This mini-lesson covers Topic 5 — Enzymes: enzymes as biological catalysts, the active site and its complementary shape, enzyme specificity, and how temperature and pH affect enzyme activity — including denaturation.
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Enzymes as catalysts
Enzymes are biological catalysts
A catalyst is a substance that increases the rate of a chemical reaction and is not changed by the reaction. Enzymes are proteins that act as biological catalysts.
Enzymes are involved in all metabolic reactions in living organisms.
They are important because they let reactions happen fast enough at body temperature to sustain life — without them, reactions would be far too slow.
Being catalysts, they are reused many times and are not used up.
Watch out: enzymes are proteins. This is why extreme temperature or pH can wreck them — it damages their protein structure (denaturation, coming up).
Quick check
What is an enzyme?
?Which statement about enzymes is correct?
Active site & specificity
The active site
An enzyme works because the shape of its active site is complementary to its substrate — like a key fitting a lock. When the substrate fits into the active site, the reaction happens and products are released.
The substrate fits the active site; products form; the enzyme is unchanged and reused.
Supplement — specificity: each enzyme is specific to one substrate because only that substrate has the complementary shape to fit its active site, forming an enzyme–substrate complex.
Supplement · Quick check
Why so specific?
?Amylase breaks down starch but has no effect on protein. Why is amylase specific to starch?
Effect of temperature
Temperature & enzyme activity
Enzyme activity depends strongly on temperature:
As temperature rises towards the optimum, activity increases — particles have more kinetic energy, so enzyme and substrate collide more often and more successfully.
Above the optimum, activity falls sharply: high temperature changes the shape of the active site so the substrate no longer fits. The enzyme is denatured — this is permanent.
Activity peaks at the optimum, then falls fast as the enzyme denatures.
Supplement: explain the rise in terms of kinetic energy and more frequent effective collisions; explain the fall in terms of a change in the shape and fit of the active site (denaturation).
Quick check
Too hot to work
?An enzyme is heated well above its optimum temperature and stops working, even after cooling down. What has happened?
Effect of pH
pH & enzyme activity
Each enzyme also has an optimum pH at which it works fastest. Moving away from the optimum slows the enzyme; a large change can denature it by changing the active site's shape.
Different enzymes have different optimum pH values. For example, pepsin in the stomach works best in acidic conditions (about pH 2), while enzymes in the small intestine prefer slightly alkaline conditions.
Activity peaks at the optimum pH and drops on either side.
Supplement: explain the effect of pH in terms of the shape and fit of the active site and denaturation when pH is far from the optimum.
Sort it
What happens to the enzyme?
Tap a condition, then tap the effect it has on enzyme activity.
⬆ Speeds up
⬇ Slows down
💥 Denatures
Calculate
Your turn — rate of reaction
1An enzyme breaks down 24 mg of substrate in 3 minutes at its optimum temperature. Calculate the average rate of reaction in mg per minute.
mg/min
Hint: rate = amount ÷ time = 24 ÷ 3.
Match it
Match the term to its meaning
Tap a term on the left, then its meaning on the right.
Term
Meaning
Recap
The big ideas to know
Enzyme: a protein that acts as a biological catalyst; speeds reactions, not used up
Active site: shape is complementary to the substrate → products form (lock-and-key idea)
Specificity (Supp): only a complementary substrate fits, forming an enzyme–substrate complex
Temperature: rises to an optimum (more kinetic energy, more effective collisions), then denatures
pH: each enzyme has an optimum pH; far from it → active site changes shape → denatured
Denaturation: permanent change to the active site's shape
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