What are enzymes?
Thousands of chemical reactions happen in your cells every second. All of them together are called metabolism. Most would be far too slow at body temperature without help.
Enzymes are the helpers. They are biological catalysts. A catalyst is a substance that speeds up a reaction but is not used up.
- Almost all enzymes are proteins (long folded chains of amino acids).
- The substance an enzyme works on is its substrate. What comes out are the products.
- Many enzyme names end in -ase: amylase breaks amylose/starch, protease breaks protein, lipase breaks lipids (fats).
- Some enzymes break big molecules (digestion); others build big molecules (making proteins, DNA, starch).
- Some need a helper called a cofactor (a metal ion) or coenzyme (often made from a vitamin).
Active site, specificity and how enzymes work
The active site is a small pocket on the enzyme. Its shape matches the substrate.
- The substrate enters the active site.
- They form an enzyme-substrate complex.
- The reaction happens and the products leave.
- The enzyme is free and unchanged, ready for the next substrate.
Lock and key model
The active site is like a lock and the substrate is the key: only one shape fits. This explains specificity — each enzyme works on one substrate (or a small group).
Induced fit model
A newer, more exact idea: the active site is a little flexible. When the substrate enters, the site moulds itself tightly around it, like a glove around a hand. This squeezes bonds and helps them break or form.
Lowering activation energy
Every reaction needs some energy to get started, the activation energy. Enzymes give the reaction an easier path with a lower energy hill, so many more molecules react each second. Enzymes do not change the products or the overall energy change.
Factors that affect enzyme activity
Temperature
As it gets warmer, molecules move faster and meet more often, so the rate rises. Each enzyme has an optimum temperature (about 37 °C for human enzymes). Above it, the enzyme vibrates so much that its shape breaks: it is denatured and the rate drops fast. Denaturing is usually permanent. At low temperature enzymes are only slowed, not destroyed; that is why fridges keep food fresh.
pH
Each enzyme has an optimum pH. Pepsin in the stomach works best at about pH 2 (acid); salivary amylase at about pH 7; trypsin in the small intestine at about pH 8. A pH far from the optimum changes the charges in the active site and denatures it.
Substrate concentration
More substrate means more collisions, so the rate rises. But once every active site is busy all the time, the rate stops rising: the enzymes are saturated. Then only more enzyme can raise the rate.
Enzyme concentration and inhibitors
More enzyme gives a faster rate (if there is plenty of substrate). Inhibitors slow enzymes: some block the active site (competitive), others bind elsewhere and change the shape (non-competitive). Many medicines and poisons work this way.
Measuring enzyme activity (assays)
To measure how fast an enzyme works, we follow how fast the substrate disappears or the product appears.
- Amylase and starch: mix amylase with starch at a set temperature. Every 30 seconds put a drop on a tile with iodine. Iodine turns blue-black while starch is left and stays orange-brown when starch is gone. The shorter the time, the faster the enzyme.
- Catalase and hydrogen peroxide: catalase (found in potato and liver) breaks hydrogen peroxide into water and oxygen. Count bubbles or measure the oxygen volume in a syringe each minute.
- Protease and milk or gelatin: the cloudy milk protein clears as it is digested.
Rate = amount changed ÷ time. For a fair test, change only one factor (for example temperature) and keep volume, concentration and pH the same.
Enzymes in life and industry
- Digestion: amylase (mouth, pancreas), pepsin (stomach), trypsin and lipase (small intestine) break food into small molecules the blood can absorb.
- Washing powders: proteases and lipases remove stains at 30–40 °C, saving energy.
- Food: pectinase gets more juice from fruit; lactase makes lactose-free milk; rennet (chymosin) makes cheese; amylase helps bread and syrups.
- Medicine and tests: glucose oxidase in blood-sugar test strips for diabetes.
- Biofuels and textiles: cellulases break plant fibres; enzymes soften denim instead of harsh chemicals.
Industry likes enzymes because they work at mild temperatures and pH and are very specific, so there is less waste. The drawbacks: they can be denatured and can be costly, so they are often fixed onto beads (immobilised) and reused.
Try it: the pineapple and jelly test
Fresh pineapple contains bromelain, a protease. Gelatin is a protein. Put fresh and boiled pineapple on set jelly. Predict which one turns the jelly runny, then test. Explain your result using the words active site and denatured.
Key formulas and definitions
- Enzyme + Substrate ⇌ Enzyme–Substrate complex → Enzyme + Products
- Rate of reaction = amount of product formed (or substrate used) ÷ time
- Starch --amylase--> maltose; Protein --protease--> amino acids; Fat --lipase--> fatty acids + glycerol
- 2 H₂O₂ --catalase--> 2 H₂O + O₂
- Key terms: active site, specificity, optimum, denaturation, activation energy, inhibitor, cofactor
Worked examples
1. Why does chewing bread for a long time make it taste sweet?
Saliva contains amylase. Amylase breaks starch (not sweet) into maltose, a sugar (sweet).
2. In a catalase test, 30 cm³ of oxygen was collected in 60 s. Find the rate.
Rate = 30 ÷ 60 = 0.5 cm³ of oxygen per second.
3. Amylase and starch were mixed at 20 °C, 40 °C and 70 °C. The starch disappeared (iodine stayed orange) after 6 min, 2 min and never. Explain.
At 40 °C, near the optimum, molecules collide often and the enzyme is intact: fastest. At 20 °C there are fewer collisions: slower. At 70 °C the enzyme is denatured, the active site has lost its shape, so starch is never broken.
4. Pepsin works in the stomach but stops working when food moves into the small intestine. Why?
Pepsin's optimum pH is about 2. The small intestine is slightly alkaline (about pH 8), which changes the active site and stops pepsin.
5. Adding more substrate first raises the rate, then the rate stays flat. Why?
At first more substrate means more collisions. Later all active sites are busy (saturated), so the rate is limited by the amount of enzyme.
Common mistakes
- Saying enzymes are "killed" by heat. Enzymes are not alive; they are denatured (their shape changes).
- Saying cold denatures enzymes. Cold only slows them; they work again when warmed.
- Thinking the enzyme is used up in the reaction. It leaves unchanged and is reused many times.
- Saying the enzyme and substrate have the "same shape". Their shapes are complementary: they fit together, like a key and its lock.