Unit 1.4: Enzymes
Part 1: Enzyme Structure and Action
1. What are enzymes?
Key definition
Enzymes are biological catalysts: they increase the rate of a reaction without being used up or
permanently changed.
• Mainly globular proteins with a specific 3D shape. Control metabolic reactions, can
work inside or outside cells, can be reused, and do not become part of the
product.
Type Where it works Example
Intracellular Inside cells Catalase breaks down hydrogen peroxide
inside cells
Extracellular Outside cells Digestive enzymes in the gut; enzymes
secreted by fungi onto food
2. Activation energy
• Activation energy is the initial energy input needed to break existing bonds and
start a reaction.
• Enzymes lower the activation energy, so reactions happen faster and at lower
temperatures.
Watch out! Marks get lost here
• Enzymes do not provide energy for the reaction and do not change the overall energy
released. They only lower the activation energy needed to start it.
, Say it like this in the exam
Enzymes lower the activation energy of a reaction, allowing it to occur more quickly at a lower
temperature.
3. Enzyme structure and the active site
• Enzymes are globular proteins: a polypeptide chain folds into a precise 3D shape,
held by hydrogen bonds, ionic bonds, disulphide bridges and hydrophobic
interactions between R groups.
• The active site is a groove, cleft or pocket formed by a small number of amino
acids, where the substrate binds and the reaction happens.
Examples: lactose is the substrate for lactase; hydrogen peroxide is the substrate for
catalase; starch is a substrate for amylase.
4. Enzyme specificity
• Each active site has a specific shape, so only a substrate with a complementary
shape can bind. This usually means one enzyme catalyses one type of reaction.
Watch out! Marks get lost here
• Do not write 'the substrate is the same shape as the active site'. Write that it is
complementary to the active site: the two shapes fit together, like matching puzzle pieces.
5. Enzyme-substrate complexes
1. The enzyme and substrate move close together and collide successfully.
2. The substrate binds to the active site, forming an enzyme-substrate complex.
3. Temporary interactions convert the substrate into product.
4. The product leaves the active site; the enzyme is unchanged and can be reused.
Enzyme + substrate ⇌ enzyme-substrate complex → enzyme + product
Key definition
An enzyme-substrate complex is the temporary structure formed when a substrate binds to the
active site of an enzyme.
6. Lock-and-key vs induced fit
Feature Lock-and-key (older model) Induced fit (modern, accepted
model)
Part 1: Enzyme Structure and Action
1. What are enzymes?
Key definition
Enzymes are biological catalysts: they increase the rate of a reaction without being used up or
permanently changed.
• Mainly globular proteins with a specific 3D shape. Control metabolic reactions, can
work inside or outside cells, can be reused, and do not become part of the
product.
Type Where it works Example
Intracellular Inside cells Catalase breaks down hydrogen peroxide
inside cells
Extracellular Outside cells Digestive enzymes in the gut; enzymes
secreted by fungi onto food
2. Activation energy
• Activation energy is the initial energy input needed to break existing bonds and
start a reaction.
• Enzymes lower the activation energy, so reactions happen faster and at lower
temperatures.
Watch out! Marks get lost here
• Enzymes do not provide energy for the reaction and do not change the overall energy
released. They only lower the activation energy needed to start it.
, Say it like this in the exam
Enzymes lower the activation energy of a reaction, allowing it to occur more quickly at a lower
temperature.
3. Enzyme structure and the active site
• Enzymes are globular proteins: a polypeptide chain folds into a precise 3D shape,
held by hydrogen bonds, ionic bonds, disulphide bridges and hydrophobic
interactions between R groups.
• The active site is a groove, cleft or pocket formed by a small number of amino
acids, where the substrate binds and the reaction happens.
Examples: lactose is the substrate for lactase; hydrogen peroxide is the substrate for
catalase; starch is a substrate for amylase.
4. Enzyme specificity
• Each active site has a specific shape, so only a substrate with a complementary
shape can bind. This usually means one enzyme catalyses one type of reaction.
Watch out! Marks get lost here
• Do not write 'the substrate is the same shape as the active site'. Write that it is
complementary to the active site: the two shapes fit together, like matching puzzle pieces.
5. Enzyme-substrate complexes
1. The enzyme and substrate move close together and collide successfully.
2. The substrate binds to the active site, forming an enzyme-substrate complex.
3. Temporary interactions convert the substrate into product.
4. The product leaves the active site; the enzyme is unchanged and can be reused.
Enzyme + substrate ⇌ enzyme-substrate complex → enzyme + product
Key definition
An enzyme-substrate complex is the temporary structure formed when a substrate binds to the
active site of an enzyme.
6. Lock-and-key vs induced fit
Feature Lock-and-key (older model) Induced fit (modern, accepted
model)
