Enzyme Mechanism and Metabolism

What are Enzymes?

Without special molecules to speed things up, the chemical reactions keeping you alive would happen far too slowly to sustain life. These crucial molecules are called enzymes, which function as biological catalysts.

All enzymes are made of protein (specifically, they are globular proteins folded into precise 3D shapes).
They speed up the rate of chemical reactions in living organisms without being changed or used up themselves.
Because they are not consumed during the reaction, a single enzyme molecule can be reused thousands of times per second.

The Mechanism of Enzyme Action

For any chemical reaction to start, a certain amount of energy is needed, known as the activation energy. Enzymes speed up reactions by lowering this activation energy, providing an alternative reaction pathway that requires less energy.

According to collision theory, reactant molecules must collide with sufficient energy and in the correct orientation for a reaction to occur. Enzymes facilitate this by providing a specific environment for the reaction:

Random Collision: Substrate and enzyme molecules move randomly due to kinetic energy. A successful collision occurs when a substrate molecule hits the enzyme's active site.
Enzyme-Substrate Complex (ESC): The substrate binds to the active site, forming a temporary enzyme-substrate complex.
Reaction: The enzyme holds the substrate in a way that allows bonds to be broken or formed more easily.
Product Release: The substrate is converted into products, which are released from the active site. The enzyme remains unchanged and is ready to catalyse another reaction.

This can be summarised as: $Enzyme + Substrate \rightarrow Enzyme\text{-}Substrate\ Complex \rightarrow Enzyme + Product$

The Active Site and Specificity

Every enzyme has a highly specific active site. This is a small pocket or groove on the surface of the enzyme with a unique 3D shape.

The lock and key hypothesis is a model used to explain enzyme specificity:

The enzyme is like a 'lock' and the substrate is the 'key'.
Only a substrate with a complementary shape can fit into the active site.
If the shapes are not complementary, no ESC will form and no reaction will occur. This is why enzymes are highly specific, usually only catalysing one particular reaction.
If conditions change drastically (for example, temperatures or pH values far from the optimum), the shape of the active site changes permanently — a process called denaturation. A denatured enzyme can no longer bind its substrate and so loses its catalytic function.

Enzymes in Metabolism

Metabolism is the sum of all the chemical reactions happening in a cell or the body. Enzymes control these reactions, which can be divided into two types:

Anabolism (Building up): Enzymes join smaller molecules together to form larger, more complex ones.
- Example: Joining glucose molecules to form starch (in plants) or glycogen (in animals).
- Example: Combining fatty acids and glycerol to make lipids.
Catabolism (Breaking down): Enzymes break down large molecules into smaller ones.
- Example: Digestion, such as amylase breaking down starch into maltose.
- Example: Deamination in the liver, where excess amino acids are broken down to form urea.
- Example: Cellular respiration, where glucose is broken down to release energy.

Intracellular and Extracellular Enzymes

Enzymes are also classified by where they perform their work:

Intracellular enzymes: These work inside the cell where they are made. For example, DNA polymerase is an enzyme that works inside the nucleus to catalyse the synthesis of new DNA strands.
Extracellular enzymes: These are produced inside cells but are secreted to work outside of them. For example, digestive enzymes like amylase are made in the salivary glands but work in the mouth to break down starch.

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Exam Tips

Common Mistake
Students often state that extreme heat 'kills' enzymes. Enzymes are molecules, not living cells, so they cannot die. You must state that the enzyme undergoes denaturation and the 'shape of the active site has changed'.
2
In any exam question about enzyme specificity, you MUST use the exact phrase 'complementary shape'. OCR examiners will explicitly penalise answers that describe the active site and substrate as being the 'same shape' or simply saying they 'fit together'.
3
When asked to describe or explain the mechanism of enzyme action for 3-4 marks, always explicitly state that an 'enzyme-substrate complex' forms. This is almost universally a standalone marking point on the mark scheme.
4
If an exam question asks you to link liver function to metabolism, be sure to specifically mention its role in the catabolic breakdown of excess amino acids to form urea.

Key Terms(16)

Enzyme: A biological catalyst made of protein that speeds up chemical reactions in living organisms without being changed or used up.
Biological catalyst: A substance found in living organisms that increases the rate of a chemical reaction without being consumed.
Activation energy: The minimum amount of energy required for a chemical reaction to successfully occur.
Substrate: The specific molecule or molecules upon which an enzyme acts and converts into products.
Active site: The specific 3D region on an enzyme's surface where the substrate binds and the reaction takes place.
Complementary: Describes two shapes that fit together perfectly, such as the relationship between a specific substrate and an enzyme's active site.
Enzyme-substrate complex: A temporary intermediate molecule formed when a substrate successfully binds to the active site of its specific enzyme.
Lock and key hypothesis: A model of enzyme action explaining that enzymes are specific because only one precisely shaped substrate (the key) fits into the active site (the lock).
Collision theory: The principle that chemical reactions only occur when reactant particles physically collide with sufficient kinetic energy and in the correct orientation.
Denaturation: An irreversible change in the 3D shape of an enzyme's active site, caused by extreme temperatures or pH, which prevents the substrate from binding.
Metabolism: The sum of all the biochemical reactions occurring within a cell or a living organism.
Anabolism: Metabolic reactions that require energy to synthesize complex molecules from simpler, smaller ones.
Catabolism: Metabolic reactions that release energy by breaking down large, complex molecules into simpler ones.
Product: A molecule produced at the end of an enzyme-catalysed reaction.
Intracellular enzymes: Enzymes that function inside the cells that produce them, such as those involved in DNA replication.
Extracellular enzymes: Enzymes that are secreted from the cell where they are made to work outside the cell, such as digestive enzymes.

