Temperature and pH Effects on Enzymes
Enzymes are biological catalysts whose activity is highly sensitive to their environment. For the ESAT, you must understand how temperature and pH changes alter the rate of enzyme-catalysed reactions by affecting the enzyme's structure and the kinetic energy of molecules.
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- As temperature increases from a low value, enzyme activity increases because substrate and enzyme molecules have more kinetic energy, leading to more frequent successful collisions.
- Every enzyme has an optimum temperature at which its activity is maximal. For most human enzymes, this is around 37°C.
- Above the optimum temperature, the enzyme starts to denature. The high thermal energy breaks bonds holding the protein in its specific 3D shape, permanently changing the active site and rapidly decreasing the reaction rate.
- Each enzyme also has an optimum pH. Deviations from this pH alter the charges on the amino acids in the active site, disrupting the enzyme-substrate interaction and reducing the reaction rate.
- Extreme pH values, far from the optimum, cause irreversible denaturation, just as high temperatures do. The bonds maintaining the enzyme's tertiary structure are broken.
- The effect of low temperature is generally reversible. If the temperature is returned to the optimum, the enzyme will regain its function, whereas denaturation by high temperature or extreme pH is permanent.
Formulae
Rate of reaction = (Amount of product formed) / time OR Rate of reaction = (Amount of substrate used) / time To calculate the rate of an enzyme-catalysed reaction from experimental data, often presented in graphs or tables. Units might be, for example, cm3/s or g/min.
Definitions
- Optimum Temperature/pH
- The specific temperature or pH value at which an enzyme exhibits its maximum rate of catalytic activity.
- Denaturation
- The irreversible change in the specific three-dimensional structure of a protein (like an enzyme), particularly the active site, caused by factors such as extreme heat or pH. This results in a loss of biological function.
- Active Site
- The specific region of an enzyme with a unique shape where the substrate molecule binds and the chemical reaction is catalysed.
Worked example
The graph below shows the activity of two different digestive enzymes, Pepsin and Trypsin, at various pH levels. At which pH is the rate of reaction for Trypsin exactly 50% of its maximum rate, while Pepsin has almost no activity? Assume the maximum rate for Trypsin is 10 arbitrary units.
- 1
Step 1:
Identify the maximum rate for Trypsin.
The prompt states this is 10 arbitrary units.
This corresponds to the peak of the Trypsin curve, which occurs at pH 8.
- 2
Step 2:
Calculate 50% of Trypsin's maximum rate.
50% of 10 units is 10 × 0.5 = 5 arbitrary units.
- 3
Step 3:
Locate this rate on the y-axis (Rate = 5) and find the corresponding pH value(s) on the Trypsin curve.
Following the line for Rate = 5 across to the Trypsin curve, we see it intersects at two points:
one on the rising slope and one on the falling slope.
- 4
Step 4:
Read the pH values for these intersection points from the x-axis.
The intersections occur at approximately pH 7 and pH 9.5.
- 5
Step 5:
Check the activity of Pepsin at these pH values.
The Pepsin curve shows a rate of essentially zero at both pH 7 and pH 9.5.
- 6
Step 6:
Select the answer that fits all conditions.
Both pH 7 and pH 9.5 are possible answers.
However, ESAT questions usually have a single correct option provided.
If 'pH 9.5' were an option, it would be a correct choice based on the graph.
Answer: pH 9.5 (or pH 7, depending on the options provided in a multiple-choice context)
Common mistakes
- ×Forgetting that low temperature only inactivates enzymes temporarily (lower kinetic energy), whereas high temperatures cause permanent denaturation. A question might ask what happens if an enzyme is cooled down and then warmed back to its optimum.
- ×Misinterpreting graphs with multiple curves. Forgetting which line corresponds to which enzyme or condition, or mixing up the x and y axes.
- ×Confusing the shapes of pH and temperature curves. While both are often 'bell-shaped', the reason for the decline in rate at low temperatures (less kinetic energy) is different from the decline at low pH (disruption of active site charges/denaturation).
- ×Assuming all enzymes have the same optimum conditions. For example, assuming the optimum pH is 7 or the optimum temperature is 37°C for an enzyme that works in the stomach (like pepsin, optimum pH ~2).
No-calculator tips
- ✓When asked to find an 'optimum' value from a graph, find the peak of the curve first by eye, then carefully read the corresponding value from the axis.
- ✓To find a percentage of a maximum rate (e.g., 50%), find the peak value on the y-axis, mentally calculate the required percentage, and then trace a horizontal line from that new value across to the curve.
- ✓When comparing two enzymes, focus on key points: where they cross, their peak values (optimums), and the range of conditions over which they are active.