Nib asked
How do enzymes lower activation energy?
An enzyme grips the reaction's high-energy transition state more tightly than the substrate, which lowers the activation-energy hill without changing the start or end.
9 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the hill
One enzyme turns a reaction with a half-time of seventy-eight million years into one of eighteen milliseconds. Its trick is lowering the energy hill every reaction must climb.
2activation energy
The top of the hill is the transition state, a strained half-broken arrangement. The height from reactants to that peak is the activation energy.
3lower path
With the enzyme, the reaction takes a lower path. The peak is lower, so far more molecules have enough energy to cross it every second.
4same start and end
Notice what did not move: same start, same end. The enzyme adds no energy and does not change the balance of reactants and products, only the speed.
a common mix-upEnzymes do not add energy or change which side of a reaction is favored; they only lower the barrier.
5the active site
How? The enzyme folds into a pocket called the active site, shaped to match the reaction's substrate and, even more, its transition state.
6squeeze to the peak
The substrate binds, and the enzyme squeezes and bends it toward the transition-state shape, gripping that shape most tightly. Stabilizing the peak is what lowers it.
7reused
Afterwards the products leave and the enzyme is back exactly as it was, ready to grab the next substrate. It is not used up.
8how fast
That is why a little enzyme goes a long way: a single carbonic anhydrase molecule can handle about a million reactions every second.
9the whole idea
So, the whole idea: the enzyme stabilizes the transition state, the hill drops, the reaction speeds up, and the start and end stay put.
Enzymes don't push reactions uphill; they carve a lower pass through the hill, again and again, without being used up.