Enzymes change the route, not the destination
A chemical reaction requires reactants to reach a high-energy transition state. The activation energy is the energy barrier between reactants and that state. Enzymes provide an alternative reaction pathway with a lower activation energy, so a larger fraction of collisions can produce products at a given temperature.
An enzyme does not make an energetically impossible reaction favourable, supply energy to the products or change the position of equilibrium. It accelerates forward and reverse reactions and helps the system reach equilibrium sooner.
The active site is chemically precise
An active site is a three-dimensional region formed by amino acid side chains that may be far apart in the primary sequence. Shape, charge, polarity and temporary interactions determine which substrates bind and how they are oriented.
The induced-fit model recognises that an enzyme is flexible. Substrate binding can shift the active site's shape, placing catalytic groups in better positions and stressing particular substrate bonds. Specificity is therefore chemical and dynamic, not simply a rigid lock-and-key outline.
Substrate collides with the active site.
Specific interactions form an enzyme–substrate complex.
The transition state is stabilised.
Products leave; the enzyme is reusable.
Why rate rises and then levels off
At low substrate concentration, adding substrate increases the frequency of successful active-site encounters. As concentration rises, more active sites are occupied. Eventually almost every enzyme molecule is working whenever possible, and the reaction approaches its maximum rate.
Adding more substrate beyond saturation has little effect. Adding more enzyme can raise the maximum rate if sufficient substrate is available. This distinction is central to graph-based questions.
| Factor | Initial effect | Limit or decline |
|---|---|---|
| Substrate concentration | More enzyme–substrate collisions | Active sites become saturated |
| Enzyme concentration | More active sites available | Substrate becomes limiting |
| Temperature | Faster molecular motion | Excess heat disrupts structure |
| pH | Changes ionisation and interactions | Extreme pH alters active-site structure |
Temperature and pH act through structure
Increasing temperature initially raises kinetic energy and collision frequency. Beyond an optimum range, thermal motion disrupts weak interactions that maintain the enzyme's tertiary structure. The active site loses the arrangement required for catalysis: the enzyme is denatured.
pH changes the ionisation of amino acid side chains. That can alter substrate binding, catalytic chemistry or the ionic interactions stabilising protein structure. Different enzymes have different pH optima because their environments and active-site chemistry differ.
Competitive and non-competitive inhibition
A competitive inhibitor binds at the active site and competes with substrate. Increasing substrate concentration can reduce its effect because substrate molecules occupy a greater fraction of active sites. The same maximum rate may be reached, but more substrate is required.
A pure non-competitive inhibitor binds elsewhere and reduces the proportion of functional enzyme molecules. Adding more substrate cannot fully restore the original maximum rate. Real enzymes can show mixed or irreversible inhibition, but MDCAT comparisons usually focus on these core patterns.
Active-site binding; overcome by sufficiently high substrate.
Binding elsewhere; maximum rate is reduced.
Worked reasoning: distinguish two rate changes
At one low substrate concentration, both competitive and non-competitive inhibition can reduce reaction rate. That single reading does not identify the mechanism. Measure a range of concentrations while keeping enzyme amount, temperature and pH constant. An untreated preparation provides the comparison.
In ideal competitive inhibition, sufficiently high substrate can approach the untreated Vmax. In pure non-competitive inhibition the maximum remains lower. Do not generalise that every inhibitor binding away from the active site leaves Km unchanged: mixed inhibition is more general. Measure initial rates to reduce complications from substrate depletion and product accumulation. The evidence is the pattern across a curve, not one slow reaction.
The ideas to carry forward
- Enzymes lower activation energy through an alternative pathway.
- Active sites depend on three-dimensional chemistry and induced fit.
- Saturation limits rate at high substrate concentration.
- Competitive inhibition differs from non-competitive inhibition in site and kinetic effect.
Answer first. Then reveal the marking logic.
01Why does an enzyme not change the equilibrium position of a reversible reaction?2 marks · show the biological link
Answer: It lowers the activation barrier for both forward and reverse directions, so equilibrium is reached faster without changing the relative energies of reactants and products.
02Which observation supports competitive inhibition?2 marks · show the biological link
Answer: The inhibited reaction can approach the original maximum rate when substrate concentration becomes sufficiently high.
03Can equal rates imply different mechanisms?2 marks · show the biological link
Answer: Yes. Different kinetic parameters can produce the same rate at one concentration.
04Why include an untreated control?2 marks · show the biological link
Answer: It establishes activity under the same conditions without inhibitor.
05Does doubling enzyme double Km?2 marks · show the biological link
Answer: No. In the simple model Vmax doubles while Km stays constant.
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