Factors that affect enzyme action

Enzyme activity is profoundly influenced by several environmental and intrinsic factors. Understanding these factors is crucial for investigating the progress of enzyme-catalysed reactions. When studying one factor, it is vital to keep all other variables constant to ensure valid results.

• Temperature

  • Low Temperatures: At low temperatures, enzyme and substrate molecules have less kinetic energy. They move more slowly, leading to a lower frequency of collisions between substrate molecules and the enzyme's active site, resulting in a low rate of reaction. Each collision is also less likely to result in a reaction.

  • Increasing Temperature (up to optimum): As temperature increases, molecules gain more kinetic energy and move faster. This leads to more frequent and energetic collisions between enzyme and substrate molecules, increasing the likelihood of successful reactions and forming more enzyme-substrate complexes, thus speeding up the reaction. For many enzyme-controlled reactions, the rate approximately doubles for every 10°C rise in temperature, a property known as the temperature coefficient (Q10).

  • Optimum Temperature: Every enzyme has an optimum temperature at which it works fastest. For most human enzymes, this is around 37 °C. However, enzymes from other organisms (e.g., bacteria in hot springs) can have much higher optimum temperatures.

  • High Temperatures (above optimum): If the temperature rises too high, the enzyme's molecules vibrate more. This increased vibration breaks some of the bonds (especially hydrogen bonds) that hold the enzyme's specific tertiary structure in shape. This causes the active site to change shape, a process called denaturation. Once denatured, the enzyme and substrate no longer fit together, preventing the formation of enzyme-substrate complexes and causing the reaction to slow down or stop completely. Denaturation is often irreversible.

• pH

  • Optimum pH: All enzymes have an optimum pH value at which they work fastest. Most human enzymes work best around pH 7 (neutral). However, exceptions exist, such as pepsin in the stomach, which has an optimum pH of about 2.

  • Deviation from Optimum pH: Changes in pH (either too high or too low) affect the concentration of hydrogen ions, which can disrupt the ionic bonds and hydrogen bonds that hold the enzyme's tertiary structure in place. This alters the shape of the active site, reducing the chances of the substrate fitting, leading to denaturation and loss of function. Unlike temperature, the effects of pH on the active site are normally reversible if the pH is restored to the optimum, provided the change was not too extreme.

  • Buffer Solutions: To maintain constant pH in experiments, buffer solutions are used, which resist changes in pH.

• Enzyme Concentration

  • The more enzyme molecules present in a solution, the more active sites are available. This increases the likelihood of a substrate molecule colliding with an active site and forming an enzyme-substrate complex, thus increasing the rate of reaction.

  • However, if the amount of substrate is limited, there comes a point where all available substrate molecules are being processed, and adding more enzyme will have no further effect on the reaction rate, as substrate concentration becomes the limiting factor.

• Substrate Concentration

  • At lower substrate concentrations, the rate of reaction increases in direct proportion to the substrate concentration. This is because more substrate molecules mean more frequent collisions with enzyme active sites and more enzyme-substrate complexes are formed.

  • At higher substrate concentrations, the enzyme molecules become saturated with substrate. At this saturation point, all active sites are continuously occupied, and the reaction rate reaches its maximum possible rate, known as Vmax. Adding more substrate beyond this point will not increase the rate further.

  • The Michaelis–Menten constant (Km) is defined as the substrate concentration at which the reaction rate is half of Vmax (½Vmax). Km is a measure of the enzyme's affinity for its substrate; a lower Km indicates a higher affinity.

• Enzyme Inhibitors

  • Inhibitors are molecules that bind to enzymes and reduce their activity.

  • Competitive Inhibitors: These molecules have a similar shape to the enzyme's normal substrate and compete to bind to the active site, thereby blocking substrate binding. The degree of inhibition depends on the relative concentrations of the inhibitor and substrate. Increasing the substrate concentration can overcome competitive inhibition as the substrate out-competes the inhibitor for the active site.

  • Non-competitive Inhibitors: These molecules have a different shape from the substrate and bind to the enzyme at a site other than the active site (often an allosteric site). This binding causes a change in the active site's shape, preventing proper substrate binding and catalytic activity. Increasing substrate concentration will not reverse the inhibition because the inhibitor is not competing for the active site