3.1.2 The digestive system provides an interface with the environment. Digestion involves enzymic hydrolysis producing smaller molecules that can be absorbed and assimilated.

Enzyme action

• Enzymes as catalysts lowering activation energy through the formation of enzyme-substrate complexes.
• The lock and key and induced fit models of enzyme action.

Enzyme properties

• The properties of enzymes relating to their tertiary structure. Description and explanation of the effects of temperature, competitive and non-competitive inhibitors, pH and substrate concentration.
• Candidates should be able to use the lock and key model to explain the properties of enzymes. They should also recognise its limitations and be able to explain why the induced fit model provides a better explanation of specific enzyme properties.

Enzymes are biological catalysts that serve to speed up metabolic reactions.

• Catalysts: molecules that speed up a reaction but remain unchanged at the end of it, and can thus be ‘reused’ and catalyse thousands of reactions over and over again 
• Enzymes are globular proteins 
• The breakdown of hydrogen peroxide by catalase enzyme is an example of an enzyme-catalysed reaction
  • Substrate: molecules that are made to react by the enzyme 
  • Products: the result of the reaction 

How do enzymes catalyse reactions?

• For a reaction to proceed, the reactant particles need to have enough energy - this minimum level of energy is known as the activation energy 
• Some particles will not have enough energy and thus will not react, making the reaction slower than it could potentially be 
• Enzymes speed up reactions by offering an alternative reaction pathway with lower activation energy. This means that more particles have enough energy to react. 
• This means that many enzyme-catalysed reactions can occur quickly at relatively low temperatures (e.g. 37ºC in the human body) 

Enzyme structure

• All enzymes have an active site: the place on the enzyme that the substrate binds to, in order for the reaction to be catalysed 
• This active site is complementary in shape to the intended substrate of the enzyme, making the enzyme specific to its substrates.
  • The exact shape of the active site is determined by the folding and shape of the enzyme. 
  • Enzymes are proteins, so this means that the active site is determined by the tertiary structure of the protein.
  • The tertiary structure of the protein is determined by the order of amino acids (primary structure), which is determined by DNA.🧬 

• The substrate can bind temporarily to the active site by forming bonds between the substrate and the active site, allowing time for the reaction to occur. 
• When the substrate and the enzyme are bound together, they form what is known as an enzyme-substrate complex. 
• Once the reaction is complete, the products are released from the enzyme and the enzyme is free to repeat the process again 

The process of enzyme action
In GCE AQA Biology, you need to understand two models of enzyme action:

1. Lock and key model 🔐
2. Induced fit model 

1. Lock and key model

• In the lock and key model, the active site is assumed to be complementary in shape to only 1 substrate, and the substrate fits exactly into the active site 
  • The substrate is the lock, and the active site of the enzyme the key 🔑
  • This accounts for the specific reaction that each enzyme catalyses
• Limitation
  • This model assumes that the “key” (the enzyme) is rigid and unchanging, making it unable to accommodate other substrates 
  • However, this rigidity means that scientists would not be able to explain how certain enzymes have been seen to change the shape of their active site slightly when other molecules bound to the enzyme at places away from the active site 
  • A more flexible model, where the enzyme can change its shape and thus widen the range of substrates it can fit, is needed 

2. Induced-fit model

• The induced-fit model allows for flexibility. It states that the enzyme can change its active site shape when there is an environmental change around it 
  • For example, when the substrate approaches the enzyme, the closeness of it causes the enzyme to change the shape of its active site slightly 
  • This enables the enzyme to better fit the substrate, gripping the substrates more tightly 
  • This strains and weakens the bonds in the substrate molecule, lowering the energy needed to break it, and thus making the reaction more likely to happen 
• The induced fit model is better able to account for the wide range of substrates and reactions that a single enzyme can catalyse 

Factors affecting enzyme activity
You should be able to describe and explain how the following factors affect enzyme activity in GCE AQA Biology:

1. Temperature 🔥
2. pH 🧪
3. Substrate concentration 
4. Inhibitors 

1. Temperature

• Initially, as the temperature increases, enzyme activity increases, increasing the rate of reaction. This is because the particles and enzymes possess greater kinetic energy, and thus successfully collide with each other more frequently. 
• As the temperature continues to increase, enzyme activity (and rate of reaction) eventually peaks at the optimum temperature. 
• Beyond the optimum temperature, enzyme activity and thus rate of reaction decrease. This is because the particles and molecules in the active site start to vibrate too quickly, straining the bonds and thus deforming the active site. 
• Eventually, beyond a certain point, the active site is permanently deformed and will not work again. The enzyme is denatured.

2. pH
Similarly to temperature, enzyme activity increases up to the optimum pH, peaks at the optimum pH, then decreases thereafter.

• pH refers to the concentration of hydrogen ions, which are charged 
• A change in the pH thus affects the charges on amino acids that make up the active site 
• This causes the substrate to become unable to bind
• An extremely significant change in pH (i.e. extremely far away from the optimum pH) may break the bonds maintaining the active site, denaturing the enzyme. 

Note that a neutral pH is not necessarily the optimum pH - different enzymes have different optimum pHs. Enzymes like stomach proteases have an optimum pH of 1-2, allowing them to function optimally in the acidic stomach environment.

3. Substrate concentration

• Initially, as substrate concentration increases, the rate of enzyme activity also increases. This is because there are increasing chances of collision between the substrate molecules and the enzyme. 
• Eventually, if enzyme concentration is fixed, further increase in substrate concentration does not increase enzyme activity, and enzyme activity plateaus. This occurs when all the enzymes are being occupied (this is known as saturation). 

4. Inhibitors
Inhibitors: any molecule that prevents the enzyme from binding to the substrate and catalysing the reaction.
GCE AQA Biology requires knowledge of two types of inhibitors:

• Competitive
• Non-competitive

Graphical representation of the types of inhibition:

We can distinguish between the 2 kinds of inhibitors with the following graph:

Wanna Boost Up Your GCE AQA Biology? Check out our study tips here!

Written by Justine | Biology Specialist @ Tuttee

References

Toole, G., & Toole, S. (2015). Aqa biology A level. Oxford: Oxford University Press.​

Images from:

1. https://www.topperlearning.com/answer/draw-a-graph-depicting-the-concept-of-activation-energy-during-an-enzyme-catalysed-reaction/j8prfooee
2. https://biologyreader.com/active-site-of-an-enzyme.html 
3. https://www.quora.com/How-does-temperature-affect-enzyme-activity
4. https://socratic.org/questions/how-does-ph-affect-enzyme-activity
5. https://www.rsb.org.uk/images/07_Enzymes.pdf 
6. https://www.khanacademy.org/science/biology/energy-and-enzymes/enzyme-regulation/a/enzyme-regulation
7. https://www.quora.com/What-is-the-difference-between-competitive-and-noncompetitive-inhibitors
8. http://www.richardsonthebrain.com/enzymes
9. http://igcse-biology-2017.blogspot.com/2017/06/213-understand-how-enzyme-function-can.html