How do cellular processes work?
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.
Examples in context
Example 1. Amylase activity in the Australian wheat industry. Grain testing labs at the Australian Export Grains Innovation Centre measure alpha-amylase activity in wheat using the Falling Number test, a standard quality check at Geelong export terminals. Amylase digests starch into maltose; high amylase from pre-harvest sprouting damages flour for bread making. The test heats a flour-water slurry to gelatinise starch, then measures viscosity as amylase digests it. Higher temperatures speed amylase up to its optimum of about 65 degrees C, then denature it above 75 degrees C. Soaked grain shows high amylase activity (low Falling Number) and is downgraded from milling to feed quality. The test is a direct biological assay of enzyme rate.
Example 2. Lactose intolerance and lactase enzyme at Royal Children's Hospital. Lactose intolerance affects about 65 percent of Aboriginal Australians and significant proportions of other ethnic groups. Royal Children's Hospital paediatricians explain that lactose (a disaccharide) cannot be digested without lactase, an enzyme in small-intestine epithelium that hydrolyses lactose into glucose and galactose. Lactase has a specific active site that recognises lactose by shape and bond polarity. Without lactase, bacteria in the colon ferment lactose into hydrogen and acids, causing bloating and diarrhoea. Treatment is dietary lactose avoidance or oral lactase supplements (Lacteeze tablets) taken before dairy meals, illustrating substrate-enzyme specificity.
Try this
Q1. Define an enzyme and explain why each enzyme catalyses only one (or a few) specific reactions. [3 marks]
- Cue. Biological catalyst that lowers activation energy; active site shape and chemistry are complementary to substrate, so only specific substrates bind (induced-fit model).
Q2. An enzyme has a measured rate of 25 units at pH 5, 60 units at pH 7, and 10 units at pH 9. (a) State the optimum pH. (b) Explain in terms of protein structure why activity drops at pH 5 and pH 9. [2+2 marks]
- Cue. (a) pH 7. (b) Extreme pH alters ionisation of side chains in the active site, changing shape; tertiary structure disrupted.
Q3. Refer to enzyme inhibition. (a) Distinguish competitive from non-competitive inhibition. (b) Predict the effect of a competitive inhibitor when substrate concentration is very high. (c) State one Australian drug or pesticide that acts as an enzyme inhibitor and identify the target enzyme. [2+2+2 marks]
- Cue. (a) Competitive binds active site, blocks substrate; non-competitive binds elsewhere, distorts active site. (b) High substrate outcompetes inhibitor; rate approaches uninhibited Vmax. (c) Glyphosate (Roundup) inhibits EPSP synthase in plants; aspirin inhibits cyclooxygenase.
Exam-style practice questions
Practice questions written in the style of VCAA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
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.
2025 VCAA-style2 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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