Previous Subtopic: Enzymatic reaction experimentsEnzymatic reaction experiments Next NoteFactors Affecting the Rate of Enzyme Reactions

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Key Terms(16)

Enzyme: A biological catalyst made of protein that speeds up chemical reactions in living organisms without being changed or used up.
Biological catalyst: A substance found in living organisms that increases the rate of a chemical reaction without being consumed.
Activation energy: The minimum amount of energy required for a chemical reaction to successfully occur.
Substrate: The specific molecule or molecules upon which an enzyme acts and converts into products.
Active site: The specific 3D region on an enzyme's surface where the substrate binds and the reaction takes place.
Complementary: Describes two shapes that fit together perfectly, such as the relationship between a specific substrate and an enzyme's active site.
Enzyme-substrate complex: A temporary intermediate molecule formed when a substrate successfully binds to the active site of its specific enzyme.
Lock and key hypothesis: A model of enzyme action explaining that enzymes are specific because only one precisely shaped substrate (the key) fits into the active site (the lock).
Collision theory: The principle that chemical reactions only occur when reactant particles physically collide with sufficient kinetic energy and in the correct orientation.
Denaturation: An irreversible change in the 3D shape of an enzyme's active site, caused by extreme temperatures or pH, which prevents the substrate from binding.
Metabolism: The sum of all the biochemical reactions occurring within a cell or a living organism.
Anabolism: Metabolic reactions that require energy to synthesize complex molecules from simpler, smaller ones.
Catabolism: Metabolic reactions that release energy by breaking down large, complex molecules into simpler ones.
Product: A molecule produced at the end of an enzyme-catalysed reaction.
Intracellular enzymes: Enzymes that function inside the cells that produce them, such as those involved in DNA replication.
Extracellular enzymes: Enzymes that are secreted from the cell where they are made to work outside the cell, such as digestive enzymes.

Enzyme Mechanism and Metabolism

What are Enzymes?

All enzymes are made of protein (specifically, they are globular proteins folded into precise 3D shapes).
They speed up the rate of chemical reactions in living organisms without being changed or used up themselves.
Because they are not consumed during the reaction, a single enzyme molecule can be reused thousands of times per second.

The Mechanism of Enzyme Action

Random Collision: Substrate and enzyme molecules move randomly due to kinetic energy. A successful collision occurs when a substrate molecule hits the enzyme's active site.
Enzyme-Substrate Complex (ESC): The substrate binds to the active site, forming a temporary enzyme-substrate complex.
Reaction: The enzyme holds the substrate in a way that allows bonds to be broken or formed more easily.
Product Release: The substrate is converted into products, which are released from the active site. The enzyme remains unchanged and is ready to catalyse another reaction.

This can be summarised as: $Enzyme + Substrate \rightarrow Enzyme\text{-}Substrate\ Complex \rightarrow Enzyme + Product$

The Active Site and Specificity

Every enzyme has a highly specific active site. This is a small pocket or groove on the surface of the enzyme with a unique 3D shape.

The lock and key hypothesis is a model used to explain enzyme specificity:

The enzyme is like a 'lock' and the substrate is the 'key'.
Only a substrate with a complementary shape can fit into the active site.
If the shapes are not complementary, no ESC will form and no reaction will occur. This is why enzymes are highly specific, usually only catalysing one particular reaction.
If conditions change drastically (for example, temperatures or pH values far from the optimum), the shape of the active site changes permanently — a process called denaturation. A denatured enzyme can no longer bind its substrate and so loses its catalytic function.

Enzymes in Metabolism

Metabolism is the sum of all the chemical reactions happening in a cell or the body. Enzymes control these reactions, which can be divided into two types:

Anabolism (Building up): Enzymes join smaller molecules together to form larger, more complex ones.
- Example: Joining glucose molecules to form starch (in plants) or glycogen (in animals).
- Example: Combining fatty acids and glycerol to make lipids.
Catabolism (Breaking down): Enzymes break down large molecules into smaller ones.
- Example: Digestion, such as amylase breaking down starch into maltose.
- Example: Deamination in the liver, where excess amino acids are broken down to form urea.
- Example: Cellular respiration, where glucose is broken down to release energy.

