IGCSE Biology 0610: Molecular Explanation of Temperature and Denaturation

A particular protease enzyme has an optimum temperature of 45°C.

(a) State the class of large biological molecule that all enzymes, including this protease, are made of. [1]

(b) The enzyme is tested at 20°C and at 45°C. At both temperatures, the enzyme's active site has the same, undamaged shape. Explain, in terms of the enzyme's three-dimensional shape and the kinetic energy of the molecules involved, why the rate of reaction is much lower at 20°C than at 45°C. [2]

(c) The same enzyme is then heated to 70°C for ten minutes and cooled back down to 45°C. Explain, in terms of the bonds that hold the enzyme's three-dimensional shape together, why the enzyme's rate of reaction at 45°C is now much lower than it was before the heating step. [3]

Part (a): The type of molecule enzymes are made of

All enzymes are proteins, made up of long chains of amino acids folded into a specific three-dimensional shape.

Part (b): Why the rate is lower at 20°C than at 45°C

The question states that the enzyme’s active site has the same, undamaged shape at both 20°C and 45°C, so the difference in rate cannot be explained by a change in shape. Instead, it is explained by kinetic energy: at the lower temperature of 20°C, the enzyme and substrate molecules have less kinetic energy, so they move more slowly and collide with each other less frequently. Fewer collisions mean fewer enzyme-substrate complexes form each minute, so the rate of reaction is lower. At 45°C, the optimum temperature, the molecules have more kinetic energy, collide more often, and the rate of reaction is higher.

Part (c): Why heating to 70°C permanently lowers the rate

A protein’s three-dimensional shape, including the shape of its active site, is held in place by bonds between different parts of the folded amino acid chain. At 70°C, far above the enzyme’s optimum of 45°C, the atoms making up the enzyme vibrate much more vigorously, and this extra vibration is enough to break some of these bonds.

Once these bonds are broken, the enzyme’s folded shape, and therefore the shape of its active site, is permanently disrupted. The enzyme is denatured: its active site is no longer complementary to the substrate, so it can no longer form enzyme-substrate complexes efficiently.

Crucially, cooling the enzyme back down to 45°C only reduces the vibration of the atoms again. It does not automatically re-form the original bonds in their original positions or restore the original folded shape. The change caused by the extreme heat is permanent, so even at 45°C, the ideal temperature for an undamaged enzyme, this particular sample now has a much lower rate of reaction than it did before it was heated to 70°C.

Final answers

(a) Protein.
(b) At 20°C the molecules have less kinetic energy and collide less often, even though the active site shape is unaffected, so the rate is lower than at 45°C.
(c) Heating to 70°C breaks the bonds holding the enzyme’s three-dimensional shape together, permanently changing (denaturing) the active site; cooling back to 45°C does not re-form these bonds, so the rate stays much lower than before.