Enzymes: Revision Notes

Condensed for the final weeks. For the full explanation, use the Enzymes study guide.

What an enzyme is

A biological catalyst — a protein that speeds up a reaction by lowering the activation energy, and is not used up in the process.

Because enzymes are proteins, their function depends on the precise 3D shape of the active site.

Specificity — the lock-and-key model

Each enzyme has an active site with a shape complementary to one specific substrate. The substrate fits, an enzyme–substrate complex forms, the reaction occurs, and products are released. The enzyme is unchanged and reused.

Only a substrate of the correct shape fits — which is why enzymes are specific. This specificity is also why a single organism needs thousands of different enzymes: one enzyme cannot catalyse every reaction, so each metabolic reaction in the body relies on its own dedicated enzyme with a complementary active site.

Temperature

Below optimum:  rate INCREASES with temperature
                more kinetic energy -> more frequent successful collisions

At optimum:     maximum rate (about 37 C in humans)

Above optimum:  rate FALLS SHARPLY
                bonds holding the tertiary structure break
                active site changes shape -> DENATURED
                substrate no longer fits -> no complexes form

Denaturation is permanent. Cooling does not restore activity. The enzyme is not “killed” — it was never alive.

pH

Each enzyme has an optimum pH. Either side of it, activity falls; extremes denature the enzyme by disrupting the bonds maintaining the active site’s shape.

Enzyme	Optimum pH	Where
Pepsin	~2	Stomach (acidic)
Amylase	~7	Mouth, small intestine
Trypsin	~8	Small intestine (alkaline)

Pepsin working best at pH 2 is the standard example that optimum ≠ neutral — a question that simply assumes every enzyme works best at pH 7 will misjudge digestive enzymes, since the human digestive system itself spans a wide pH range from the strongly acidic stomach to the mildly alkaline small intestine, and each enzyme’s optimum reflects the specific environment it evolved to work in.

Following the progress of a reaction

Enzyme-catalysed reactions can be tracked in two ways: measuring the disappearance of a reactant (substrate) or the appearance of a product, over time.

EXAMPLE 1  Amylase + starch
           Sample the mixture at regular time intervals, test each
           sample with iodine solution -- time how long until the
           blue-black colour no longer forms (starch is gone)

EXAMPLE 2  Catalase + hydrogen peroxide
           Catalase breaks down H2O2 into water and oxygen gas
           Collect the oxygen produced in a gas syringe, and time how
           much gas is collected in fixed time intervals

A steeper graph of product formed against time means a faster reaction; the graph levels off (plateaus) once the reaction is complete or the substrate has run out.

Substrate concentration (beyond this syllabus)

This sub-topic is not part of the O Level 5090 enzymes outcomes (enzyme action and the effects of temperature and pH). It is included here as extension material only.

Rate increases with substrate concentration until all active sites are occupied. Beyond that the rate plateaus — the enzyme is the limiting factor, and adding more substrate changes nothing. Adding more enzyme, rather than more substrate, would raise the plateau again, since it provides more active sites for the (now excess) substrate to bind to.

Exam traps

• Enzymes are denatured, never “killed” or “dead”.
• Denaturation is irreversible.
• Say the active site changes shape so the substrate no longer fits — not just “the enzyme stops working”.
• Enzymes lower activation energy; they do not “give energy” to the reaction.
• Distinguish the temperature explanation (kinetic energy, then denaturation) from the specificity explanation (lock and key). Questions test one or the other.
• Describing a catalyst as something that is “used up” in the reaction — a catalyst, enzymes included, is chemically unchanged at the end and can be reused.
• Forgetting that a levelling-off (plateau) in a rate-of-reaction graph can mean either that the reaction has finished or that a factor (substrate, or one of the active sites) has become limiting — the two are not automatically the same thing and a question may ask you to distinguish them.

Self-test

1. Define an enzyme.
2. Explain why enzymes are specific.
3. Describe what happens to an enzyme above its optimum temperature, in four steps.
4. Why does pepsin have an optimum pH of about 2?
5. Why does rate plateau at high substrate concentration?
6. Describe one practical method for following the progress of an enzyme-catalysed reaction.
7. What does a steeper product-against-time graph indicate about a reaction?

Answers: 1. A protein that acts as a biological catalyst, speeding up a reaction by lowering activation energy without being used up. 2. The active site has a shape complementary to one particular substrate, so only that substrate can bind and form an enzyme–substrate complex. 3. Bonds maintaining the tertiary structure break; the active site changes shape; the substrate can no longer bind; no enzyme–substrate complexes form, so the reaction stops — the enzyme is denatured. 4. It works in the stomach, where hydrochloric acid makes conditions strongly acidic; its structure is adapted to that environment. 5. All the active sites are occupied, so the enzyme concentration becomes the limiting factor and extra substrate cannot be processed any faster. 6. Any valid example, e.g. mixing catalase with hydrogen peroxide and collecting the oxygen produced in a gas syringe at regular time intervals. 7. A faster reaction rate at that point in time.

For the full explanation, including how reaction progress is investigated experimentally, see the Enzymes study guide.