BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 7. ENZYMOLOGY

7.4.Mechanism of Enzyme Action

7.4.2.Stages of Enzymatic Catalysis

Formation of the enzyme-substrate complex. The high Specificity of Enzymes led to the hypothesis proposed in 1890 that the Active Site of an enzyme is complementary to the substrate, matching it like a "lock and key." Following the interaction of the substrate ("key") with the active site ("lock"), chemical Conversion of the substrate into the product takes place. In this model, the active site was viewed as a stable, rigidly predetermined Structure.

In 1959, another variation of the lock-and-key hypothesis was proposed to explain events at the enzyme's active site. According to this hypothesis, the active site is a flexible structure with respect to the substrate. Upon interacting with the enzyme's active site, the substrate induces a conformational change in the site, leading to The formation of an enzyme-substrate complex favorable for chemical Modification of the substrate. Meanwhile, the substrate molecule also undergoes conformational changes, ensuring higher efficiency of the enzymatic reaction. This "induced fit hypothesis" later received solid experimental support.

Sequence of events during Enzymatic Catalysis. The process of enzymatic catalysis can be conventionally divided into the following stages (Fig. 7.12).

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Fig. 7.12. Stages of enzymatic catalysis:

I - stage of approach and orientation of the substrate relative to the enzyme's active site; II - formation of the enzyme-substrate complex (ES) via induced fit; III - substrate deformation and formation of the unstable enzyme-product complex (EP); IV - dissociation of the complex (EP) with the release of reaction products from the enzyme's active site and the recovery of the free enzyme

The first, second, and fourth stages of catalysis are brief and depend on the Substrate Concentration (for The First stage) and the Ligand binding constants at the enzyme's active site (for the first and third stages). Changes in the Thermodynamics of the chemical reaction during these stages are negligible. The Third Stage is the slowest; its duration depends on the activation energy of the chemical reaction. At this stage, bonds within the substrate molecule are broken, new bonds are formed, and the product molecule is synthesized.

The Role of the active site in enzymatic catalysis. Research has established that an enzyme molecule is typically much larger than the substrate molecule undergoing chemical conversion by that enzyme. Only a small portion of the enzyme molecule comes into contact with the substrate—usually 5 to 10 amino acid residues that form its active site. The role of the remaining amino acid residues is to ensure the proper conformation of the enzyme molecule for optimal course of the chemical reaction.

At all stages of enzymatic catalysis, the active site cannot be regarded merely as a passive binding site for the substrate. It is a complex molecular "machine" that employs various chemical mechanisms to facilitate The conversion of the substrate into the product.

Within the enzyme's active site, substrates are positioned such that the reactive Functional groups of the substrates lie directly adjacent to one another. This property of the active site is known as the Proximity and Orientation effect of reactants. Such ordered arrangement of substrates leads to a decrease in Entropy and, consequently, a reduction in the activation energy (E_a), which determines the catalytic activity of enzymes. The enzyme's active site also facilitates the destabilization of interatomic bonds within the substrate molecule, easing the progression of the chemical reaction and product formation. This property of the active site is referred to as the substrate deformation effect (Fig. 7.12).