Enzymes
Section: Biology | Syllabus: Cambridge IGCSE Combined Science (0653)
Catalysts and Enzymes
- A catalyst is a substance that increases the rate of a chemical reaction and is not changed by the reaction
- An enzyme is a protein that is involved in all metabolic reactions, where it functions as a biological catalyst
- Enzymes are proteins - their function depends entirely on their precise 3D shape
- Enzymes are not changed by the reaction - they can be reused many times
Why Enzymes are Essential
All living processes - digestion, respiration, photosynthesis, protein synthesis, DNA replication - are chemical reactions. Without enzymes, these reactions would be far too slow to sustain life - body temperature alone is not enough to drive them at the required rate. Each process needs a specific enzyme to proceed fast enough to keep the organism alive.
- Amylase digests starch, in the mouth and small intestine
- Proteases (e.g. pepsin, trypsin) digest proteins, in the stomach and small intestine
- Lipase digests fats, in the small intestine
- Many enzymes (e.g. in glycolysis) drive aerobic respiration, in the cytoplasm and mitochondria
- Many enzymes (e.g. RuBisCO) drive photosynthesis, in the chloroplasts
- Catalase breaks down hydrogen peroxide, in all cells - H₂O₂ is a toxic by-product of respiration
Adding pureed potato (which contains catalase) to hydrogen peroxide produces bubbles of oxygen and a foam. This shows an enzyme breaking down a toxic metabolic by-product (H₂O₂ → H₂O + O₂) rapidly - a reaction that would occur far too slowly without the enzyme.
How Enzymes Work - The Active Site
Each enzyme has a specially shaped region called the active site where the substrate binds and the reaction takes place.
- The shape of the active site is complementary to the shape of the substrate (like a lock and key)
- The substrate binds to the active site, and the reaction occurs: the substrate is converted into products
- Products are released; the enzyme is unchanged and can be reused
The active site of an enzyme has a shape complementary to its substrate, like a lock and key
Common Mistakes
- ❌ Describing the active site and substrate as having the "same shape"
- ✓ the active site and substrate have complementary shapes - they fit together because one is the exact shape that fits the other (like a lock and a key), not because they are identical shapes. Writing "same shape" in an exam will lose marks
Enzyme Action in Detail (Supplement)
When the substrate binds to the active site, the temporary structure formed is the enzyme-substrate complex. After the reaction, products are released and the enzyme is free to bind another substrate molecule.
- Substrate - the molecule that the enzyme acts on
- Enzyme-substrate complex - the temporary combination of enzyme and substrate during the reaction
- Product - the molecule(s) produced; released from the active site when the reaction is complete
Enzyme Specificity (Supplement)
Each enzyme only catalyses one specific reaction, because the active site has a unique 3D shape complementary to only one specific substrate.
- Only a substrate with the complementary shape can bind to the active site
- A molecule with a different shape cannot fit - no enzyme-substrate complex forms, so no reaction is catalysed
Worked Example: Amylase vs Protease
- Question: amylase catalyses the breakdown of starch. Explain why amylase cannot break down proteins.
- Answer: amylase's active site is complementary only to starch. Protein molecules have a different shape and cannot fit into amylase's active site, so no enzyme-substrate complex can form. By the same reasoning, proteases cannot act on starch
Most enzyme names end in -ase and reflect their substrate: amylase (starch/amylose), protease (proteins), lipase (lipids).
Effect of Temperature on Enzyme Activity
Temperature has a characteristic two-phase effect on enzyme activity. There is always an optimum temperature at which activity is greatest.
- As temperature increases towards the optimum, enzyme activity increases
- At the optimum temperature, activity is at its maximum - for most human enzymes, this is around 37°C
- Above the optimum temperature, activity decreases rapidly and eventually stops, because the enzyme becomes denatured
Enzyme activity rises to a peak at the optimum temperature, then falls sharply as the enzyme is denatured
Common Mistakes
- ❌ Saying an enzyme is "killed" by high temperature
- ✓ enzymes are never "killed" - they are denatured. Denaturation is a permanent change in the enzyme's 3D shape that stops the substrate binding, but the enzyme molecule itself still exists
- ❌ Thinking low temperature has the same permanent effect as high temperature
- ✓ at low temperatures, enzymes slow down but are not denatured - warming the enzyme back up restores activity, which is why refrigeration and freezing preserve food. At high temperatures, denaturation is permanent and irreversible - cooling will not restore activity
Explaining the Effect of Temperature (Supplement)
- Denaturation is a permanent change in the 3D shape of an enzyme, caused by high temperature or extreme pH. The active site's shape changes irreversibly, so the substrate can no longer bind - the enzyme still exists, but can no longer function
- Increasing temperature up to the optimum: a higher temperature gives molecules more kinetic energy, so they move faster and collide more often - this increases the frequency of effective collisions between enzyme and substrate, forming more enzyme-substrate complexes per unit time
- At the optimum temperature: the rate of effective collisions is at its maximum, and the active site's shape and fit are perfectly maintained
- Above the optimum: excess heat breaks the bonds holding the enzyme's 3D shape together. The active site's shape and fit change and are no longer complementary to the substrate, so fewer enzyme-substrate complexes can form - the enzyme is denatured
Effect of pH on Enzyme Activity
Each enzyme has an optimum pH. Deviating from this optimum - in either direction - reduces activity, and an extreme pH causes denaturation.
- At the optimum pH, enzyme activity is at its maximum
- Above or below the optimum pH, activity decreases
- At an extreme pH, the enzyme becomes denatured and activity stops completely
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