A pocket of a few amino acids, holding one molecule in exactly the right position — and a reaction that would take years happens in milliseconds.
AP · U3.1CED 3.1–3.2Interactive lesson
Concept 01 · CED 3.1
A barrier, not a push
Most reactions that should happen do not, because nothing has enough energy to get started. An enzyme does not provide that energy — it lowers the amount needed.
The problem
Activation energy
Bonds must be strained before they break. At body temperature few molecules can manage it unaided.
The solution
A lower route
The active site strains bonds and orients reactants, so far more collisions succeed.
The limit
ΔG is untouched
Where the reaction ends up is set by thermodynamics. The enzyme only decides how quickly it gets there.
Note carefully what the pink curve changes and what it does not. The peak is lower, so the reaction proceeds far faster — but the substrate and product sit at exactly the same energies, so ΔG is identical. An enzyme changes the route, never the destination.Diagram · interactive
Anatomy of an enzyme
Tap each part. Almost every property here descends from tertiary structure.
Diagram
A pocket in a folded protein
Tap the body, the active site, the substrate, the R groups, the fold, or the summary panel.
Signature interactive · CED 3.2
One catalytic cycle
Five steps, and at the end the enzyme is exactly as it started. Watch the site close in step three — that is induced fit.
Concept 02 · CED 3.2
What a catalyst cannot do
Half the marks in this topic come from knowing the limits rather than the mechanism.
Can
Increase rate
Often by a factor of a million or more, by lowering the activation energy barrier.
Cannot
Change equilibrium
Forward and reverse reactions are sped equally, so the equilibrium position does not move.
Cannot
Be consumed
The enzyme is regenerated every cycle, which is why cells need so little of it.
If an exam question says an enzyme made a reaction happen that otherwise would not, or that the enzyme was used up, or that the products contain more energy than before, something in the statement is wrong. Only the first of these three is a real property of catalysis.Concept map · interactive
How it all hangs together
Tap a node to light up its connections. The pink dashed link is the boundary of what catalysis can do.
Concept map
Tap a node
Read down the left for structure, across the right for consequence.
Retrieval · drag and drop
Fill the gaps
Drag a term into a gap, or tap a term and then tap a gap. Two terms belong nowhere.
active sitesubstrateactivation energyinduced fitspecificcomplementarycatalystunchangeddenaturedhydrolysis
An enzyme binds its in a pocket called the
, whose shape is
to that molecule — which is why each enzyme is
to one reaction. Binding causes the site to close more tightly, a
model known as . The enzyme works by lowering the
of the reaction. Because it is a
, it emerges from the reaction
and can be used again immediately.
Check yourself · AP-style
Six questions
Single best answer. You get the reasoning as soon as you commit.
Biology by Bradford