1. Describe and explain the similarities and differences between competitive and non-competitive enzyme
inhibition.
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2. Figure1 shows the effects of increasing substrate concentration on enzyme activity with and without two
types of inhibitor, competitive and non-competitive.
Figure 1
Sketch a line on both graphs to indicate the effect of increasing inhibitor concentration in each case. Explain
the position and shape of each line.
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3.
In humans, the enzyme sucrose hydrolyses sucrose. This reaction occurs in the small intestine at 37°C.Explain why
sucrose only hydrolyses sucrose, and why this reaction can take place at normal body temperature.
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1.Similarities between competitive and non-competitive inhibition:
The inhibitor binds to the enzyme (surface); [1 mark]
 There is a reduction in the enzyme’s rate (of catalysis); [1 mark]
 The rate of initial reaction is inversely proportional to inhibitor concentration /
as inhibitor concentration increases, rate of reaction decreases; [1 mark]
Differences between competitive and non-competitive inhibition:
 The specific site of binding differs, competitive inhibitors bind to the active
site AND non-competitive inhibitors bind to an allosteric site/site away from
active site; [1 mark]
 So, in competitive inhibition, the active site is blocked AND in non-competitive
inhibition, the active site is distorted / changes shape; [1 mark]
 Increasing substrate concentration will decrease the effect of competitive
inhibition BUT will not affect non-competitive inhibition; [1 mark]
2.Competitive inhibition graph:
 A curve under the dotted line - not reaching Vmax OR achieving Vmax further to
the right;
[1 mark]
Explanation:
 Higher substrate concentrations required to achieve a certain reaction
rate OR the highest rate can be achieved but at higher substrate
concentrations; [1 mark]
Non-competitive inhibition graph:
 A curve under the dotted line - plateauing well below Vmax / never achieving
Vmax ;
[1 mark]
Explanation:
 The inhibitor can bind (allosterically) to both free enzyme and enzymesubstrate complex meaning all enzyme function can be removed, even at very
high substrate concentrations; [1 mark]
[Total: 4 marks]
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3. The 3-D shape / tertiary structure of the enzyme / active site; [1 mark]
Is complementary to sucrose’s molecular shape; [1 mark]
The reaction can take place at a normal body temperature because...
 Sucrase lowers the activation energy required; [1 mark]
 By stressing the substrate / induced fit / forming an enzyme-substrate
complex; [1 mark]
[Total: 4 marks]
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