Enzymes

Cambridge definition (memorise verbatim):

"An enzyme is a biological catalyst (made of protein) that speeds up a chemical reaction without being used up. Each enzyme is SPECIFIC to one substrate."

Three keywords Cambridge marks:

1. Catalyst — speeds up reactions but isn't consumed.
2. Protein — that's why heat denatures them (proteins lose their 3D shape).
3. Specific — each enzyme works on only one substrate.

Why enzymes matter. Most metabolic reactions in cells would happen FAR too slowly without catalysts. Enzymes lower the activation energy → reactions proceed at body temperature. They make life possible.

Naming conventions. Most enzyme names end in -ase. Substrate name + -ase:

• Amylase: digests AMYLose (starch).
• Maltase: digests MALTOSE.
• Lipase: digests LIPIDS.
• Protease: digests PROTEIN.
• Catalase: catalyses (the breakdown of hydrogen peroxide).

Worked qualitative. Why doesn't an enzyme catalyse multiple different reactions? Because of its specific 3D ACTIVE SITE — only one substrate fits. Different reactions need different enzymes.

The model.

1. The enzyme has a region called the ACTIVE SITE — a precise 3D shape.
2. The SUBSTRATE has a complementary shape that fits the active site (like a key in a lock).
3. The substrate binds to form an enzyme-substrate complex.
4. The reaction takes place (substrate is broken down, joined, or otherwise transformed).
5. The product(s) leave; the enzyme is unchanged and ready for another substrate.

Why this explains specificity. Only ONE substrate has the right shape to fit a particular active site. The enzyme can't catalyse other reactions because the substrates don't fit.

Worked qualitative. Why doesn't amylase digest protein? Amylase's active site is shaped to fit STARCH molecules. Protein has a completely different shape — won't fit the active site → no binding → no reaction.

More modern: induced-fit model. A small refinement: the active site CHANGES SHAPE slightly when the substrate binds, fitting more snugly (like a glove around a hand). Cambridge IGCSE accepts both lock-and-key and induced-fit; lock-and-key is the standard.

Cambridge tip. Always say the substrate "fits" the active site, NOT "fits the enzyme". The active site is the specific binding region.

Sketch the curve. A typical enzyme rate vs temperature graph:

• 0°C: very low rate (particles barely move).
• 0-37°C: rate INCREASES with temperature.
  • Why: more kinetic energy → more frequent and energetic collisions between enzyme and substrate.
• At optimum (~37°C for human enzymes): MAXIMUM rate.
• Above optimum: rate DECREASES SHARPLY.
  • Why: heat breaks the bonds holding the enzyme's 3D shape.
  • The active site CHANGES SHAPE — substrate no longer fits.
  • The enzyme is DENATURED.
• Above ~50°C: most enzymes fully denatured. Very low or zero activity.

Why the curve is asymmetric.

• Below optimum: gradual rise (just KE effect).
• Above optimum: sharp drop (denaturation).
• Cold INACTIVATES (reversible — warm them back up). Heat DENATURES (irreversible).

Worked qualitative. Why does refrigeration preserve food? At ~4°C, enzymes (in food and in microbes) work very slowly → spoilage happens slowly → food lasts longer. Take it out and warm up → enzymes work normally → spoilage resumes.

Cambridge tip. Always mention the BIPHASIC behaviour. Below optimum: kinetic energy explanation. Above: denaturation explanation. Both halves needed for full marks.

Each enzyme has its own optimum pH matching where it works in the body.

Enzyme	Where it works	Optimum pH
Amylase (salivary)	Mouth	~7 (neutral)
Pepsin	Stomach	2 (very acidic)
Pancreatic amylase	Small intestine	~8 (slightly alkaline)
Trypsin	Small intestine	~8
Lipase	Small intestine	~8

Curve. Like the temperature graph but symmetric (or close to it).

• Far from optimum: rate very low.
• At optimum: max.
• Returning to neutral pH from extreme: enzyme may RECOVER (some pH denaturation is reversible, unlike heat).

Why pH affects enzymes.

• Extreme pH disrupts the bonds holding the enzyme's 3D shape (especially ionic bonds).
• Active site changes → substrate no longer fits → reaction stops.

Worked qualitative. Why does the stomach use acid? Two reasons:

• Kills swallowed bacteria.
• Provides the optimum pH (2) for PEPSIN to work.

Worked qualitative. Why does pepsin stop working when food enters the small intestine? The pancreas releases bicarbonate to neutralise the acid. pH rises to ~8. At pH 8, pepsin is denatured (or close to it). Different enzymes (trypsin, etc.) take over, optimised for the alkaline environment.

Cambridge tip. Don't say "enzymes work best at neutral pH". Different enzymes have different optima. Always SPECIFY the enzyme.

1. Biological washing powders / detergents.

• Contain proteases (digest blood, sweat, food protein stains).
• Contain lipases (digest oily / greasy stains).
• Effective at LOW temperatures (~30-40°C) — saving energy compared to hot wash.
• Can't be too hot — would denature.

2. Brewing.

• Yeast enzymes ferment sugars to ethanol and CO₂.
• Used for beer, wine, bread.

3. Food industry.

• Lactase added to milk → digests lactose → lactose-free milk.
• Sucrase / invertase splits sucrose → fructose + glucose for sweets.
• Pectinase clarifies fruit juice.

4. Medical.

• Diabetes test strips use glucose oxidase to detect glucose in urine.
• Some clot-busting drugs are enzymes (streptokinase).

Cambridge tip. When asked "give a use of enzymes outside the body", pick a clear example with a short explanation. "Biological washing powders contain protease that digests blood-stains in clothes" is better than "in clothes washing".