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the role of enzymes and coenzymes in facilitating biochemical reactions, including factors affecting enzyme activity (temperature, pH, substrate concentration) and the effect of competitive and non-competitive inhibitors
A focused answer to the VCE Biology Unit 3 dot point on enzymes. Covers active site and induced fit, factors affecting rate (temperature, pH, substrate concentration), competitive vs non-competitive inhibition, and the role of coenzymes and cofactors.
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What this dot point is asking
VCAA wants the structure-function link for enzymes (active site, induced fit), the factors that change the rate of an enzyme-catalysed reaction, the difference between competitive and non-competitive inhibition, and the role of coenzymes and cofactors.
The answer
An enzyme is a biological catalyst, almost always a protein, that speeds up a specific biochemical reaction by lowering its activation energy. Enzymes are not consumed in the reaction and can be reused.
Active site and induced fit
Every enzyme has an active site: a pocket or cleft formed by the tertiary fold of the polypeptide. The R groups lining this site give it a specific shape and chemistry that match one substrate (or a small family of related substrates). This is the basis of enzyme specificity.
VCAA uses the induced fit model, not the older lock and key model.
- The substrate enters the active site.
- The active site changes shape slightly to mould around the substrate, forming the enzyme-substrate complex.
- The induced fit strains substrate bonds and positions catalytic R groups, stabilising the transition state and lowering activation energy.
- Product(s) form and are released; the enzyme returns to its original shape.
Factors affecting enzyme activity
Temperature. As temperature rises, kinetic energy and successful collisions increase, so rate rises. Above the optimum (around 37 degrees Celsius for human enzymes), the weak bonds that hold tertiary structure break, the enzyme denatures, and rate falls sharply. Denaturation is usually irreversible.
pH. Each enzyme has an optimum pH at which its R groups carry the charges needed for substrate binding and catalysis. Pepsin works near pH 2 (stomach); trypsin near pH 8 (small intestine); most cytosolic enzymes near pH 7. Changes in pH alter ionic and hydrogen bonding within the enzyme, distort the active site, and reduce rate. Extreme pH also denatures the enzyme.
Substrate concentration. At low substrate concentrations, rate rises linearly with substrate because most active sites are empty. As more substrate is added, more active sites are occupied, and the rate approaches a maximum (Vmax) when all active sites are saturated. Beyond this point, adding more substrate has no further effect.
Enzyme concentration. Provided substrate is in excess, rate rises linearly with enzyme concentration because more active sites are available.
Inhibitors
A competitive inhibitor has a shape similar to the substrate and binds the active site. While it occupies the site, the real substrate cannot bind, so rate falls. Adding more substrate displaces the inhibitor and restores Vmax. The apparent Km (substrate concentration for half Vmax) rises.
A non-competitive inhibitor binds an allosteric site (a different site on the enzyme). The enzyme changes shape, distorting the active site so the substrate either cannot bind productively or cannot be catalysed. Adding more substrate does not overcome the inhibition: Vmax is reduced.
Some inhibition is irreversible (for example, heavy metals or organophosphates that form covalent bonds with R groups in the enzyme).
Coenzymes and cofactors
Many enzymes need a non-protein partner to be active.
- A cofactor is an inorganic ion (for example, Mg2+, Zn2+, Fe2+) bound at or near the active site. It often participates directly in catalysis.
- A coenzyme is a small organic molecule, often derived from a vitamin (for example, NAD+, NADP+, FAD, coenzyme A). Coenzymes typically transfer chemical groups, electrons or hydrogen atoms between reactions.
- A prosthetic group is a non-protein partner permanently bound to the enzyme (for example, the haem group in catalase).
In cellular respiration, NAD+ and FAD accept electrons and hydrogen ions in glycolysis and the Krebs cycle and deliver them to the electron transport chain as NADH and FADH2. In photosynthesis, NADP+ is reduced to NADPH in the light-dependent reactions.
Worked example
A student measures the rate of catalase breaking down hydrogen peroxide at three temperatures: 10, 37 and 60 degrees Celsius. Rate is highest at 37 degrees, lower at 10 degrees (less kinetic energy), and almost zero at 60 degrees (enzyme denatured). If the student then adds a competitive inhibitor at 37 degrees, the rate drops, but doubling the substrate concentration mostly restores the original rate. If a non-competitive inhibitor is added instead, doubling the substrate does not restore the rate, because Vmax has fallen.
Common traps
Saying "the enzyme is killed." Enzymes are molecules, not organisms. They are denatured or inhibited.
Confusing optimum with average. The optimum is a single temperature or pH at which rate is highest, not the range over which the enzyme works.
Calling lock and key the current model. VCAA expects induced fit. The lock and key model is only mentioned as a historical contrast.
Mixing up coenzyme and cofactor. Coenzymes are organic (NAD+, FAD, NADP+, coenzyme A). Cofactors are inorganic ions.
Confusing competitive and non-competitive. Competitive: same site as substrate, overcome by more substrate, Vmax unchanged. Non-competitive: allosteric site, not overcome by more substrate, Vmax lowered.
In one sentence
An enzyme is a protein catalyst whose active site uses induced fit to bind a specific substrate and lower activation energy, with reaction rate set by temperature, pH and substrate concentration, modulated by competitive or non-competitive inhibitors, and assisted by coenzymes and cofactors that supply or carry chemical groups.
Past exam questions, worked
Real questions from past VCAA papers on this dot point, with our answer explainer.
2022 VCE3 marksUsing the induced fit model, explain how an enzyme catalyses a specific biochemical reaction.Show worked answer →
A 3-mark answer needs the active site, the induced fit step, and the lowering of activation energy.
- Active site and specificity. An enzyme has a three-dimensional active site whose shape and chemistry are complementary to a specific substrate. Only substrates that fit can bind.
- Induced fit. When the substrate binds, the active site changes shape slightly to wrap around it, forming the enzyme-substrate complex. This brings reactive groups into the correct orientation and strains substrate bonds.
- Lowering activation energy. The induced fit destabilises substrate bonds and stabilises the transition state, so the reaction proceeds at a much lower activation energy. Products are released and the enzyme returns to its original shape, ready for another substrate.
Markers reward explicit naming of induced fit (not lock and key) and an explicit link to activation energy.
2024 VCE3 marksDistinguish between competitive and non-competitive inhibition of enzyme activity.Show worked answer →
A 3-mark answer needs binding site, effect on the active site, and the substrate concentration response.
Competitive inhibitor. Binds at the active site because its shape resembles the substrate. It blocks substrate binding while it is there. Adding more substrate outcompetes the inhibitor, so the maximum reaction rate (Vmax) can still be reached.
Non-competitive inhibitor. Binds at an allosteric site (a different site on the enzyme). This changes the enzyme's overall shape, distorting the active site so the substrate either cannot bind or cannot be catalysed. Adding more substrate does not overcome the inhibition, so Vmax is lowered.
A clear table or labelled diagram is acceptable. Markers reward the binding location, the effect on active site shape, and the substrate concentration response.
2026 VCE2 marksExplain why most human enzymes lose function above 45 degrees Celsius.Show worked answer →
A 2-mark answer needs the bond level affected and the structural consequence.
Heat increases the kinetic energy of the polypeptide. Weak bonds that stabilise tertiary structure (hydrogen bonds, ionic bonds and hydrophobic interactions between R groups) are disrupted. The enzyme denatures: it unfolds, the active site loses its specific shape, the substrate can no longer bind, and catalysis stops. Denaturation is usually irreversible because the protein cannot refold correctly.
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