzyme that catalyses the transfer of a phosphate group from ATP to an intracellular protein. Fig. 3.1 is a diagram to show how TKR is involved in cell signalling.

(a)Most enzymes are specific to one reaction. With reference to Fig. 3.1, explain how the structure of an enzyme provides its specificity.[2]
(b)The drug GNF-5 is used in the treatment of some cancers. GNF-5 affects the activity of TKR by binding to the enzyme component of the complex. Researchers investigated the effect of GNF-5 on the activity of TKR using different concentrations of ATP solution. In an experiment the activity of TKR was measured with no GNF-5 and with GNF-5. The results are shown in Fig. 3.2. The researchers concluded that GNF-5 acts as an inhibitor of the enzyme component of TKR and that it is a competitive inhibitor. Use Fig. 3.2 to provide evidence for these conclusions.[3]
(c)(i)State how the activity of T315L differs from TKR when no GNF-5 was present.[1]
(c)(ii)State how the effect of GNF-5 on T315L differs from the effect of GNF-5 on TKR.[2]

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Mark scheme

Marking points from the official mark scheme (May/June 2022). The mark or mark type is shown in brackets.

[2] 1 active site, shape / tertiary structure ;
[2] 2 complementary to, substrate(s) / (intracellular) protein / ATP ;
[3] any three from:
[3] 1 at low(er) concentrations / AW, of, substrate / ATP, lower rate of activity with GNF-5 ;
[3] 2 as substrate concentration increases, effect of GNF-5 decreases ;
[3] 3 maximum rate of reaction / Vmax / described, is the same ;
[3] 4 Michaelis-Menten constant / Km, is higher with inhibitor ; ora
[3] accept comparative data quotes to support answers
[3] for ATP concentration, must use µmol dm³ at least once
[1] without inhibitor, activity of T315L
[1] any one from:
[1] 1 no activity at 0.1 µmol dm-³ (ATP) ;
[1] 2 maximum rate of reaction / Vmax, is reached / AW, (for T315L) at a higher concentration (of ATP);
[1] 3 Michaelis-Menten constant / Km, is higher (for T315L); ora
[1] accept comparative data quotes to support answers
[1] for ATP concentration, must use µmol dm-3 at least once
[2] with inhibitor, activity of T315L
[2] any two from:
[2] 1 activity does not reach TKR, maximum rate of reaction /Vmax;
[2] 2 idea that drug / GNF-5, acts as a non-competitive inhibitor of T315L;
[2] 3 drug / GNF-5, has less of an effect at low concentrations on activity (of enzyme / T315L); ora
[2] accept comparative data quotes to support answers

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