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Enzymes - Effects of temperature and pH

Grade 12IGCSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Enzymes are biological catalysts, primarily proteins, that speed up chemical reactions by lowering the activation energy (ΔG‡\Delta G^{\ddagger}).

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The 'Lock and Key' and 'Induced Fit' models explain how the substrate binds to the specific three-dimensional shape of the active site.

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As temperature increases, molecules gain more kinetic energy (EkE_k), leading to a higher frequency of successful collisions between the enzyme and the substrate.

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The optimum temperature is the point at which the rate of reaction is at its maximum; for most human enzymes, this is approximately 37∘C37^\circ C.

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High temperatures (typically above 45∘C45^\circ C) provide enough thermal energy to break hydrogen and ionic bonds, causing the enzyme to denature and the active site to lose its complementary shape.

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The pHpH scale measures the concentration of hydrogen ions (H+H^+); extreme pHpH values interfere with the charges on the amino acids in the active site, leading to denaturation.

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Each enzyme has an optimum pHpH; for example, Pepsin works best at pH≈2pH \approx 2 (acidic), while Trypsin works best at pH≈8pH \approx 8 (slightly alkaline).

📐Formulae

Rate of Reaction=1time (s)\text{Rate of Reaction} = \frac{1}{\text{time (s)}}

Q10=Rate at (T+10)∘CRate at T∘CQ_{10} = \frac{\text{Rate at } (T + 10)^\circ C}{\text{Rate at } T^\circ C}

pH=−log⁡10[H+]pH = -\log_{10}[H^+]

Rate=Δ[Product]Δt\text{Rate} = \frac{\Delta [\text{Product}]}{\Delta t}

💡Examples

Problem 1:

An experiment measures the time taken for amylase to break down starch at 20∘C20^\circ C as 8080 seconds, and at 30∘C30^\circ C as 4040 seconds. Calculate the Q10Q_{10} value for this range.

Solution:

The rate at 20∘C20^\circ C (R1R_1) is 180=0.0125 s−1\frac{1}{80} = 0.0125 \text{ s}^{-1}. The rate at 30∘C30^\circ C (R2R_2) is 140=0.025 s−1\frac{1}{40} = 0.025 \text{ s}^{-1}. Using Q10=R2R1Q_{10} = \frac{R_2}{R_1}, we get Q10=0.0250.0125=2Q_{10} = \frac{0.025}{0.0125} = 2.

Explanation:

The Q10Q_{10} value of 22 indicates that the rate of reaction doubles for every 10∘C10^\circ C increase in temperature within this specific range.

Problem 2:

Predict and explain the effect on the rate of reaction if a protease enzyme with an optimum pHpH of 2.02.0 is moved to a buffer solution of pH 7.0pH\ 7.0.

Solution:

The rate of reaction will decrease significantly or drop to zero.

Explanation:

Moving the enzyme from pH 2.0pH\ 2.0 to pH 7.0pH\ 7.0 changes the concentration of [H+][H^+] ions. These ions interact with the RR-groups of the amino acids in the enzyme, disrupting the ionic and hydrogen bonds that maintain the protein's tertiary structure. This causes the active site to denature, meaning it can no longer form enzyme-substrate complexes with the protein substrate.

Effects of temperature and pH - Revision Notes & Key Diagrams | IGCSE Grade 12 Biology