Enzymes: Biology 0610 (Cambridge O Level / IGCSE)

A student extracts a protease enzyme from kiwi fruit and adds a small, equal sample of the enzyme to three separate test tubes:

• test tube 1 contains egg white (a protein)
• test tube 2 contains vegetable oil (a fat)
• test tube 3 contains a paste made from cornflour (a carbohydrate)

All three test tubes are left at the same temperature for 30 minutes. Only the contents of test tube 1 show any sign of the protein being broken down into smaller, soluble fragments. Test tubes 2 and 3 show no change at all.

Which statement best explains why the enzyme has this effect only on the egg white?

A food scientist is developing a protein-tenderising powder based on papain, a protease enzyme extracted from papaya fruit. The scientist places identical raw chicken breast cubes into five water baths at different temperatures, adds the same mass of papain powder to each cube, and leaves each cube in its water bath for 20 minutes. The scientist then blots each cube dry and measures how much mass it has lost, since papain breaks down some of the protein in the chicken into smaller fragments that dissolve into the surrounding water.

(a) State the independent variable and the dependent variable in this investigation. [2]

(b) Describe the pattern shown by the results in the table. [2]

(c) The scientist repeats the investigation, but this time first heats one chicken cube, with papain powder already added, to 90°C for 5 minutes, then cools it back down and leaves it at 40°C for the remaining 20 minutes. Predict how the mass lost by this cube will compare with the mass lost by the cube kept at 40°C throughout, and explain your answer. [2]

Rennin, also called chymosin, is an enzyme used in cheese-making. When rennin is added to milk, it acts on casein, a protein dissolved in the milk, causing the casein to clump together and form solid curds. Rennin has no effect on lactose, a sugar that is also dissolved in milk.

(a) State the type of biological molecule that acts as the substrate for rennin. [1]

(b) Using the term active site, describe why rennin causes casein to clump together but has no effect on lactose. [2]

(c) A cheese-maker accidentally uses a batch of rennin that was left inside a hot delivery van for several hours and has since lost its normal shape. Suggest what will happen when this rennin is added to milk, and explain your answer. [2]

Two research groups each extract a protease enzyme that digests protein. Enzyme G is extracted from bacteria that live in the human gut. Enzyme H is extracted from bacteria that live in a geothermal hot spring, where the water is naturally very hot. Each group measures the rate of protein digestion produced by its enzyme at a range of temperatures, using the same concentration of enzyme and the same protein substrate every time.

(a) State the optimum temperature shown by these results for enzyme G and for enzyme H. [2]

(b) Explain, in terms of kinetic energy and the frequency of effective collisions between enzyme and substrate molecules, why the rate for enzyme G increases between 10°C and 37°C. [3]

(c) Explain why the rate for enzyme G has fallen to zero by 65°C, while enzyme H still has a high rate of reaction at this temperature. [3]

A student is investigating a protease enzyme extracted from a species of bacteria that lives naturally in a soda lake, a body of water with a very alkaline pH of about 10. The student prepares five test tubes, each containing the same volume of buffer solution set to a different pH, adds the same amount of enzyme and the same protein substrate to each tube, and records how long it takes for the protein to be completely broken down in each tube. A shorter time means a faster rate of reaction.

(a) State which pH gives the fastest rate of reaction, and explain how you can tell this from the times recorded. [2]

(b) Explain, in terms of the shape of the enzyme's active site, why hardly any protein is broken down in the test tube at pH 4. [3]

(c) The student did not measure the exact volume of buffer solution added to each test tube, or the exact starting mass of protein substrate used. Suggest two reasons why this makes it difficult to conclude confidently that pH alone is responsible for the different times recorded. [2]

Catalase is an enzyme found in liver and potato cells. It speeds up the breakdown of hydrogen peroxide into water and oxygen gas.

A student places a small piece of fresh liver into a test tube and adds a sample of hydrogen peroxide solution. Bubbles of oxygen gas form quickly. Once the bubbling stops, the student pours away the liquid, leaving the same piece of liver in the test tube, and adds a fresh sample of hydrogen peroxide solution. Bubbles of oxygen gas form quickly again, at about the same rate as before.

Which statement best explains why the same piece of liver can break down a second sample of hydrogen peroxide, without any new enzyme being added?

Sucrase is an enzyme found in the small intestine. It catalyses the breakdown of the sugar sucrose into two smaller sugars, glucose and fructose.

(a) Using the terms active site and substrate, describe how a sucrase molecule and a sucrose molecule interact to form an enzyme-substrate complex. [2]

(b) Starch is a much larger carbohydrate molecule that is also present in food in the small intestine. Explain why sucrase cannot break down starch. [2]

(c) State what happens to a sucrase molecule once it has released the glucose and fructose products, and explain why this allows one sucrase molecule to break down many sucrose molecules over time. [2]

A group of students investigates how the rate of reaction of a human digestive enzyme changes with temperature, testing temperatures from 0°C to 80°C. This enzyme has an optimum temperature of 37°C, the normal temperature inside the human body.

Which description best matches the general shape of the graph obtained when the rate of reaction (y-axis) is plotted against temperature (x-axis) across this range?

Pepsin is a protease enzyme released in the stomach, where hydrochloric acid keeps the contents strongly acidic at about pH 2. Pepsin's optimum pH is about pH 2. Trypsin is a different protease enzyme, released into the small intestine, where the contents are slightly alkaline at about pH 8 because acid from the stomach has been neutralised. Trypsin's optimum pH is about pH 8.

(a) Explain why pepsin can digest protein efficiently in the stomach, while trypsin cannot function well there. [2]

(b) Trypsin is accidentally exposed to the strongly acidic conditions of the stomach (pH 2) for a long period of time. In terms of the bonds that hold an enzyme's three-dimensional shape together, explain what happens to trypsin's active site, and why this change cannot be reversed simply by moving the trypsin into the less acidic conditions of the small intestine. [3]

(c) Suggest one advantage to the body of pepsin having an optimum pH that matches the stomach, rather than sharing the same optimum pH as trypsin. [1]

A particular protease enzyme has an optimum temperature of 45°C.

(a) State the class of large biological molecule that all enzymes, including this protease, are made of. [1]

(b) The enzyme is tested at 20°C and at 45°C. At both temperatures, the enzyme's active site has the same, undamaged shape. Explain, in terms of the enzyme's three-dimensional shape and the kinetic energy of the molecules involved, why the rate of reaction is much lower at 20°C than at 45°C. [2]

(c) The same enzyme is then heated to 70°C for ten minutes and cooled back down to 45°C. Explain, in terms of the bonds that hold the enzyme's three-dimensional shape together, why the enzyme's rate of reaction at 45°C is now much lower than it was before the heating step. [3]