Enzymes

What enzymes are and how they act

Content standard 5.2 defines an enzyme as a biological catalyst, a protein that speeds up the rate of a biochemical reaction without being used up itself. Every enzyme has a specific three-dimensional region called the active site, whose shape matches only one substrate (or a small group of similar substrates), which is why enzymes are described as specific.

Lock-and-key versus induced fit

The lock-and-key theory describes the active site as a rigid shape that fits its substrate exactly, like a key fitting a lock. The induced fit theory refines this idea: the active site is flexible and changes shape slightly as the substrate binds, moulding around it to fit more precisely.

Both models explain enzyme specificity, but induced fit is the more accurate modern explanation.

Factors affecting enzyme activity

Denaturation and how it is examined

Denaturation happens when heat, extreme pH, or other harsh conditions permanently change the shape of an enzyme's active site, so the substrate can no longer bind and the enzyme stops working. Denaturation is irreversible.

Exam questions commonly ask you to sketch or interpret a graph of reaction rate against temperature or pH, to explain what happens to an enzyme at high temperature, or to describe the lock-and-key or induced-fit model with a labelled diagram.

Worked exam-style question

Question. A student investigates the effect of temperature on the activity of amylase by mixing amylase with starch solution at 10°C, 37°C and 70°C, then testing a sample of each mixture with iodine solution every minute until it no longer turns blue-black. At 10°C the mixture took 12 minutes to stop turning blue-black; at 37°C it took 3 minutes; at 70°C it still turned blue-black after 20 minutes.

(a) State what is meant by an enzyme. (b) Explain why the starch was digested fastest at 37°C.

(c) Explain why the starch was not digested at 70°C, even after 20 minutes. (d) Predict what would happen to the time taken at 37°C if the concentration of amylase were doubled, and explain your prediction.

Model answer. (a) An enzyme is a biological catalyst, a protein that speeds up the rate of a reaction without being used up, by binding substrate at its active site. (b) 37°C is close to amylase's optimum temperature, so molecules have enough kinetic energy for frequent, successful collisions between the active site and starch, without the enzyme being denatured.

(c) At 70°C, the high heat energy breaks the bonds holding the enzyme's three-dimensional shape, permanently changing the shape of its active site. The amylase is denatured, so starch molecules can no longer bind, and no digestion occurs.

(d) The time taken would decrease (digestion would be faster), because doubling the enzyme concentration provides more active sites, allowing more starch molecules to be broken down per minute, provided the substrate is not limiting.

Practice question

Try this. Pepsin is a digestive enzyme that works best at pH 2 in the stomach. Predict what happens to pepsin's activity if it passes into the small intestine, where the pH is about 8, and explain your prediction.

Exam tip

Key terms

These terms from Chapter 5 are easy to confuse, keep them distinct:

• Enzyme, a protein that acts as a biological catalyst.
• Substrate, the molecule an enzyme acts on.
• Active site, the specific region of an enzyme where the substrate binds.
• Denaturation, a permanent change in an enzyme's shape that stops it working.
• Metabolism, the sum of all chemical reactions occurring in an organism, most of which are enzyme-controlled.

Source:SRC-DSKP-EN

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