Enzymes

These are compounds that accelerate chemical reactions in the body. Almost all chemical reactions in the body occur with the participation of biological catalysts. If these catalysts were not present in the cells, the chemical reactions would proceed very slowly (several hours, months) or may not occur at all, therefore such cells could not exist. Many enzymes are proteins. The chemical compound that changes their structure is called a substrate. The new compound formed after the reaction is called a product. Enzymes are protein biological catalysts, therefore they have certain general catalyst properties and specific properties: enzymes are neither consumed nor produced during the reaction, they only increase the speed of the reaction without altering its equilibrium. Properties specific to enzymes: they only accelerate a specific chemical reaction and do not produce side reaction products; they are specific to substrates. Some enzymes are strictly specific to a substrate, while others can catalyze the conversion of several compounds. Enzymes are sensitive to changes in temperature and acidity of the environment (pH); enzyme activity is regulated, i.e. it depends on the concentration of certain compounds in the cell. During chemical reactions, the substrate turns into a product. The change in concentration of the substrate or product per unit of time is called the rate of the chemical reaction. A reaction in which one substrate turns into one product is called a first-order reaction, as its rate depends on the concentrations of the substrate or product raised to the first power. There is a substrate half-life period, indicating how long it takes for the initial substrate concentration to decrease by half. Depending on how many substrates are involved in chemical reactions, they are classified into single-substrate, two-substrate, and three-substrate reactions. There are few reactions involving a larger number of substrates. A reaction occurs only when two molecules collide, not only with sufficient energy but also in a favorable spatial orientation. Thus, each reaction is characterized by a certain free energy barrier that only sufficiently active molecules overcome. This state of molecules is called the active, or transitional, state. Molecules in the transitional state have altered chemical bonds or their electronic shell structure is partially changed, so when two such molecules collide, a reaction occurs. At a certain moment in the reaction, only some molecules are in the transitional state, from which they can return to the original, non-active state. The reaction equilibrium constant does not depend on the reaction energy barrier (i.e., activation energy) and speed, but depends on the differences in the initial state of reacting materials and the final state of the products’ free energy. Enzymes reduce the standard free activation energy ∆G°^‡ of reacting materials, i.e. the reaction energy barrier, because they increase the number of substrate molecules with enough energy to reach the transitional state. Therefore, in the presence of enzymes, both the forward and reverse reaction rates increase equally, but the standard free energy change of substrates and products and the reaction equilibrium constant remain unchanged. The way enzymes reduce the energy barrier can be explained based on the expression of the II law of thermodynamics:

∆G°^‡ =∆H°^‡ – T x ∆S°^‡ .

A reaction is thermodynamically considered spontaneous if its ∆G° is negative.

A negative ∆G° value occurs in the following cases:

∆H°^‡ is negative and ∆S° is positive; ∆H° and ∆S° are negative;

∆H° and ∆S° are positive (entropy-driven reactions are rare).

Thus, during enzymatic reactions, both or one of these thermodynamic parameters change. Influence of enzymes on ∆H°^‡: enzymes reduce enthalpy (∆H°^‡) by creating several intermediate substrate states. Each of these states is unstable, so they quickly convert into another and ultimately into the reaction product. The energy of intermediate substrate states is lower than the energy of the transitional state, thus reducing the energy barrier of the enzymatic reaction. Influence of enzymes on ∆S°^‡: the spatial orientation of reacting molecules is crucial for a chemical reaction to occur. For example, the product of a second-order reaction is formed only when molecules occupying a specific spatial position collide. If their spatial position is unfavorable for the reaction, the colliding molecules repel each other, so the product does not form. Enzymes bind substrates in a way that optimizes their spatial orientation for the reaction to occur. Thus, the enzyme reduces the negative value of activation entropy, i.e. ∆S increases.

Source | Glossary of Most Commonly Used Biomedical Terms and Concepts | Lithuanian University of Health Sciences | Academician Professor Antanas Praškevičius, Professor Laima Ivanovienė