Enzymes and factors affecting activity (SACE Stage 2 Biology)
“Explain how enzymes catalyse reactions via the active site and how temperature, pH, concentration and inhibitors affect their activity”
Enzymes are protein catalysts that lower the activation energy of reactions by binding a specific substrate at their active site. Their activity is changed by temperature, pH, substrate concentration, enzyme concentration and inhibitors, because each of these affects either the rate of enzyme-substrate collisions or the shape of the active site.
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What this dot point is asking
You need to explain how enzymes work using the active-site model and explain how the main factors change the rate of an enzyme-controlled reaction. This dot point builds directly on protein structure: an enzyme's function depends on its 3D shape.
What an enzyme is
An enzyme is a biological catalyst, usually a protein, that speeds up a chemical reaction without being used up. Enzymes work by lowering the activation energy (the energy needed to start a reaction), so reactions proceed faster at the relatively low temperatures inside cells.
Each enzyme is specific: it catalyses only one reaction or type of reaction. This specificity comes from the active site, a region whose shape is complementary to a particular substrate.
How enzymes catalyse reactions
- The substrate binds to the enzyme's active site, forming an enzyme-substrate complex.
- The active site strains or positions the substrate so the reaction occurs more easily, lowering activation energy.
- The product(s) are released, and the unchanged enzyme is free to bind another substrate.
A refinement of the simple lock-and-key idea. The active site is not perfectly rigid; it changes shape slightly to fit the substrate more snugly when the substrate binds, which helps catalyse the reaction.
Factors affecting enzyme activity
Temperature
As temperature rises, molecules move faster and collide more often, so the reaction rate increases up to an optimum temperature (around 37 degrees Celsius for many human enzymes). Above the optimum, heat breaks the bonds maintaining the enzyme's tertiary structure, so the enzyme denatures, the active site changes shape, and activity falls sharply.
pH
Each enzyme has an optimum pH at which its active site is the correct shape. For example, pepsin in the stomach works best around pH 2, while many cellular enzymes prefer about pH 7. Moving away from the optimum disrupts ionic and hydrogen bonds, changing the active site and reducing activity; extreme pH denatures the enzyme.
Substrate concentration
Increasing substrate concentration increases the reaction rate because more substrate molecules collide with active sites. Eventually all active sites are occupied (saturation), so the rate levels off at a maximum; adding more substrate then has no effect.
Enzyme concentration
If substrate is plentiful, increasing enzyme concentration increases the rate because more active sites are available to bind substrate.
Inhibitors
Inhibitors reduce enzyme activity. Competitive inhibitors resemble the substrate and bind to the active site, blocking it; their effect can be reduced by adding more substrate. Non-competitive inhibitors bind elsewhere on the enzyme, changing the active site's shape so the substrate no longer fits.
Reading an enzyme-temperature curve
A graph of reaction rate against temperature rises from 0 to a peak at 40 degrees Celsius, then falls steeply to zero by 60 degrees Celsius.
- The rise to 40 degrees: increasing temperature gives more frequent, higher-energy collisions, so rate increases.
- The peak at 40 degrees: this is the optimum temperature for this enzyme.
- The steep fall after 40 degrees: the enzyme is denaturing. The active site loses its shape, fewer enzyme-substrate complexes form, and rate drops to zero once all enzyme is denatured.
A common exam point: the fall after the optimum is steeper than the rise before it, because denaturation rapidly destroys function.
- Saying enzymes are "killed"
- Enzymes are not alive; at high temperature they denature, not die.
- Confusing the two inhibitor types
- Competitive inhibitors bind the active site and can be overcome by more substrate; non-competitive inhibitors bind elsewhere and cannot.
- Claiming rate keeps rising with substrate
- Rate plateaus once enzymes are saturated, because all active sites are occupied.
- Calling denaturation reversible
- Denaturation is usually permanent because the precise fold cannot reform.
Exam-style questions
Questions in the style of SACE Board exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
SACE 2019-style3 marksROCK is an enzyme that activates a second enzyme, LIM kinase (LIMK). Explain why ROCK is the only enzyme that is able to activate LIMK.Show worked answer →
For 3 marks, build the argument from enzyme specificity.
Enzymes are specific: each enzyme has a uniquely shaped active site determined by its tertiary structure.
The active site of ROCK is complementary in shape to LIMK (its substrate), so only ROCK can bind LIMK and form an enzyme-substrate complex.
Other enzymes have differently shaped active sites that are not complementary to LIMK, so they cannot bind it and cannot activate it. This lock-and-key (or induced-fit) specificity means only ROCK activates LIMK.
SACE 2019-style3 marksThe drug fasudil is an inhibitor of the enzyme ROCK. Explain one way in which a drug such as fasudil could reduce the activity of the enzyme ROCK.Show worked answer →
Describe one inhibition mechanism for the marks.
Competitive inhibition: fasudil has a shape similar to ROCK's normal substrate, so it binds to and blocks the active site. While the inhibitor occupies the active site, the substrate cannot bind, so fewer enzyme-substrate complexes form and ROCK activity falls.
(A non-competitive answer is equally valid: fasudil binds to an allosteric site away from the active site, changing the enzyme's shape so the active site no longer fits the substrate, reducing activity.)
SACE 2018-style3 marksProcaspase 3 is activated by the enzyme caspase 8. Explain how caspase 8 increases the rate of activation of procaspase 3.Show worked answer →
Treat caspase 8 as an enzyme and procaspase 3 as its substrate.
Caspase 8 is an enzyme (a biological catalyst), and procaspase 3 is its substrate.
Caspase 8 binds procaspase 3 at its active site, forming an enzyme-substrate complex, and catalyses its conversion to active caspase 3.
By lowering the activation energy of the reaction, caspase 8 increases the rate at which procaspase 3 is converted to caspase 3, compared with the uncatalysed rate.
Practise this
Sources & how we know this
- Stage 2 Biology subject outline (2025 onwards) — SACE Board
- Stage 2 Biology: external assessment — SACE Board