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3.2.23 · Enzymes

Rate- pH — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 7 parts Lesson 10 of 17 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

3.2.2 Rate: pH — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how pH affects enzyme activity and how buffers are used in investigations.Definition — every enzyme has an optimum pH at which it works best.Link to location — where an enzyme works indicates its…

  • Bonds affected
  • Acid or alkali
  • Active site
  • Denaturation
  • Temperature
  • Buffer solution

Part 1 of 5Optimum pH

Definition
  • Definition every enzyme has an optimum pH at which it works best.
Link to location
  • Link to location where an enzyme works indicates its optimum.
  • Pepsin works in the stomach at about pH 2 and has an optimum of pH 2.

Part 2 of 5Effect of extreme pH

Bonds affected
  • Bonds affected hydrogen and ionic bonds hold the tertiary structure of the enzyme together.
Acid or alkali
  • Acid or alkali excess H+ or OH- ions break these bonds.
Active site
  • Active site its shape changes, so enzyme–substrate complexes form less easily.
Denaturation
  • Denaturation eventually no complexes can form and the enzyme is completely denatured.

Part 3 of 5Using buffers in investigations

1
Use buffer solutions, each with a specific pH.
2
Buffers maintain the pH even if the reaction would otherwise change it.
3
Add the same measured volume of each buffer to the reaction mixture.
4
Measure the rate at each pH.

Part 4 of 5Temperature compared with pH

Temperature
  • Temperature changes the speed of molecules and so the collision rate, and can denature enzymes.
pH
  • pH does not change collision rate; it disrupts the ability of the substrate to bind to the active site.

Part 5 of 5More from the specification

1Active sites conformation and product formation
  • Active sites conformation and product formation optimal pH maintains enzyme active sites structure, providing a means to maximize catalytic product yield.
2Enzyme Active Sites and Catalytic Mechanism
  • Enzymes are biological catalysts with a specific tertiary active site shape;
    • collision with complementary substrate molecules forms an enzyme-substrate complex, lowering the activation energy.
3Effect of pH on Enzyme Activity
  • Deviations from an enzyme's optimum pH alter the ionization of acidic and basic amino acid R-groups at the active site, disrupting ionic bonds and leading to denaturation and loss of catalytic function.
4Competitive vs Non-Competitive Inhibitors
  • Competitive inhibitors resemble substrate molecules and bind reversibly to the active site,
  • while non-competitive inhibitors bind to an allosteric site, altering active site conformation and reducing maximal rate (Vmax).
5Michaelis-Menten Constant (Km) and Immobilised Enzymes
  • The Michaelis-Menten constant (Km) represents the substrate concentration at which reaction rate is half of Vmax;
    • immobilised enzymes bound to inert matrices exhibit enhanced stability against pH and temperature shifts.
6Enzyme-substrate complex
  • The temporary structure formed when a substrate binds to an enzyme's active site.
7Inhibitor
  • A molecule that reduces enzyme activity.
8Lactose
  • A disaccharide of glucose and galactose.
9Affinity
  • How readily haemoglobin binds oxygen.
10Substrate concentration
  • The amount of substrate per unit volume.
11Enzyme concentration
  • The amount of enzyme per unit volume.
12Successful collision
  • A collision with enough energy and correct orientation.
13Non-competitive inhibitor
  • An inhibitor that binds elsewhere and changes the active site.
14Immobilised enzyme
  • An enzyme fixed to a surface or trapped in a matrix so it can be reused.

Key terms

TermMeaning
Optimum pHThe pH at which an enzyme has maximum activity.
Buffer solutionA solution that maintains a specific pH.
DenaturationLoss of the active site shape due to broken bonds.
Ionic bondAn attraction between oppositely charged R groups.
PepsinA stomach enzyme with an optimum of pH 2.
Hydrogen bondA weak bond helping to hold tertiary structure.

Avoid these mistakes

1
Saying pH changes the speed of molecules — it changes charges and bonding in the enzyme.
2
Forgetting to add the same volume of buffer to each tube — buffer volume must be controlled.
3
Saying all enzymes have the same optimum pH — optimum pH differs, e.g. pepsin pH 2.
4
Saying denaturation by pH removes peptide bonds — hydrogen and ionic bonds break.

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