Step by Step
Ea
Activation energy — the energy hill every reaction must climb
Every chemical reaction requires an initial input of energy to proceed — called the activation energy (Ea). Even exergonic (energy-releasing) reactions need this initial push. Think of it as the energy required to break existing bonds before new ones can form. Without enough activation energy, reactions proceed too slowly to sustain life.
Memory trick: Activation energy = the hill a ball must roll over before rolling down. Enzymes make the hill shorter.
Cat
Enzymes are biological catalysts
Enzymes lower the activation energy of a reaction — they do NOT change the free energy (ΔG) of the reaction, the equilibrium position, or whether the reaction is exergonic or endergonic. They simply make it happen faster. Enzymes are not consumed in the reaction — they are released unchanged and can catalyze the same reaction repeatedly.
TS
Transition state stabilization
Enzymes lower Ea by stabilizing the transition state — the unstable, high-energy intermediate between substrates and products. The active site of the enzyme binds the substrate and distorts it toward the transition state, reducing the energy needed to reach it. This is the key mechanism of enzyme catalysis.
Memory trick: The enzyme hugs the transition state — making it stable long enough for the reaction to proceed.
ΔG
What enzymes do NOT change
Enzymes do not alter: the overall ΔG of the reaction, the equilibrium constant (Keq), whether the reaction is spontaneous, or the final concentrations of products and reactants at equilibrium. They only change the rate at which equilibrium is reached.
Applied Walkthrough
1
Without an enzyme: glucose + oxygen → CO₂ + H₂O + energy. This reaction is spontaneous (ΔG < 0) but at body temperature proceeds far too slowly to power cellular activity.
2
Glycolytic enzymes lower the activation energy of each step — the same overall reaction now proceeds millions of times faster.
3
The ΔG of complete glucose oxidation is the same with or without enzymes (~−2870 kJ/mol). Enzymes just remove the kinetic barrier.
4
Each enzyme is released unchanged at the end of its reaction and immediately catalyzes the next molecule of substrate.
Exam Application
Exams test whether enzymes change ΔG (they don't), whether they change equilibrium (they don't), and the mechanism by which they speed reactions (transition state stabilization, lowering Ea). Energy diagrams showing Ea with and without enzyme are common — know which curve is which and what the enzyme changes vs what it doesn't.
⚠ Common Trap
Students think enzymes make reactions spontaneous — they don't. If a reaction is endergonic (ΔG > 0), an enzyme cannot make it proceed spontaneously. Enzymes only speed up reactions that are already thermodynamically favorable. Also: enzymes lower the activation energy of BOTH the forward and reverse reactions equally — they don't favor one direction over the other.
✓ Quick Self-Check
1. What is activation energy?
The minimum energy required to initiate a chemical reaction — the energy needed to reach the transition state.
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2. How do enzymes speed up reactions?
By lowering the activation energy — specifically by stabilizing the transition state in the active site.
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3. Do enzymes change the ΔG of a reaction?
No — enzymes do not change ΔG, the equilibrium constant, or whether the reaction is spontaneous. They only change the rate.
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4. Are enzymes consumed in the reactions they catalyze?
No — enzymes are released unchanged after each reaction and can catalyze the same reaction repeatedly.
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5. Can an enzyme make an endergonic reaction proceed spontaneously?
No — enzymes cannot change whether a reaction is thermodynamically favorable. An endergonic reaction (ΔG > 0) requires energy input regardless of enzyme presence.
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