Paper 4 · November 2003 · Enzymes

Catalase activity was measured over the pH range 3 to 8. As the pH rises from pH3 towards pH7, the rate of the reaction increases. What happens to the ionisable groups of catalase's active site as the pH rises over this range?

AThey are destroyed permanently and cannot recover

BThey are completely unaffected by the pH change

CThey are progressively modified

DThey stop being ionisable altogether

Explanation

Between pH3 and pH7 the ionisable groups of the active site are progressively modified by the changing pH, which is what goes on to disrupt the ionic bonding that maintains the enzyme's shape.

Derived from ZIMSEC Biology 9190/4 Paper 4 (Practical Test), November 2003, Q1

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More questions from this paper

As the ionisable groups of catalase's active site are modified between pH3 and pH7, what specifically is disrupted, allowing the reaction rate to rise?
Between pH3 and pH7, disruption of the ionic bonding that maintains catalase's shape is linked to a rising reaction rate, without yet denaturing the enzyme. What effect does this disruption have on the active site over this range?
Above pH7 (towards pH8), the rate of the catalase reaction falls. What happens to catalase's ability to form enzyme-substrate (ES) complexes over this range?
Table 1.1 in this practical dilutes a 0.005 mol/dm3 glucose stock solution with distilled water to make five standards, keeping the total volume at 5 cm3. Row 1 mixes 1 cm3 stock with 4 cm3 water (0.001 mol/dm3); row 2 mixes 2 cm3 stock with 3 cm3 water (0.002 mol/dm3); row 3 mixes 3 cm3 stock with 2 cm3 water (0.003 mol/dm3). Following this same pattern, how many cm3 of the 0.005 mol/dm3 glucose stock and how many cm3 of distilled water are needed to make the 0.004 mol/dm3 standard?
Using the same 5 cm3-total dilution series as Table 1.1 (1 cm3 stock + 4 cm3 water = 0.001 mol/dm3, up to 3 cm3 stock + 2 cm3 water = 0.003 mol/dm3), how many cm3 of the 0.005 mol/dm3 glucose stock and how many cm3 of distilled water are needed to make the 0.005 mol/dm3 standard itself?

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