Mode of action of enzymes
Enzymes: nature's catalysts
- Most reactions in a cell would be far too slow to keep you alive.
- Enzymes speed them up — by millions of times — without being used up.
- They are globular proteins, and their shape is everything.
Which best describes an enzyme?
Enzymes are globular protein catalysts: they speed reactions and are not used up, so each one works repeatedly.
Where enzymes work
- Intracellular enzymes work inside the cell that made them — e.g. catalase, which breaks down harmful hydrogen peroxide.
- Extracellular enzymes are secreted to work outside the cell — e.g. amylase, released into the gut to digest starch.
- Either way, an enzyme is a catalyst: it speeds a reaction and is not used up, so it works again and again.
Amylase secreted into the gut to digest starch is an example of an enzyme that is:
Amylase is secreted to work outside the cell — extracellular. Catalase, by contrast, works inside the cell (intracellular).
The active site and specificity
- Each enzyme has a pocket called the active site. The molecule it acts on is the substrate.
- The substrate binds to form an enzyme–substrate complex; the reaction happens, then the products leave and free the site.
- An enzyme is specific — its active site is complementary to just one substrate's shape, and no other.
An enzyme is specific because:
Specificity comes from the active site's shape being complementary to one substrate and no other.
Lock-and-key vs induced fit
- Lock-and-key: the active site is a fixed shape; only a substrate with the matching shape fits, like a key in a lock.
- Induced fit: the active site is not quite the right shape at first — when the substrate binds, the site moulds around it for a tight grip.
- Induced fit matches the evidence better.
The induced-fit hypothesis differs from lock-and-key because:
In induced fit the active site is not a perfect fit at first; it moulds around the substrate when it binds.
Lowering activation energy
- Every reaction needs a starting "push" of energy — the activation energy.
- An enzyme lowers the activation energy, so the reaction can run fast at the cell's normal temperature instead of needing high heat.
How does an enzyme speed up a reaction?
Enzymes lower the activation energy, so the reaction proceeds quickly at normal cell temperatures.
You've got it
- enzymes = globular protein catalysts; not used up; intracellular (catalase) or extracellular (amylase)
- active site + substrate → enzyme–substrate complex → products leave
- specific: active site is complementary to one substrate (lock-and-key / induced fit)
- enzymes work by lowering the activation energy