An enzyme lowers the hump, never the finish line: same ΔG, same equilibrium, reached faster.
An enzyme changes how fast a reaction happens, never where it ends up. It holds the substrate in the right pose and lowers the energy hump. A reaction that would take weeks can finish in seconds.
Key terms
- Activation energy — the energy hump reactants must get over before they can become products.
- Active site — the pocket where the substrate binds. Its shape and chemical groups decide which substrates fit.
- Induced fit — the active site shifts shape as the substrate binds, closing around it.
- Catalyst — speeds up a reaction and comes out unchanged, so one enzyme molecule works again and again.
- Saturation — nearly every enzyme molecule is busy. Adding more substrate barely raises the rate.
- Cofactor — a non-protein helper, such as a metal ion, that some enzymes need in order to work.
How it works
- The substrate binds in the active site through weak interactions: hydrogen bonds, ionic attractions, hydrophobic contacts.
- The site closes around it. That motion lines up catalytic groups at the bonds that will change, and it can strain the bonds that must break.
- Together these lower the activation energy, so the reaction runs far faster.
- The products are held only weakly, so they drift off. The enzyme is unchanged and binds the next substrate.
- The endpoint never moves. ΔG and the final mix of reactants and products are the same with or without the enzyme.
- With a fixed amount of enzyme, more substrate means a faster rate until nearly every enzyme is busy. Then extra substrate barely changes the rate; extra enzyme raises it.
Why it matters
The next topic asks what temperature, pH, and inhibitors do to enzymes, and most of those answers come back to the shape of the active site. Photosynthesis and respiration depend on dozens of enzymes, too. Expect rate tables. Read the rows against each other: change one thing, compare the rates, and ask whether binding, the chemistry, or the amount of working enzyme changed.
Exam traps
| Trap | Actually |
|---|
| Enzymes add energy, so the reaction releases more. | They lower the activation energy. ΔG and the equilibrium stay exactly the same. |
| As substrate rises, the rate plateaus, so the substrate ran out or the enzyme broke. | Nearly every enzyme molecule is busy. That's saturation — adding enzyme would raise the plateau. |
| If an enzyme binds its substrate normally, it works normally. | Binding and catalysis are separate. A site that can't close around the substrate binds normally but converts it slowly. |
| Any ion with the same charge can stand in for a cofactor. | Cofactors are specific. An enzyme built around Mg²⁺ can fail with Ca²⁺, even though both carry a 2+ charge. |
Worked example
An energy diagram for one reaction shows the reactants at 120 kJ/mol and the products at 95 kJ/mol. The peak of the hump is at 200 kJ/mol without an enzyme and 150 kJ/mol with one. A student says the enzyme makes the reaction release 75 kJ/mol instead of 25. Evaluate the claim.
- Free energy released = reactants − products = 120 − 95 = 25 kJ/mol. The enzyme doesn't move either level.
- Activation energy = peak − reactants. Without the enzyme: 200 − 120 = 80. With it: 150 − 120 = 30.
- The enzyme cut the hump by 50 kJ/mol. The student added that 50 to the 25 released.
The claim is wrong. The reaction releases 25 kJ/mol with or without the enzyme. Only the activation energy drops, from 80 to 30 kJ/mol.
ΔG depends only on where the reaction starts and ends, and an enzyme changes neither. Lowering the hump makes the reaction faster, not more exergonic. The near miss, 75, comes from counting the saved activation energy as extra energy released.