Question
What are the main factors that affect enzyme activity? Explain the effect of temperature, pH, and substrate concentration on enzyme-catalysed reactions.
Solution — Step by Step
As temperature rises from 0°C to about 37–40°C, enzyme activity increases. Higher kinetic energy means more enzyme-substrate collisions per second, so the reaction speeds up.
But push beyond the optimum temperature and the enzyme denatures — the weak hydrogen bonds and van der Waals forces holding the tertiary structure together break. The active site shape changes permanently, and the substrate no longer fits.
Each enzyme has an optimum pH at which charged amino acid residues in the active site carry the right ionic form to bind substrate. Salivary amylase works best at pH 7, pepsin at pH 2, and trypsin at pH 8.
Shifting pH away from the optimum changes ionisation of -COOH and -NH₂ groups in the active site. This distorts its shape, weakening enzyme-substrate complementarity and reducing the rate.
At low , rate increases almost linearly with substrate concentration — more substrate means more productive collisions with free active sites.
At high , all active sites are occupied. The enzyme is working flat-out (this is ). Adding more substrate does nothing; the rate plateaus. The curve is a rectangular hyperbola, described by the Michaelis–Menten equation.
If substrate is in excess, increasing enzyme concentration increases rate proportionally — you simply have more active sites available.
In most cells, enzyme concentration is carefully regulated rather than left to free variation.
Competitive inhibitors resemble the substrate and block the active site. Adding more substrate can displace them — is unchanged but increases.
Non-competitive inhibitors bind the allosteric site, changing active site shape. More substrate does not help — decreases but stays the same.
Why This Works
Enzymes are biological catalysts — they lower activation energy by orienting substrates correctly in the active site. The active site is a precise three-dimensional pocket shaped by the enzyme’s tertiary structure.
Anything that changes tertiary structure (heat, wrong pH, heavy metal ions) destroys function. Anything that changes how often enzyme meets substrate (temperature within safe range, , ) changes the rate of productive collisions.
The Michaelis–Menten equation captures this neatly:
where is the substrate concentration at half-maximum velocity. A low means the enzyme has high affinity for its substrate.
Alternative Method
For boards, many students use a simple table approach to answer this question under time pressure:
| Factor | Increase effect | Reason |
|---|---|---|
| Temperature (up to optimum) | Rate increases | More kinetic energy → more collisions |
| Temperature (beyond optimum) | Rate drops to zero | Denaturation of enzyme |
| pH (at optimum) | Maximum rate | Ideal ionisation of active site |
| pH (away from optimum) | Rate decreases | Distorted active site |
| (low) | Rate increases | More substrate fills free active sites |
| (high, saturating) | Rate constant () | All active sites occupied |
This table format earns full marks in CBSE 3-mark and 5-mark questions if you explain the reason column clearly.
Common Mistake
Many students write “high temperature increases enzyme activity” without the critical qualifier “up to the optimum.” In exams, this costs you marks. Always specify that beyond the optimum, the enzyme denatures and activity falls to zero — not just slows down. Denaturation is permanent (in most cases), which is why we say the enzyme is “destroyed” rather than “inhibited.”
In NEET and CBSE Class 12, expect a graph-based question showing the bell-shaped temperature curve or the rectangular hyperbola for vs rate. Practice reading these graphs carefully — the examiner often asks “what happens at point X on the graph?” rather than asking you to describe the curve from scratch.