The unicellular fungus Kluyveromyces lactis is found in dairy products. It is a safe microorganism to culture for the extraction of the enzyme lactase. Lactase catalyses the breakdown of lactose, a sugar found in milk. The reaction catalysed by lactase is summarised in Fig. 3.1. [Figure 3.1]

1. (a)Describe the reaction that is catalysed by lactase. Use Fig. 3.1 to help you. In your answer, identify R and product S.[4]
2. (b)(i)On a commercial scale, immobilised lactase can be used to produce lactose-free milk. One of the products of the reaction shown in Fig. 3.1 acts as an inhibitor of lactase. This is an example of product inhibition. Suggest why product inhibition is useful in K. lactis when lactase is acting as an intracellular enzyme, but can be a disadvantage when extracted lactase is used free in solution for the production of lactose-free milk.[2]
3. (b)(ii)Suggest how using immobilised lactase in a commercial application helps to reduce the problem of product inhibition.[1]
4. (b)(iii)The first large-scale production of lactose-free milk with an immobilised enzyme used lactase trapped in cellulose triacetate fibres. Suggest one feature of cellulose triacetate that makes it useful as an immobilising material.[1]
5. (c)For a commercial application using an enzyme, the progress of the enzyme-catalysed reaction needs to be studied. Outline how the progress of an enzyme-catalysed reaction can be investigated experimentally.[3]

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