A sugar cube can sit in air for years without reacting with oxygen, yet your cells break down sugar in seconds at 37 °C. The difference is enzymes, the most important job of the proteins you just met.
Key idea
Enzymes are proteins that speed up reactions by lowering the activation energy. The substrate fits the enzyme's active site. The enzyme is not used up: it releases the products unchanged and can work again and again.
Every reaction, even one that releases energy overall, needs a push to get started: the activation energy, the hill the molecules must climb before they can roll downhill. An enzyme gives the reaction an easier path with a lower hill. It does not change where the reaction starts or ends, so the energy released (or absorbed) overall stays the same.
- The substrate binds to the active site, a pocket with just the right shape and chemistry.
- The enzyme changes shape slightly to grip the substrate (induced fit), straining its bonds.
- The reaction happens; the products are released.
- The enzyme, unchanged, binds a new substrate. Many enzymes handle thousands of substrate molecules every second.
Why it works
Enzymes are specific because the active site has a shape that only certain molecules fit, like a key in a lock. Many enzyme names end in -ase and name the substrate: lactase breaks lactose, lipase breaks lipids, protease breaks proteins. An inhibitor can block an enzyme, either by sitting in the active site (competitive) or by binding elsewhere and changing its shape (noncompetitive).
Worked example: Read the energy diagram
Without an enzyme, a reaction's reactants sit at 40 kJ and the peak is at 120 kJ. With the enzyme, the peak is at 70 kJ. The products sit at 10 kJ. Find each activation energy and the energy released.
- Without the enzyme: $120 - 40 = 80$ kJ.
- With the enzyme: $70 - 40 = 30$ kJ.
- Energy released either way: $40 - 10 = 30$ kJ. The enzyme does not change it.
Answer: 80 kJ and 30 kJ; 30 kJ released in both cases
Common mistake
Thinking enzymes are used up in the reaction, or that they supply the energy for it. An enzyme comes out of each reaction exactly as it went in, which is why a tiny amount does so much work. It supplies no energy: it only lowers the energy barrier. Check on a diagram: the starting and ending levels are the same with or without the enzyme; only the peak moves.