Factors Affecting the Rate of Enzyme Reactions

What does the rate of an enzyme reaction mean?

Enzymes catalyse reactions when substrate molecules collide successfully with their active sites. An enzyme–substrate complex forms, the reaction occurs and the products leave. The enzyme is unchanged and can catalyse another reaction.

The reaction rate describes how much change occurs per unit time. It can be measured from the amount of product formed or substrate used:

$\text{rate}=\frac{\text{amount of product formed}}{\text{time}}$

or

$\text{rate}=\frac{\text{amount of substrate used}}{\text{time}}$

The units depend on the measurement, such as $\text{cm}^3\text{/s}$ for gas production. If an investigation records the time taken to reach the same endpoint each time, rate can be represented by $1/\text{time}$, with units of $\text{s}^{-1}$. A shorter time therefore means a faster reaction.

Temperature changes movement and enzyme shape

At a low temperature, enzyme and substrate molecules have relatively little kinetic energy. They move slowly and collide less frequently, so fewer enzyme–substrate complexes form each second and the reaction is slow.

Raising the temperature gives the molecules more kinetic energy. Collisions occur more frequently, and a greater proportion have enough energy for a reaction, so the rate rises towards an optimum temperature.

Above the optimum, heat disrupts bonds that maintain the enzyme's three-dimensional shape. The active site changes shape and is no longer complementary to the substrate. Fewer enzyme–substrate complexes can form, so the rate falls rapidly. The enzyme has been denatured, a permanent structural change.

Cooling an enzyme is different from denaturing it: low temperature usually slows molecular movement without permanently changing the active site. Warming it again can restore the rate. Optimum temperatures vary between enzymes, so 37 °C is not a universal optimum.

pH affects the active site

Each enzyme has an optimum pH at which its active site has the best shape for binding its substrate. Moving above or below this optimum reduces the rate. Conditions far from the optimum can disrupt bonds within the protein, changing the active site's shape and denaturing the enzyme.

A graph of rate against pH therefore usually has a peak around the optimum. Different enzymes can have different optimum pH values because they work in different chemical environments.

Substrate concentration can make active sites limiting

At a low substrate concentration, relatively few substrate molecules are available. Increasing the concentration puts more substrate particles into the same volume, producing more frequent collisions with active sites. More enzyme–substrate complexes form each second, so the rate increases.

Eventually, substrate molecules occupy the available active sites almost continuously. The enzymes are working as fast as the conditions allow, so adding still more substrate has little effect and the graph reaches a plateau. Enzyme concentration has become the limiting factor.

Enzyme concentration changes the number of active sites

Increasing enzyme concentration provides more active sites. If plenty of substrate is available, more enzyme–substrate complexes can form at the same time, so doubling the enzyme concentration can double the initial rate.

This proportional increase cannot continue if the substrate supply is fixed. Eventually there are too few substrate molecules to occupy the additional active sites, and the graph levels off because substrate concentration has become limiting. Thus the effect of one factor depends on the values of the others.

Comparing the four factors

Temperature and pH can alter the enzyme itself, so their graphs usually show a maximum at an optimum followed by a decrease. In contrast, increasing substrate or enzyme concentration initially raises the frequency of enzyme–substrate complex formation. Their graphs level off when the other reactant becomes limiting; increasing concentration does not normally denature the enzyme.

To interpret any graph, first identify the factor on the horizontal axis and the reaction rate on the vertical axis. Then describe the pattern and explain it using collisions, active-site availability, limiting factors or denaturation. Other variables must be controlled when one factor is investigated, because changing several conditions would make the cause of a rate change unclear.

Rate

$\text{rate}=\frac{\text{amount of product formed or substrate used}}{\text{time}}$

For a fixed endpoint, $\text{rate}\propto 1/\text{time}$.

Temperature

• Low temperature: less kinetic energy, fewer successful collisions, slower rate.
• Rising temperature: rate increases to the optimum.
• Above optimum: active site changes shape; enzyme is denatured and rate falls rapidly.
• Low temperature does not usually denature an enzyme.

pH

• Each enzyme has an optimum pH.
• Moving away from it reduces the rate.
• Extreme pH can change the active site's shape and denature the enzyme.

Substrate concentration

• Initially: more substrate means more frequent collisions and a faster rate.
• Plateau: active sites are occupied; enzyme concentration is limiting.

Enzyme concentration

• With excess substrate: more enzymes provide more active sites, so rate rises and may be directly proportional to enzyme concentration.
• With a fixed substrate supply: the rate eventually plateaus because substrate becomes limiting.

Graph distinction

Temperature and pH usually give peaked curves. Substrate concentration rises then plateaus. Enzyme concentration is initially linear and may plateau if substrate becomes limiting.