C.7.6

=The Effect of Temperature, pH and Heavy Metal Ions on Enzyme Activity=

Enzyme action depends on its shape, and so the effect of temperature, pH, and heavy metal ions on enzymes depends on how these factors effect the shape of the enzyme.

Temperature
Increase in temperature causes **initial increase** in **rate** of reaction up to a **maximum** or **optimum temperature**, because **more reactant molecules will have energy greater than the activation energy** and the **frequency of collisions increases**. The maximum or optimum temperature for enzymes is **up to 40 °C**.

longer functions as a catalyst, because it has become **denatured (or destroyed**). The high temperatures **break down hydrogen bonds holding the protein structure** in place causing the **active site** to be **altered**. Every enzyme has a temperature range of optimum activity.
 * Above 40°C**, the **rate** of reaction rapidly **decreases** until the reaction stops. At this point the enzyme no



As the pH increases, the rate of reaction increases from zero until it reaches a maximum. The pH value that gives the maximum rate of reaction is called the **optimum pH**. Above the optimum pH the rate of reaction falls rapidly back to zero.
 * pH Value**

Enzymes are electrically charged because R-groups may ionize when they dissolve in water. Different pH values cause differences in charges on the amino acid residues, affecting the bonds between them, and changing the tertiary structure. The pH therefore determines the changes in the enzyme. Enzymes will function only at given pH's depending on their electrical charge. Most enzymes function best at pH values close to neutral (when the pH is 7). They don't function well if if the pH value is at either extreme. Change in the pH will affect the polar and non-polar intramolecular attractive and repulsive forces and alter the shape of the enzyme and the active site as well to the point where the substrate molecule could no longer fit, and the chemical change would be inhibited from taking place as efficiently or not at all.The diagram below illustrates the general relationship between pH and enzyme activity; however, note that some enzymes show little similarity to the diagram.



Heavy metal ions poison enzymes; they have the ability of inhibiting enzyme activity completely. They react with -SH groups, replace the hydrogen atom with a heavy metal ion, and change the tertiary structure.
 * Heavy Metal Ions**

These three factors have the effect of changing the tertiary structure of the enzymes, rendering it useless to catalisize reactions.