Principles of Biochemistry

8.2.2 Enzyme kinetics

The energy content of individual molecules in a population at constant temperature varies greatly and may be represented by a bell-shaped curve.

Some molecules are very rich in energy, some are very poor, but most have an energy content near a mean. A chemical reaction, such as A and P, takes place because a certain fraction of A molecules at any given instant possesses more internal energy than the rest of the population, sufficient to bring them to the top of the “energy hill” to a reactive form called the transition state .

The activation energy of a reaction is the amount of energy in calories required to bring all the molecules in 1 mol of a substance at a given temperature state at the top of the energy barrier. At this point there is equal probability for them to undergo reaction to from the products or to fall back into the pool of unreacted A molecules.

The rate of any chemical reaction is proportional to the concentration of the transition- state species. Thus the rate of a chemical reaction will be very high if a large fraction of A molecules is in the energy-rich transition state but very low if only a small fraction of A is in the transition state.

There are two general ways in which the rate of a chemical reaction can be increased.

One is to increase the temperature, which increases the thermal motion of the molecules and thus increases the fraction having sufficient internal energy to enter the transition state. Usually the rate of a chemical reaction is approximately doubled by a 10oC rise in temperature.

The second way to accelerate a chemical reaction is to add a catalyst. Catalysts accelerate chemical reactions by finding a lower “pass” over the energy barrier. The catalyst, designated C, combines transiently with the reactant A to produce a new complex or compound CA whose transition state has a much lower activation energy than the transition state of A in the un catalyzed reaction. The catalyst releases the free catalyst, which can then combine with another molecule of A and repeat the cycle. In this way catalysts lower the activation energy of chemical reaction, allowing a much larger fraction of the molecules in a given population to react per unit time than the absence of the catalyst.