The activation energy barrier is also known as the threshold energy barrier. This term was introduced in 1889 by S. Arrhenius. From a biochemical point of view, it can be defined as the minimal amount of energy that is necessary to launch a chemical reaction, in other words, the energy that has to be overcome for the reaction to take place. A chemical reaction, for example, A→P is proceeding because a certain fraction of “A” molecules at any moment possesses a greater amount of energy than the rest of the molecules and this energy is sufficient enough to make the substance perform a transition into its active state. Such a state provides the possibility of breakage or formation of a new chemical bond, which leads to the synthesis of product “P”. The activation energy can be expressed in kilojoules or calories and it represents the energy that is needed for all molecules of one mole of the substance to be transferred to their active state under the conditions of a certain temperature and pH level. Any chemical reaction has a transitional state, which is characterized by a high level of available energy and can be described as a state of interacting molecules that corresponds to the peak of the activation barrier. The reaction speed is proportional to the concentration of the molecules, which are in the transitional state. As the temperature rises, the energy of thermal molecule movement increases. As a result, the number of molecules that can reach the transitional state goes up as well. In many reactions, if the temperature is increased by 100C, the speed of the reaction doubles. Enzymes can also lower the threshold energy barrier by destabilizing the ground state of the substance or stabilizing the transition state. Destabilization can be carried out by the enzyme that interacts with the substance and destabilizes it via electrostatic interactions, geometric interactions, or desolvation. Catalysts speed up chemical reactions, lowering the available activation energy. The bonding of reactants with catalysts leads to a new transitional state that is described by lower available energy, comparing to the transitional state of non-catalyzed reaction. The formation of reaction products is accompanied by restoring of free catalysts.
The above fundamentals of chemical reaction kinetics apply to enzymatic reactions as well. However, such reactions have one particular difference that is not common to non-enzymatic reactions. This difference is known as substrate saturation. With low substrate concentration, the reaction rate increases proportionally to the substrate concentration. However as the concentration of the substrate increases, the reaction rate grows slower and slower, and the balance is disrupted. With a further increase of substrate concentration, the reaction rate becomes constant and does not depend on substrate concentration any longer. At this, the enzyme is becoming saturated by the substrate, and the concentration of the enzyme becomes the limiting factor for the reaction. Although the effect of saturation is common to all enzymes, certain amounts of substrate concentration under which the saturation can occur may vary significantly for various enzymes.
Sometimes the rate of a reaction may lower with increasing temperature. This occasion results in a negative value of activation energy Ea. Such reactions are usually without a barrier, and the proceeding of the reaction depends on the molecule’s capture in a potential well. If the temperature rises, the probability that colliding molecules will capture one another decreases. This situation may not be directly interpreted as the potential energy barrier height.
As such, activation energy can be described as a potential barrier’s height that separates two minima of the reactant’s and the reaction product’s potential energy. For the reaction to proceed at a certain rate, there must be a sufficient number of molecules, the energy of which is equal to or higher than the activation energy.
References
Campbell, Neil, and Reece Jane. Biology. Benjamin Cummings: 7th edition, 2004.