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Topic 5 · 5.1

Enzymes

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Describe a catalyst and describe enzymes as proteins that act as biological catalysts in all metabolic reactions.
  • Explain why enzymes are needed, and describe enzyme action in terms of the active site’s shape being complementary to the substrate.
  • Investigate and describe the effects of temperature and pH, using the terms optimum and denaturation.
  • Explain enzyme action, specificity and the effects of temperature and pH in detail EXTENDED.

📚The biology

Enzymes as catalysts

A catalyst is a substance that increases the rate of a chemical reaction and is not changed by the reaction. Enzymes are proteins that act as biological catalysts in all metabolic reactions. Without them, reactions at body temperature would be far too slow to sustain life.

Three stages. An enzyme with a notch-shaped active site and a separate substrate. The substrate fitted into the active site, forming an enzyme-substrate complex. The enzyme, unchanged, with two product molecules leaving; the enzyme can be used again.
The substrate fits the active site because their shapes are complementary. The products leave and the enzyme is reused.

EXTENDED The substrate binds to the active site, forming an enzyme-substrate complex; the reaction happens and the products are released. Enzymes are specific: each active site has a shape complementary to only one substrate (or a few similar ones), so amylase breaks down starch but not protein.

Temperature

A graph of rate of reaction against temperature from 0 to 70 degrees Celsius. The rate rises gradually to a peak, the optimum temperature at about 40 degrees, then falls steeply to zero by about 55 to 60 degrees. The rising side is labelled more kinetic energy, more effective collisions; the falling side, active site changes shape, denatured.
A typical human enzyme. The rise is gradual; the fall after the optimum is steep and permanent. (Illustrative curve.)

As temperature rises the rate increases, up to the optimum temperature. Above the optimum the enzyme is denatured: its active site changes shape, so the substrate no longer fits. Denaturation is permanent. At low temperatures the enzyme works slowly but is not denatured.

EXTENDED Up to the optimum, enzyme and substrate molecules have more kinetic energy, move faster, and so there are more frequent effective collisions and more enzyme-substrate complexes form. Above it, bonds holding the enzyme’s shape break: the shape and fit of the active site are lost.

Left: an enzyme with the substrate fitting its active site. An arrow labelled high temperature or wrong pH leads to the right: the enzyme has a distorted shape and the substrate, crossed out, no longer fits.
Denaturation: the active site loses its shape, so no enzyme-substrate complexes can form.

pH

Each enzyme has an optimum pH. Moving far from it changes the shape of the active site, and the enzyme is denatured.

A graph of rate of reaction against pH from 0 to 14 with three curves: pepsin peaking at pH 2, salivary amylase peaking at pH 7 and trypsin peaking at about pH 8.5.
Pepsin works in the acid stomach, amylase in the neutral mouth, trypsin in the alkaline small intestine. (Illustrative curves.)

EXTENDED Changes in pH affect the bonds that hold the enzyme in shape, changing the shape and fit of the active site and causing denaturation.

Investigating enzyme activity

Amylase digests starch. Mix amylase with starch suspension in a water bath, and every 30 s put a drop of the mixture on to iodine solution on a spotting tile. When the drop no longer turns blue-black, all the starch is gone: that time is the end-point.

A spotting tile with seven drops of iodine solution, tested at 0, 30, 60, 90, 120, 150 and 180 seconds. The first five are blue-black; from 150 seconds they stay orange-brown.
End-point here: 150 s. The rate can be found as 1 ÷ time.

✏️Worked example

The time for amylase to digest all the starch was measured at five temperatures: 20 °C, 300 s; 30 °C, 150 s; 40 °C, 90 s; 50 °C, 210 s; 60 °C, still blue-black after 600 s. (a) Calculate the rate at 40 °C, using rate = 1 ÷ time. [1] (b) State the range in which the optimum temperature lies. [1] (c) Explain the result at 60 °C. [2] (d) EXTENDED Explain why the rate at 30 °C is higher than at 20 °C. [3]

(a) \( 1 \div 90 = 0.011 \) per second (s−1).

(b) Between 30 °C and 50 °C (the fastest measured was 40 °C, but the true optimum could be either side of it).

(c) The amylase was denatured: its active site changed shape, so starch could no longer fit and was not digested.

(d) The molecules have more kinetic energy and move faster, so there are more frequent effective collisions between enzyme and substrate, and more enzyme-substrate complexes form per second.

Check it. Shorter time means faster rate: 90 s is the shortest, so 40 °C has the highest rate (0.011 s−1 against 0.0067 at 30 °C and 0.0033 at 20 °C).
“The enzyme is killed.” Enzymes are molecules, not living things. Say denatured, and say what changes: the shape of the active site.

📝Practise

In the style of the multiple-choice, theory and practical papers. EXTENDED marks Supplement content.

1. (Multiple choice.) What is a catalyst? A: a substance that slows a reaction and is used up. B: a substance that increases the rate of a reaction and is not changed by it. C: a protein that is changed by the reaction. D: the product of a reaction.
B.
2. (Theory.) Explain why living organisms need enzymes. [2]
Metabolic reactions would be too slow at body temperature to sustain life; enzymes speed them up to the rate needed.
3. (Theory.) Two tubes of the same enzyme are kept at 5 °C and 80 °C, then both are moved to 37 °C. Predict what happens to the activity of each, and explain. [3]
5 °C: activity increases at 37 °C — the enzyme was only slowed by low temperature, not denatured. 80 °C: no activity — it was denatured; the change to its active site is permanent.
4. (Theory.) Pepsin digests protein in the stomach. Suggest why pepsin stops working when the stomach contents enter the small intestine. [2]
The small intestine is alkaline, far from pepsin’s acidic optimum pH, so its active site changes shape (it is denatured) and protein no longer fits.
5. (Practical.) In the starch–amylase investigation, state two variables that must be kept constant, and explain why the tubes are kept in a water bath. [3]
Any two: volume and concentration of amylase; volume and concentration of starch; pH; interval between samples. The water bath keeps the temperature constant (and at the chosen value).
6. (Theory.) EXTENDED Explain why lipase breaks down fats but not starch. [2]
Enzymes are specific: the active site of lipase has a shape complementary to fat molecules only, so starch cannot fit and no enzyme-substrate complex forms.
7. (Theory.) EXTENDED Put these in order to describe enzyme action: products released; substrate collides with the active site; enzyme-substrate complex forms; reaction occurs. [1]
Substrate collides with the active site → enzyme-substrate complex forms → reaction occurs → products released.
8. (Theory.) EXTENDED Explain, in terms of shape and fit, why an enzyme’s activity falls at a pH far from its optimum. [2]
The pH changes the shape of the active site, so it is no longer complementary to the substrate; fewer enzyme-substrate complexes form (the enzyme is denatured).

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • Royal Society of Biology — the effect of temperature on amylase, a class practical
  • Learn Genetics (University of Utah) — animations of enzymes and active sites