Factors Affecting Enzyme Activity and Graphs

Revision notes

Temperature

Increasing temperature initially increases molecular motion and the frequency of potentially productive encounters. Rate rises while the enzyme remains suitably folded. Above a condition-dependent optimum, structural disruption can reduce activity sharply. The optimum is the measured temperature giving the highest rate under the stated assay conditions, not a universal constant of 37 °C.

Low temperature usually slows activity without denaturing the protein. Heating sufficiently can disrupt interactions maintaining the active site; cooling afterwards does not guarantee restoration. Duration of exposure matters, so comparisons should use consistent timing.

pH

pH changes the ionisation of amino-acid side chains and can alter substrate binding, catalysis and structural interactions. Each enzyme has an appropriate pH range. Extreme pH can cause denaturation, but a reversible activity change need not mean the protein has permanently unfolded. Use a suitable buffer when testing other variables, and consider whether the buffer itself affects the reaction.

Substrate concentration

At fixed enzyme concentration, increasing substrate concentration raises rate at low concentrations because productive encounters become more likely. The curve approaches a maximum when available active sites are occupied much of the time. Saturation, not substrate-induced denaturation, explains the standard plateau. A higher substrate concentration cannot indefinitely increase rate when enzyme availability limits it.

Enzyme concentration

With abundant substrate and otherwise identical initial conditions, initial rate is approximately proportional to enzyme concentration. More active sites are available. If substrate becomes limiting or another constraint appears, this relationship can fail. A whole-reaction endpoint may be identical with different enzyme amounts even though initial rates differ.

Read graphs and evaluate

A temperature or pH curve usually has an optimum; a substrate curve usually approaches a plateau; an enzyme-concentration curve is initially linear under suitable conditions. These shapes are not interchangeable. Plot independent variables on the horizontal axis and rate with units on the vertical axis. Show repeats or variability where available, avoid unsupported smooth curves through sparse results and describe both the trend and the mechanism.

Keep enzyme source, volumes, substrate concentration, pH, temperature and measurement timing controlled except for the chosen variable. Use initial rates, not inconsistent observation windows.

Worked application and data skills

Fictional initial rates at 10, 20, 30, 40 and 60 °C are 1, 2, 4, 5 and 0.5 units. The highest measured rate is at 40 °C, but the true optimum could lie between sampled temperatures. Increasing substrate from 1 to 2 units raises rate from 3 to 5; increasing it from 8 to 16 raises rate only from 9.5 to 9.8, suggesting approach to saturation.

Common misconceptions

Low temperature is not usually denaturing. A substrate plateau is not the same as a high-temperature decline. The optimum depends on assay conditions.

Examination tips

Describe the trend with data, then explain it using collisions, active sites, saturation or structural change as appropriate.

Check your understanding

Five original questions. Use Tab to move between controls, arrow keys within a radio group, and Enter or Space on buttons. Feedback appears after Check Answer. Try Again clears the question. Attempts are not saved.

Sources and scope

Scope: Unit 1 C2.3. Original BTECRevision notes, schematics and questions. Independent resource; not endorsed by Pearson.

Official Pearson specification. Scientific review references are listed in the accompanying QA report.