Enzymes and how they work
Enzymes are biological catalysts, almost all globular proteins. Each is highly specific and works by lowering the activation energy of a reaction.
What enzymes are
Life depends on thousands of reactions that must run quickly at body temperature. They are catalysed by enzymes. Almost all enzymes are globular proteins. Each enzyme is very specific: it catalyses one reaction of one substrate (the molecule it acts on). Enzymes are usually named after the substrate or the reaction, with the ending -ase. For example, maltase catalyses the hydrolysis of maltose. Enzymes that catalyse the oxidation of one substrate together with the reduction of another are called oxidoreductases.
Mechanism of enzyme action: what your textbook says
Enzymes are needed only in small amounts. Like other catalysts, they work by lowering the activation energy of a reaction, so many more molecules can react at body temperature. Your textbook's example is the hydrolysis of sucrose. With acid as the catalyst, the activation energy is 6.22 kJ mol⁻¹. With the enzyme sucrase, it is only 2.15 kJ mol⁻¹.
Extra, beyond your textbook: the lock and key model
Your textbook does not describe this model, so it is not needed for the board exam. It only helps you picture how an enzyme works.
- 1E + S ⇌ ESThe substrate (S) fits into the active site of the enzyme (E), like a key into a lock, forming an enzyme-substrate complex.
- 2ES → EPInside the complex, bonds in the substrate are broken or formed by a path with a lower activation energy.
- 3★EP → E + PThe product (P) leaves. The enzyme is unchanged and free to act again.
- 4Because each enzyme molecule is used again and again, enzymes are needed only in small amounts.
★ key step
Extra: the enzyme cycle
- back to Free enzyme (E)
Some enzyme-catalysed reactions (maltase and invertase, also called sucrase, are in your textbook; the rest are extra)
| Enzyme | Reaction |
|---|---|
| Maltase | C₁₂H₂₂O₁₁ (maltose) + H₂O → 2C₆H₁₂O₆ (glucose) |
| Invertase (sucrase) | C₁₂H₂₂O₁₁ (sucrose) + H₂O → C₆H₁₂O₆ (glucose) + C₆H₁₂O₆ (fructose) |
| Diastase | 2(C₆H₁₀O₅)n (starch) + nH₂O → nC₁₂H₂₂O₁₁ (maltose) |
| Zymase (from yeast) | C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ |
| Urease | NH₂CONH₂ + H₂O → 2NH₃ + CO₂ |
| Pepsin | Proteins → peptides (in the stomach) |
Extra, beyond your textbook: more features of enzymes
- Highly efficient: one enzyme molecule converts a very large number of substrate molecules.
- Work best at an optimum temperature, close to body temperature (about 310 K). Strong heating denatures them.
- Work best at an optimum pH: pepsin in the acid of the stomach, trypsin in the alkaline small intestine.
- Activity is raised by activators and coenzymes, and lowered by inhibitors.
Saliva contains the enzyme amylase, which starts breaking starch down into maltose. Chew a piece of chapati or plain rice for a minute and it begins to taste sweet.
2 marks: “What are enzymes? How do they speed up a reaction? Give the example your textbook uses.” Biocatalysts, almost all globular proteins, each specific to one reaction and one substrate. They lower the activation energy: sucrose hydrolysis needs 6.22 kJ mol⁻¹ with acid but only 2.15 kJ mol⁻¹ with sucrase.
1 marks: “Name the enzyme that hydrolyses maltose, and the product formed.” Maltase; it gives two molecules of glucose from each maltose molecule.
2 marks: “How are enzymes named? Give one example of each way.” After the substrate, with the ending -ase: maltase acts on maltose. After the reaction: oxidoreductases catalyse the oxidation of one substrate with the reduction of another.
Enzymes: globular proteins, very specific, names end in -ase, and they lower the activation energy. Sucrose hydrolysis: 6.22 kJ mol⁻¹ with acid, only 2.15 kJ mol⁻¹ with sucrase.