Enzyme actionAQA A-Level Biology: Subtopic test

1. 1
   Catalase, found inside liver cells, catalyses the breakdown of hydrogen peroxide, a toxic by-product of metabolism, into water and oxygen. Trypsin is made by the pancreas and secreted into the small intestine, where it catalyses the hydrolysis of proteins to shorter polypeptides.

   (a)

   Which statement correctly describes trypsin?

   [1 mark]

   • AAn intracellular enzyme, because it is made inside cells of the pancreas
   • BAn extracellular enzyme, because it acts outside the cells that made it
   • CAn intracellular enzyme, because it is a protein
   • DAn enzyme that is used up in the reaction it catalyses

   (b)

   Which property of catalase allows it to bind hydrogen peroxide?

   [1 mark]

   • AThe presence of a phosphate group on the enzyme
   • BThe concentration of substrate in the liver cell
   • CThe size of the liver cell in which it is found
   • DThe specific tertiary structure of its active site, giving a shape complementary to hydrogen peroxide

   (c)

   Explain why trypsin does not catalyse the breakdown of hydrogen peroxide.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In 1894 Emil Fischer proposed that an enzyme and its substrate fit together like a lock and key. In 1958 Daniel Koshland proposed a modified model after it was found that some enzymes can act on substrates of slightly different shapes and that the shape of an enzyme can change when its substrate binds.

   (a)

   Which statement describes the induced-fit model of enzyme action?

   [1 mark]

   • AThe active site changes shape slightly as the substrate binds, so that it becomes complementary to the substrate
   • BThe active site is a rigid shape that is exactly complementary to the substrate before it binds
   • CThe substrate changes shape so that it fits a rigid active site
   • DThe enzyme is changed permanently by the substrate it binds

   (b)

   Which statement best explains why the lock and key model was modified?

   [1 mark]

   • AEnzymes were found to be made of carbohydrate, not protein
   • BSubstrates were found never to bind to enzymes
   • CNew evidence showed that enzymes are flexible and that the shape of the active site can change when the substrate binds
   • DEnzymes were found not to lower the activation energy of reactions

   (c)

   Explain how the induced-fit model accounts for an enzyme lowering the activation energy of a reaction.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Lactase is an enzyme made by cells lining the small intestine. It is anchored in the cell surface membrane with its active site facing the gut lumen, where it hydrolyses the disaccharide lactose to glucose and galactose. Some adults make too little lactase, and can take a tablet containing lactase from a fungus with dairy foods.

   (a)

   A mutation in a different person replaces one amino acid in the active site of lactase with an amino acid with a different R group. The enzyme no longer hydrolyses lactose. Explain why.

   [3 marks]

   (b)

   Suggest how a lactase tablet can help a person with low lactase production, even though the enzyme in the tablet works outside living cells.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Hexokinase catalyses the reaction between glucose and ATP, the first step of glycolysis. Fischer (1894) proposed that an enzyme's active site is a rigid shape that exactly matches its substrate. Koshland (1958) proposed that the active site is flexible. Later, X-ray crystallography of hexokinase showed that its shape changes when glucose binds and that the active site closes around the glucose.

   (a)

   Describe and explain how the structure of an enzyme such as hexokinase enables it to lower the activation energy of a reaction.

   [6 marks]

   (b)

   Evaluate how the model of enzyme action has changed over time and why the induced-fit model is now preferred.

   [6 marks]

   Total for question 4: 12 marks

End of questions