| Date | November 2016 | Marks available | 2 | Reference code | 16N.3.hl.TZ0.14 |
| Level | HL | Paper | 3 | Time zone | TZ0 |
| Command term | Estimate and Suggest | Question number | 14 | Adapted from | N/A |
Question
Glucokinase and hexokinase are both enzymes that catalyse the conversion of glucose to glucose-6-phosphate. The enzymes differ, however, in their affinity for the substrate, as shown in the graph below.
(i) Estimate the Km values of the two enzymes.
(ii) Suggest, with a reason, which enzyme will be more responsive to changes in the concentration of glucose in the blood.
(i) Outline what is meant by product inhibition as it applies to hexokinase.
(ii) Product inhibition of hexokinase does not affect its Km value. Using this information, deduce the type of binding site that the inhibitor attaches to.
Markscheme
i
Km hexokinase: approx. 1.7 «mmol dm–3»
AND
Km glucokinase: approx. 8.5 «mmol dm–3»
Accept answers in the range 1.0-2.0 for hexokinase and 7.0-9.0 for glucokinase.
ii
glucokinase as it is not saturated «with substrate at normal concentration of blood glucose»
OR
glucokinase as its saturation increases with increased glucose concentration in the blood
Accept “at the normal levels of blood glucose concentration, relative velocity of glucokinase still dependent on concentration of glucose”
i
glucose-6-phosphate lowers enzyme activity/acts as enzyme inhibitor
ii
«inhibitor binds at» allosteric site
Accept “outside/away from active site”.
