BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 7. ENZYMOLOGY

7.4. Mechanism of Enzyme Action

7.4.1. Energetic Changes During Chemical Reactions

The METABOLISM/10.html">Mechanism of enzyme Action can be examined from two Perspectives: in terms of Changes in the energetics of Chemical Reactions and in terms of events occurring within the Active Site. All chemical reactions proceed in accordance with two fundamental Laws of Thermodynamics: the law of conservation of energy and The Second Law of thermodynamics (Entropy). According to these principles, the total energy of a chemical System and Its surroundings remains constant, while the chemical system tends toward decreased order (increased entropy). To understand the energetics of a chemical reaction, it is insufficient merely to know the Energy balance between the initial and final reactants; one must also account for energy fluctuations throughout the course of the reaction and The Role of Enzymes in the dynamics of this process. Let us consider the decomposition reaction of carbonic acid:

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Carbonic acid is a weak acid; its decomposition reaction occurs under standard conditions provided its molecules possess energy exceeding a specific threshold known as the activation energy — E_а (Fig. 7.10).

Fig. 7.10. Free energy Changes during the decomposition of carbonic acid

Activation energy refers to the additional kinetic energy required by reactant molecules to enter into a chemical reaction. Upon overcoming this energy barrier, the molecule undergoes structural changes that trigger the redistribution of chemical bonds and The formation of new compounds. Molecules possessing E_а are said to be in a Transition State. The energy difference between the initial reactant H₂СО₃ and the final products H₂О and СО₂ is termed the Standard Free Energy change of the reaction, ΔG. H₂О and СО₂ molecules are more stable than H₂СО₃, meaning they possess lower energy and practically do not react under normal conditions. The energy released As a result of this reaction is dissipated into the environment as heat.

The greater the number of molecules possessing energy that exceeds the E_а level, the faster the chemical reaction proceeds. Reaction rates can be increased by heating, which raises the energy of the reacting molecules. However, high temperatures are lethal to living organisms; therefore, Cells utilize enzymes to accelerate chemical reactions. Enzymes ensure high reaction rates under the optimal conditions existing within The Cell by lowering the E_а level. Consequently, enzymes reduce the height of the energy barrier, thereby increasing the number of reactive molecules and, ultimately, accelerating the reaction rate.

A crucial aspect of the Enzymatic Catalysis mechanism is the formation of unstable intermediate compounds—the enzyme-substrate complex ES, which undergoes conversion into an unstable transition complex EP that dissociates almost instantaneously into the free enzyme and the reaction product.

Biological catalysts (enzymes) do not alter the Free energy of substrates and products, and therefore do not affect the reaction equilibrium (Fig. 7.11). While functioning as catalysts, enzymes obey the general laws of catalysis and share all properties characteristic of non-biological catalysts; however, they also exhibit distinct properties associated with their specific structural Organization.

Fig. 7.11. Free energy changes during enzyme-catalyzed and uncatalyzed chemical reactions:

E_а — activation energy of the uncatalyzed reaction; E'_а — activation energy of the enzyme-catalyzed reaction.

The enzyme lowers the activation energy E_а, thereby reducing the height of the energy barrier. As a result, the fraction of reactive molecules increases, leading to an accelerated reaction rate.

Similarities between enzymes and non-biological catalysts:

✵ enzymes catalyze thermodynamically feasible reactions;

✵ the total energy of the chemical system remains constant;

✵ the direction of the reaction remains unchanged during catalysis;

✵ enzymes are not consumed in the course of the reaction.

Differences between enzymes and non-biological catalysts:

✵ the Rate of Enzymatic reactions is significantly higher than that of Reactions Catalyzed by non-protein catalysts;

✵ enzymes exhibit high Specificity;

✵ the enzymatic reaction takes place within the cell, i.e., at a Temperature of 37 °C, constant atmospheric pressure, and a physiological pH value;

✵ The rate of an enzymatic reaction can be regulated.