Enzymes

Watch first

CognitoEnzymes — structure, lock and key, denaturation

Key facts & methods

Enzyme structure and function

Enzymes are biological catalysts — proteins that speed up chemical reactions without being used up.
Each enzyme has an active site — a specific shape that only one substrate can fit into.
Lock and key model: the substrate is the "key" that fits exactly into the enzyme "lock".
At the active site, the substrate is converted into products which are released.
Enzyme specificity: each enzyme catalyses only one reaction (its substrate has a complementary shape to the active site).

Effect of temperature

As temperature increases: more kinetic energy → molecules collide more frequently → rate increases.
At optimum temperature: maximum rate of reaction.
Above optimum: heat disrupts bonds in the enzyme → active site changes shape → enzyme is denatured.
Denaturation is permanent — the enzyme cannot recover.
Human enzyme optimum: ~37°C. Above ~45°C, most enzymes denature.

Effect of pH

Each enzyme has an optimum pH at which it works fastest.
Pepsin (stomach): optimum pH 2 (acidic). Amylase (mouth): optimum pH 7 (neutral).
Too high or too low pH changes the ionic bonds in the active site → active site changes shape → enzyme denatured.
pH affects the charge on the active site amino acids, altering its shape.

Enzymes in digestion

Amylase: carbohydrase. Breaks starch → maltose/glucose. Produced in salivary glands, pancreas.
Protease: breaks proteins → amino acids. Produced in stomach (pepsin), pancreas, small intestine.
Lipase: breaks lipids (fats) → fatty acids + glycerol. Produced in pancreas, small intestine.
Bile: emulsifies fats (breaks into small droplets), increasing surface area for lipase.
Products are absorbed into the bloodstream through the small intestine wall.

Exam questions — 5 questions · 11 marks · AQA 8461 style

1 markEnzyme definitionAQA 8461 style

Which of the following best describes an enzyme?

A A substrate that speeds up chemical reactions

B A biological catalyst made of protein that speeds up reactions without being used up

C A molecule that is permanently changed by chemical reactions

D A protein that slows down harmful chemical reactions

3 marksDenaturation explanationAQA 8461 P1 style

Explain what happens to an enzyme when the temperature rises significantly above its optimum temperature. (3 marks)

Hint: Sequence: bonds broken → active site shape changes → substrate cannot bind → enzyme denatured (permanent).

+30 XP

1 markpH and enzymeAQA 8461 style

Why does pepsin (stomach enzyme) work best at pH 2 but not at pH 7?

A At pH 7, pepsin dissolves and cannot function

B At pH 7, the ionic bonds in pepsin's active site are changed, altering its shape so substrate cannot bind

C At pH 7, the substrate molecule changes shape and no longer fits the active site

D At pH 7, pepsin becomes too active and breaks down

4 marksDigestion enzymesAQA 8461 style

A student eats a cheese sandwich. Name the enzymes involved in digesting the carbohydrates, proteins and lipids in the sandwich, and state the products of each reaction. (4 marks)

Hint: Three food components (starch, protein, fat) → three enzymes → three sets of products. Add bile as a bonus point.

+40 XP

1 markLock and keyAQA 8461 style

Why can amylase digest starch but not proteins?

A Amylase is too small to bind to proteins

B Proteins are found in different parts of the body from amylase

C The active site of amylase is complementary in shape only to starch molecules, not protein molecules

D Proteins denature amylase before it can act on them

Enzyme structure and function

Effect of temperature

Effect of pH

Enzymes in digestion

Module complete! 🎉