Intracellular and Extracellular Enzymes

Enzymes are also classified by where they perform their work:

Intracellular enzymes: These work inside the cell where they are made. For example, DNA polymerase is an enzyme that works inside the nucleus to catalyse the synthesis of new DNA strands.
Extracellular enzymes: These are produced inside cells but are secreted to work outside of them. For example, digestive enzymes like amylase are made in the salivary glands but work in the mouth to break down starch.

Sign up to continue reading

Get full access to revision notes, key terms, and exam tips for every subtopic.

Exam Tips

Common Mistake
Students often state that extreme heat 'kills' enzymes. Enzymes are molecules, not living cells, so they cannot die. You must state that the enzyme undergoes denaturation and the 'shape of the active site has changed'.
2
In any exam question about enzyme specificity, you MUST use the exact phrase 'complementary shape'. OCR examiners will explicitly penalise answers that describe the active site and substrate as being the 'same shape' or simply saying they 'fit together'.
3
When asked to describe or explain the mechanism of enzyme action for 3-4 marks, always explicitly state that an 'enzyme-substrate complex' forms. This is almost universally a standalone marking point on the mark scheme.
4
If an exam question asks you to link liver function to metabolism, be sure to specifically mention its role in the catabolic breakdown of excess amino acids to form urea.

Key Terms(16)

Enzyme: A biological catalyst made of protein that speeds up chemical reactions in living organisms without being changed or used up.
Biological catalyst: A substance found in living organisms that increases the rate of a chemical reaction without being consumed.
Activation energy: The minimum amount of energy required for a chemical reaction to successfully occur.
Substrate: The specific molecule or molecules upon which an enzyme acts and converts into products.
Active site: The specific 3D region on an enzyme's surface where the substrate binds and the reaction takes place.
Complementary: Describes two shapes that fit together perfectly, such as the relationship between a specific substrate and an enzyme's active site.
Enzyme-substrate complex: A temporary intermediate molecule formed when a substrate successfully binds to the active site of its specific enzyme.
Lock and key hypothesis: A model of enzyme action explaining that enzymes are specific because only one precisely shaped substrate (the key) fits into the active site (the lock).
Collision theory: The principle that chemical reactions only occur when reactant particles physically collide with sufficient kinetic energy and in the correct orientation.
Denaturation: An irreversible change in the 3D shape of an enzyme's active site, caused by extreme temperatures or pH, which prevents the substrate from binding.
Metabolism: The sum of all the biochemical reactions occurring within a cell or a living organism.
Anabolism: Metabolic reactions that require energy to synthesize complex molecules from simpler, smaller ones.
Catabolism: Metabolic reactions that release energy by breaking down large, complex molecules into simpler ones.
Product: A molecule produced at the end of an enzyme-catalysed reaction.
Intracellular enzymes: Enzymes that function inside the cells that produce them, such as those involved in DNA replication.
Extracellular enzymes: Enzymes that are secreted from the cell where they are made to work outside the cell, such as digestive enzymes.

Previous Subtopic: Enzymatic reaction experimentsEnzymatic reaction experiments Next NoteFactors Affecting the Rate of Enzyme Reactions

Back to Enzyme action

Put your knowledge into practice — try past paper questions for Biology A

Key Terms(16)

Enzyme: A biological catalyst made of protein that speeds up chemical reactions in living organisms without being changed or used up.
Biological catalyst: A substance found in living organisms that increases the rate of a chemical reaction without being consumed.
Activation energy: The minimum amount of energy required for a chemical reaction to successfully occur.
Substrate: The specific molecule or molecules upon which an enzyme acts and converts into products.
Active site: The specific 3D region on an enzyme's surface where the substrate binds and the reaction takes place.
Complementary: Describes two shapes that fit together perfectly, such as the relationship between a specific substrate and an enzyme's active site.
Enzyme-substrate complex: A temporary intermediate molecule formed when a substrate successfully binds to the active site of its specific enzyme.
Lock and key hypothesis: A model of enzyme action explaining that enzymes are specific because only one precisely shaped substrate (the key) fits into the active site (the lock).
Collision theory: The principle that chemical reactions only occur when reactant particles physically collide with sufficient kinetic energy and in the correct orientation.
Denaturation: An irreversible change in the 3D shape of an enzyme's active site, caused by extreme temperatures or pH, which prevents the substrate from binding.
Metabolism: The sum of all the biochemical reactions occurring within a cell or a living organism.
Anabolism: Metabolic reactions that require energy to synthesize complex molecules from simpler, smaller ones.
Catabolism: Metabolic reactions that release energy by breaking down large, complex molecules into simpler ones.
Product: A molecule produced at the end of an enzyme-catalysed reaction.
Intracellular enzymes: Enzymes that function inside the cells that produce them, such as those involved in DNA replication.
Extracellular enzymes: Enzymes that are secreted from the cell where they are made to work outside the cell, such as digestive enzymes.