Biology · Energy, Enzymes & Metabolism

Enzymes, activation barriers and catalytic specificity

A sugar solution can remain in a bottle even though some reactions involving sugar would release free energy. Thermodynamic favourability describes an overall difference. Molecules still need a route through a transition state before the reaction can proceed at an appreciable rate.

Read the Chemistry of Life and Cells & Membranes chapters first if atoms, molecular interactions and membrane transport are unfamiliar. The necessary ideas are also introduced here. You do not need to memorise complete biochemical pathways or calculate thermodynamic quantities to follow the first reading.

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By the end, you can

Explain how an enzyme changes reaction rate without changing the overall reaction's free-energy difference.

Distinguish the barrier from the destination

An activation barrier separates reactants from a transition state along a reaction pathway. An enzyme is a biological catalyst that provides a faster pathway, often by positioning substrates, facilitating chemical transfers or stabilising the transition state. Most enzymes are proteins; some RNA molecules also catalyse reactions.

Compare the two schematic profiles. They begin with the same reactants and end with the same products, but their highest barriers differ. The horizontal axis is a reaction coordinate: progress through a molecular rearrangement, not elapsed time or distance travelled by an organism. Real enzyme mechanisms may have several intermediates and barriers.

Change the barrier while keeping the endpoints

A deliberately simple model. No curve is a measured enzyme reaction or a prediction of its numerical rate.

Read the diagram in words

Two constructed free-energy profiles share reactants at 20 units and products at 5. The uncatalysed profile peaks at 80 and the catalysed profile at 45. The forward barrier falls from 60 to 25 units while the overall free-energy change remains minus 15. The horizontal axis is reaction coordinate, not time.

Uncatalysed route
Reaction coordinate — not time	Illustrative free-energy units
0	20
0.2	20
0.5	80
0.8	5
1	5

Catalysed route
Reaction coordinate — not time	Illustrative free-energy units
0	20
0.2	20
0.5	45
0.8	5
1	5

Schematic values for the two routes
Route	Reactants	Highest point	Products	Forward barrier	Same overall change
Uncatalysed route	20	80	5	60	-15
Catalysed route	20	45	5	25	-15

Illustrative free-energy units

Binding depends on chemistry as well as shape

A substrate is a reactant acted on by an enzyme. The active site contains groups positioned to bind and transform suitable substrates. Complementary shape matters, but so do charges, polarity and other molecular interactions. The enzyme and substrate can change conformation during binding; a rigid lock-and-key drawing is only a first approximation.

Specificity does not always mean one enzyme accepts exactly one molecule. Some enzymes act on a family of related substrates. Nor does binding alone establish catalysis: an inhibitor may occupy a binding site without undergoing the enzyme's usual reaction. Ask what transformation occurs, rather than identifying specificity from a shape alone.

A catalyst works within conditions

Enzymes do not change the ΔG or equilibrium constant of the overall reaction they catalyse at a given temperature. They accelerate approach to equilibrium, in both directions of a reversible reaction. Driving a thermodynamically unfavourable process requires coupling or a change in conditions, not simply adding more catalyst.

Rate also depends on substrate availability, enzyme amount, temperature, pH, cofactors and inhibitors. Increasing temperature may initially speed a reaction but can also disrupt enzyme structure or activity. The useful range differs among enzymes and organisms. A rate increase cannot continue indefinitely just because a general collision argument predicts faster molecular motion.

Worked example

Read the schematic profile

In the constructed diagram, reactants are at 20 units and products at 5. The uncatalysed peak is at 80, so its forward barrier is 80 − 20 = 60 units. The catalysed peak is at 45, giving a 25-unit barrier. Both profiles have the same overall change: 5 − 20 = −15 units.

The lower barrier helps explain a faster reaction; the unchanged endpoints show why an enzyme does not make a different thermodynamic destination. These numbers illustrate that distinction. They do not specify a real enzyme's mechanism or allow calculation of a rate without additional information.

Check your understanding

An enzyme lowers an activation barrier. Has it supplied the free energy needed to make an uphill reaction favourable?
Show reasoning
No. Barrier lowering changes kinetics. A separate favourable coupled process or appropriate change in conditions is needed to alter the thermodynamic account.
Why is an enzyme not used up once it has catalysed one reaction?
Show reasoning
The catalyst is regenerated through its reaction cycle, although it can later be damaged, inhibited or degraded. Its participation does not mean net consumption in each catalytic event.
Two molecules fit a drawn active-site outline. Is that enough to predict identical reaction rates?
Show reasoning
No. A flat outline omits chemical interactions, conformational changes and transition-state stabilisation. Binding and catalysis must be tested.

What to carry forward

An enzyme changes the route and the rate; it does not change the overall free-energy difference for the same reaction and conditions.

References

Resources supporting this lesson’s explanations and examples.