Enzymes
Applied Science: BTEC Level 3 National Extended Diploma, topic 3.3.2
Enzymes
Enzymes are proteins that catalyse reactions. The chemical reactants that enzymes bind to are called substrates.
Biological catalyst
- A substance that speeds up a chemical reaction without being used up itself is a catalyst.
- Enzymes are proteins that catalyse biochemical reactions.
- Enzymes can act inside or outside of cells.
Activation energy
- For a chemical reaction to start, it needs a specific amount of energy. This amount of energy is different for different reactions.
- This is called the activation energy.
Activation energy and enzymes
- Enzymes lower the activation energies of chemical reactions inside the cell to increase the rate of reactions.
- Enzymes lower the activation energy by binding to the reactant molecules (substrate) and allowing chemical bond-breaking and bond-forming processes to happen more easily.
The active site
- Enzymes catalyse specific reactions.
- The active site has a specific shape for each enzyme.
- Substrates with a complementary shape to the active site of an enzyme can bind to form an enzyme-substrate complex.
- The shape of the active site is determined by the tertiary structure of the polypeptide.
Models of Enzyme Action
Models of enzyme action have changed over time. For many years, it was thought that enzymes worked in a lock and key manner. The induced fit model is now more widely accepted.
The lock and key model
- The lock and key model was originally used to explain enzyme action.
- The model proposes that the enzyme and substrate fit together perfectly.
- The substrate is a key fitting into a lock (the enzyme).
The induced fit model
- The induced fit model suggests there is a more dynamic interaction between enzyme and substrate.
- The model states that as an enzyme and substrate come together, their interaction causes a small shift in the enzyme’s structure.
- The shift means that the enzyme and substrate can bind to form an enzyme-substrate complex and catalyse a reaction.
- This model is now more widely accepted.
Enzyme-Substrate Complexes
Enzymes bind with substrates to form an enzyme-substrate complex in a specific fashion. The specificity of enzymes is determined by two things:
Active site
- Every enzyme only catalyses one specific reaction.
- Every enzyme has a specific active site that is complementary to the specific substrate.
- This jigsaw puzzle-like match between an enzyme and its substrates is what makes enzymes highly specific.
,w_800.png)
Environmental factors
- The 3D, tertiary structure of the polypetide chain determines the shape of the active site.
- Environmental changes can change the tertiary structure of the active site and can stop the enzyme from working properly.
- This is called a denatured enzyme.
Test yourself on Enzymes
What is the name of the molecules that speed up biochemical reactions?
Which of the following statements regarding enzymes is true?
What is the name of the model of enzyme action shown below?
Which of the following statements about models of enzyme action are true?
What is the name given to enzymes that have experienced a change in their active site due to environmental factors?