Effect of temperature on enzyme activity

The temperture of a system is to some extent a measure of the kinetic energy of the molecules in the system. Thus the lower the kinetic energy, the lower the temperature of the system and , likewise, the higher the kinetic energy, the greater the temperature of the system.

Increases in the temperature of a system results from increases in the kinetic energy of the system. This has several effects on the rates of reactions.

1) More energetic collisions

When molecules collide, the kinetic energy of the molecules can be converted into chemical potential energy of the molecules. If the chemical potential energy of the molecules become great enough, the activation energy of a exergonic reaction can be achieved and a change in chemical state will result. Thus the greater the kinetic energy of the molecules in a system, the greater is the resulting chemical potential energy when two molecules collide. As the temperature of a system is increased it is possible that more molecules per unit time will reach the activation energy. Thus the rate of the reaction may increase.

2) The number of collisions per unit time will increase.

In order to convert substrate into product, enzymes must collide with and bind to the substrate at the active site. Increasing the temperature of a system will increase the number of collisions of enzyne and substrate per unit time. Thus, within limits, the rate of the reaction will increse.

3) The heat of the molecules in the system will increase.

As the temperatue of the system is increased, the internal energy of the molecules in the system will increase. The internal energy of the molecules may include the translational energy, vibrational energy and rotational energy of the molecules, the energy involved in chemical bonding of the molecules as well as the energy involved in nonbonding interactions. Some of this heat may be converted into chemical potential energy. If this chemical potential energy increase is great enough some of the weak bonds that determine the three dimensional shape of the active proteins many be broken. This could lead to a thermal denaturation of the protein and thus inactivate the protein. Thus too much heat can cause the rate of an enzyme catalyzed reaction to decrease because the enzyme or substrate becomes denatured and inactive.

 Temperature optimum of an enzyme

Given the above considerations, each enzyme has a temperatuare range in which a maximal rate of reaction is achieved. This maximum is known as the temperature optimum of the enzyme.

In the above figure the temperature optima of three different enzymes is depicted. You should note that the temperature optimum of each enzyme is different.

The Curve in blue might represent an enzyme isolated from a shrimp that normally lives in the cold waters of Alaska. Thus its enzymes have evolved to work best at lower temperatures.

The curve in red might represent that obtained with porcine chymotrypsin.

Curve curve in green might represent the temperature optimum obtained with an enzyme isolated from a bacteria that normally lives in the hot springs of Yellowstone National Park. The enzymes from this bacteria would work best at temperatures that would normally denature enzymes isolated from you or me.

In addition, you should notice that not only are the optimum temperatures different, the shapes of the curves are also different


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