Enzymes: Question 7
Syllabus 5.1
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]
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Worked solution
Part (a): Forming the enzyme-substrate complex
Sucrase, like every enzyme, has a region called the active site whose shape is complementary to the shape of one particular type of molecule. Here, that molecule is sucrose, the substrate. Because sucrose’s shape matches the active site, it can fit into and bind to the active site. This is the lock-and-key idea. Once bound, the sucrase and sucrose together form an enzyme-substrate complex, which is then converted into the products.
Part (b): Why sucrase cannot act on starch
Sucrase’s active site is complementary only to the shape of sucrose. Starch is a much larger carbohydrate molecule with a different shape from sucrose, so it does not fit into sucrase’s active site. Since starch cannot bind to the active site, no enzyme-substrate complex can form, and sucrase cannot break it down. This is an example of enzyme specificity. (A different enzyme, amylase, has an active site shaped to fit starch instead.)
Part (c): Why one sucrase molecule can act repeatedly
After the enzyme-substrate complex breaks apart and the products (glucose and fructose) are released, the sucrase molecule itself is unaffected. It is not used up or permanently changed. Its active site returns to its original shape, so it is immediately ready to bind another sucrose molecule. Because the same sucrase molecule can be reused over and over again, a small amount of enzyme can break down a very large number of substrate molecules over time.
Final answers
- (a) Sucrose (the substrate) fits into sucrase’s complementary-shaped active site and binds to it, forming an enzyme-substrate complex.
- (b) Starch has a different shape from sucrose, so it does not fit sucrase’s active site and no enzyme-substrate complex can form.
- (c) The active site returns to its original shape after releasing the products, so the same sucrase molecule can repeatedly bind and break down many sucrose molecules.