3. Enzymes
A section of Biology, 9700
Listing 10 of 276 questions
Enzymes are polymers of amino acids. Complete to show the general structure of an amino acid. O C C H N When bananas are peeled, the exposed tissue gradually turns brown in the presence of oxygen in the air. This is due to an enzyme called catechol oxidase, which acts on the substrate catechol. Catechol and catechol oxidase are present in the banana tissue. The overall reaction is shown in . catechol catechol oxidase melanin oxygen A student investigated how the concentration of catechol oxidase affects the rate of this reaction. All other variables were kept constant throughout the investigation. For each concentration of catechol oxidase used, the student mixed catechol oxidase solution with catechol and recorded the time taken for the mixture to reach a standard brown colour. The rate of reaction, R, for each concentration of catechol oxidase used was then calculated using the formula: R = time to reach standard brown colour in minutes Calculate the rate of reaction when the standard brown colour was reached in 2 minutes 30 seconds. rate of reaction = min–1 is a graph showing the results of the investigation. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 rate of reaction / min–1 percentage concentration of catechol oxidase State how the results shown in show that substrate was in excess at all concentrations of catechol oxidase tested. The student carried out a further experiment to investigate how the concentration of catechol affects the initial rate of reaction. All other variables were kept constant throughout this investigation. is a graph showing the effect of varying the concentration of catechol on the initial rate of reaction. 0.0 0.00 0.20 0.40 0.60 catechol concentration / mol dm–3 0.80 1.00 1.20 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 initial rate of reaction / min–1 Explain the shape of the curve shown in . Use to calculate the value of the Michaelis–Menten constant (Km) for the reaction between catechol oxidase and catechol. Km = mol dm–3 Methylcatechol has a similar shape to catechol. Catechol oxidase can also use methylcatechol as a substrate. The Km value for the reaction using methylcatechol as the substrate was found to be much lower than the Km value for the reaction using catechol as the substrate, when the reactions were carried out under the same conditions. State what these Km values indicate about the relationship between the enzyme and the two substrates.
9700_m22_qp_22
THEORY
2022
Paper 2, Variant 2
View
Use Some bacteria that are found in soils contain the enzyme urease. Urease catalyses the hydrolysis of urea to form ammonia and carbon dioxide: urea + water carbon dioxide + ammonia Some fertilisers added to soils to help crop growth contain urea. Although some crop plants can absorb ammonium ions, most obtain their source of nitrogen as nitrate ions. Describe how urea from fertilisers becomes available to plants as nitrate ions. The activity of urease can be measured by following the increase in pH as ammonia is produced in the reaction. A student was provided with urease extracted from bacteria and solutions of urea and two chemical inhibitors, thiourea and lead nitrate. The student prepared six reaction mixtures (1 to 6) as shown in Table 5.1 in order to investigate the effect of the two chemical inhibitors on the activity of urease. Table 5.1 reaction mixture urea water thiourea lead nitrate urease boiled urease Key = present in reaction mixture = absent from reaction mixture The student recorded an increase in pH in reaction mixtures 1 and 2. The reaction was faster in 1 than in 2. The pH in the other reaction mixtures did not change. Examiner’s Use The student made some conclusions about the results from the test-tubes. Match the statements to the reaction mixtures, 1 to 6. You may use the numbers once, more than once or not at all. ‘No reaction took place because urease was denatured.’ ‘There was no reaction because there was no substrate for urease.’ ‘The reaction did not occur because there was an inhibitor present.’ Thiourea has a molecular structure that is very similar to that of urea. The student designed an experiment to find out whether thiourea is a competitive inhibitor. The student set up several reaction mixtures like 1 using increasing concentrations of urea. The student determined the initial rate of the reaction for urease at each concentration of urea. The results are shown in . initial rate of reaction concentration of urea The student then repeated the experiment using the same concentrations of urea. However, the student added the same volume and concentration of a thiourea solution to each test-tube in place of the water. Sketch a curve on to show the results that the student would expect if thiourea acts as a competitive inhibitor of urease. Explain why it is important to determine the initial rate of reaction when investigating the effect of a competitive inhibitor on an enzyme.
9700_s06_qp_2
THEORY
2006
Paper 2, Variant 0
View
Questions Discovered
276