Limiting Factors Affecting Enzymes: pH (AQA A Level Biology): Revision Note
Exam code: 7402
The effect of pH
All enzymes have an optimum pH
Enzymes are denatured at extremes of pH
Hydrogen and ionic bonds hold the tertiary structure of the protein (i.e. the enzyme) together
Below and above the optimum pH of an enzyme, solutions with an excess of H+ ions (acidic solutions) and OH- ions (alkaline solutions) can cause these bonds to break
This alters the shape of the active site, which means enzyme-substrate complexes form less easily
Eventually, enzyme-substrate complexes can no longer form at all
At this point, complete denaturation of the enzyme has occurred
Where an enzyme functions can be an indicator of its optimal environment:
E.g. pepsin is found in the stomach, an acidic environment at pH 2
Pepsin’s optimum pH is therefore pH 2
Buffer solutions
When investigating the effect of pH on the rate of an enzyme-catalysed reaction, you can use buffer solutions to measure the rate of reaction at different pH values:
Buffer solutions each have a specific pH
Buffer solutions maintain this specific pH, even if the reaction taking place would otherwise cause the pH of the reaction mixture to change
Examiner Tips and Tricks
Temperature affects both the movement of molecules (increasing collision rate) and can denature enzymes at high levels. pH does not affect collision rate, but changes in pH can disrupt substrate binding, reducing successful collisions until the active site is no longer functional.
Calculating pH
If the hydrogen ion (H+) concentration of a solution is known, the pH can be calculated using the equation:
pH = -log₁₀ [H⁺]
You can find the ‘log’ function on your calculator (‘log’ is the same as ‘log10’ so don’t worry if the calculator doesn’t say ‘log10’)
Worked Example
The hydrogen ion concentration of a solution is 1.6 x 10-4 mol dm-3. Find the pH of this solution.
The pH of the solution is:
pH = -log₁₀ [H⁺]
pH = -log₁₀ 1.6 x 10-4 = 3.796
pH = 3.8
Worked Example
The hydrogen ion concentration of a solution of sodium hydroxide is 3.5 x 10-11 mol dm-3. Find the pH of this solution.
The pH of the solution is:
pH = -log₁₀ [H⁺]
pH = -log₁₀ 3.5 x 10-11 = 10.456
pH = 10.5
Worked Example
Ethanoic acid (also known as acetic acid) is a weak acid produced by wood ants that they can spray at predators as a defence mechanism. The hydrogen ion concentration of a sample of ethanoic acid taken from some wood ants was 8.39 x 10-6 mol dm-3. Find the pH of the ethanoic acid produced by wood ants.
The pH of the solution is:
pH = -log₁₀ [H⁺]
pH = -log₁₀ 8.39 x 10-6 = 5.076
pH = 5.08
Examiner Tips and Tricks
Don’t forget the minus sign in the formula: pH = –log₁₀ [H⁺] as this is a common exam mistake. pH values should be between 0 and 14. If your answer falls outside this range, check your working. Use clues in the question to estimate if your answer is reasonable (e.g. alkalis should have high pH, weak acids around pH 4–6).
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