C1.1 Factors Affecting Enzyme Reaction Rate
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What is activation energy (Eₐ)?
The minimum energy required for a reaction to occur.
How do enzymes lower activation energy?
By destabilizing substrate bonds and stabilizing the transition state.
What is the transition state?
An unstable intermediate formed during a reaction before products appear.
What are exergonic and endergonic reactions?
Exergonic: releases energy (catabolic) Endergonic: requires energy input (anabolic)
What are the main factors affecting enzyme activity?
Temperature, pH, enzyme concentration, substrate concentration.
What is the optimum temperature?
The temperature at which enzyme activity is maximum.
What happens below the optimum temperature?
Molecular motion slows → fewer effective collisions → lower reaction rate.
What happens above the optimum temperature?
Enzyme denatures as hydrogen bonds break, changing active site shape.
Why is denaturation irreversible?
Disruption of tertiary structure prevents the enzyme from refolding properly.
What is the Q₁₀ temperature coefficient?
Measures rate change for a 10°C increase in temperature (usually 2–3× faster until denaturation).
What is the optimum pH?
The pH at which ionic and hydrogen bonds in the enzyme are most stable and active site shape is ideal.
What happens when pH deviates from optimum?
Changes in H⁺ concentration alter charge distribution → active site shape changes → enzyme denatures.
Example: Pepsin vs Trypsin vs Urease
Pepsin: pH 1–2 (stomach, acidic) Trypsin: pH 7.5–8.5 (small intestine, alkaline) Urease: pH 6.5–7.0 (neutral)
What is enzyme concentration’s effect on rate?
Rate increases linearly with enzyme concentration if substrate is not limiting.
What happens when enzyme concentration increases but substrate is limited?
Rate increases then plateaus — active sites saturated.
What is substrate saturation?
When all enzyme active sites are occupied — reaction rate cannot increase further.
How can cells control enzyme concentration?
By regulating gene expression (transcription and translation).
What is substrate concentration’s effect on rate?
Rate increases until enzyme saturation; then it levels off.
Why does reaction rate slow down over time?
Substrate is used up, product accumulates, fewer effective collisions.
What does “optimum conditions” mean?
The specific pH, temperature, and substrate concentration where enzyme rate is maximal.
What happens to enzymes at low temperature?
Reactions are too slow but reversible — enzyme not denatured.
Why are enzymes specific to their environments?
Evolutionary adaptation of enzyme structure to suit organism’s conditions (e.g., icefish enzymes optimal at -2°C).
What is the effect of extreme temperature on enzyme bonds?
Breaks hydrogen and ionic bonds, destabilizing tertiary structure.
What is the difference between enzyme-catalyzed and uncatalyzed reactions?
Catalyzed reactions have lower activation energy and faster reaction rate.