Metabolism and Energy Transformation in Body Cells - Renata Armenakovna Petrosova 2004

Enzymatic nature of metabolic reactions
Structure of enzymes

Every enzyme has a specific Structure. Like all Proteins, it depends on its Primary Structure, which determines the Tertiary and Quaternary structures—namely, the globule's shape and spatial configuration. Research has shown that enzyme molecules are significantly larger than the molecules of the substances they activate in reactions. Enzymes are generally Globular proteins. Often, they form complexes with non-protein components: metals (such as zinc, iron, manganese, and copper), low-molecular-weight Organic compounds, and Vitamins. For instance, catalase contains iron; vitamin B₃ (or PP) is a component of redox enzymes; and vitamin B₁ is part of enzymes that cleave carbon from organic molecules.

Being proteinaceous in nature, enzymes have high molecular weights ranging from several thousand up to a million. For example, Trypsin has a Molecular Weight of 40,000, whereas higher fatty acid synthetase, which catalyzes the synthesis of Fatty acids, belongs to giant enzymes with a molecular weight exceeding 1,000,000. It consists of an entire assembly line of proteins integrated into a single superstructure. High-molecular-weight enzymes typically feature a quaternary structure. For instance, the enzyme catalase, which breaks down hydrogen peroxide, consists of six subunits.

An enzyme comprises three distinct centers: the substrate-binding, active, and regulatory centers (Fig. 5). Only a small portion of the protein molecule—typically consisting of 3 to 15 amino acid residues—participates directly in the reaction. This is the catalytic or active center of the enzyme. The remaining Amino Acids determine the molecular configuration, bind the substrate, and attach additional ions. The active center is the core part of the enzyme. This is where the substrate is modified, the reaction actually takes place, and the product or products are formed. In some cases, the Functions of the active center are performed by a non-protein component, such as a vitamin, which is bound to the enzyme to form a single integrated unit.

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Fig. 5. Structure of an enzyme: 1 — substrate-binding center; 2 — active center; 3 — regulatory center

The substrate-binding center serves as an "anchor" platform for binding the enzyme to the substrate. Specific bonds form between them, allowing the enzyme to hold the substrate in place. The active and substrate-binding centers of enzymes are frequently located adjacent to each other or overlap.

For these centers to function—that is, to bind the substrate and catalyze the reaction—a specific conformation of the enzyme protein is required. An enzyme maintains its activity as long as the specific configuration of its catalytic center is preserved, which is linked to the tertiary and quaternary structures of the protein.

The configuration of a protein molecule can alter to ensure rapid access of substances to the active center or, conversely, to slow down the reaction. This function is performed by the regulatory center of the enzyme. Inorganic ions and low-molecular-weight substances can attach to it, altering the shape of the enzyme molecule either to facilitate swift binding with the substrate or, conversely, to prevent binding altogether.