3.6: Enzymes

Question 1

Define enzyme and active site:

Answer 1

Enzymes: Globular proteins which act as catalysts of chemical reactions.

Active site: Region on the surface of an enzyme to which substrates bind and which catalyses a chemical reaction involving the substrates.

Question 2

Explain enzyme-substrate specificity:

Answer 2

• Active site and substrate complement each other in terms of both shape and chemical properties (e.g. opposite charges)
• Binding to the active site brings the substrate into close physical proximity, creating an enzyme-substrate complex
• The enzyme catalyses the conversion of the substrate into a product (or products), creating an enzyme-product complex
• As the enzyme is not consumed in the reaction, it can continue to work once the product dissociates (hence only low concentrations are needed)

Lock and Key Model:
+ Enzymes and substrates share specificity (a given enzyme will only interact with a small number of specific substrates that complement the active site)
+ This explanation of enzyme-substrate interaction is described as the ‘lock and key’ model (a lock only opens in response to a specific key)

Question 3

Explain the effects of temperature, pH and substrate concentration on enzyme activity:

Answer 3

Enzyme activity increases with an increase in temperature and usually doubles with every 10 degrees rise. This is due to the molecules moving faster and colliding more often together. However at a certain point the temperature gets to high and the enzymes denature and stop functioning. This is due to the heat causing vibrations within the enzyme destroying its structure by breaking the bonds in the enzyme.

Enzymes usually have an optimum pH at which they work most efficiently. As the pH diverges from the optimum, enzyme activity decreases. Both acid and alkali environments can denature enzymes.

Enzyme activity increases with an increase in substrate concentration as there are more random collisions between the substrate and the active site. However, at some point, all the active sites are taken up and so increasing the substrate concentration will have no more effect on enzyme activity. As long as there are active sites available, an increase in substrate concentration will lead to an increase in enzyme activity.

Question 4

Define denaturation:

Answer 4

Denaturation is changing the structure of an enzyme so it can no longer carry out its function.

Question 5

Explain the use of lactase in the production of lactose-free milk:

Answer 5

Lactose is the sugar found in milk. It can be broken down by the enzyme lactase into glucose and galactose. However some people lack this enzyme and so cannot break down lactose leading to lactose intolerance. Lactose intolerant people need to drink milk that has been lactose reduced. Lactose-free milk can be made in two ways. The first involves adding the enzyme lactase to the milk so that the milk contains the enzyme. The second way involves immobilising the enzyme on a surface or in beads of a porous material. The milk is then allowed to flow past the beads or surface with the immobilised lactase. This method avoids having lactase in the milk.