Biochemical Foundations of Human Vital Activity - Volkov N.I., Nesen E.N. 2000

Biochemical Foundations of Human Vital Activity
Enzymes — biological catalysts
Properties of Enzymes

Enzymes are characterized by high catalytic Specificity, reaction specificity, and activity regulation.

Due to the high catalytic activity of enzymes, The rate of certain Chemical Reactions increases by millions of times. An example is the reversible Synthesis and Breakdown of carbonic acid, catalyzed by the enzyme Carbonic anhydrase: СО₂ + Н₂О ⇄ Н₂СО₃. A single erythrocyte carbonic anhydrase molecule can bind up to 10⁵ СО₂ molecules per second, accelerating the reaction rate by 10⁷ times in the presence of the enzyme. The synthesis of Н₂СО₃ occurs continuously in body Tissues during the binding of metabolic СО₂, whereas The breakdown of Н₂СО₃ takes place intensively in the pulmonary capillaries, where СО₂ is eliminated from the body upon exhalation.

Catalase exhibits exceptionally high activity, catalyzing the breakdown of hydrogen peroxide, a substance toxic to the Organism: 2Н₂О₂ → 2Н₂О + О₂. At a Temperature of 0 °С, a single catalase molecule decomposes 40,000 molecules of Н₂О₂ per second.

The reaction specificity of enzymes lies in their ability to catalyze The conversion of a specific substrate or act on a particular type of chemical bond within it. This enables numerous chemical reactions to proceed simultaneously within The Cell in a strictly defined order. Enzymes are classified according to absolute, relative, stereochemical, and group specificity. Absolute specificity is manifested when an enzyme catalyzes the conversion of molecules of only a single substrate. For instance, the enzyme arginase is capable of catalyzing the breakdown of Arginine alone into urea and Ornithine, whereas sucrase, maltase, and lactase break down only their respective Disaccharides. Relative specificity characterizes enzymes that catalyze the Cleavage of a specific type of chemical bond in molecules of different substances, for which the precise Structure OF THE substrate molecule is not critically important. Relative specificity is typical of digestive peptidases (Pepsin, Trypsin, Chymotrypsin), which cleave peptide bonds in various Proteins AND Peptides, as well as Phosphatases and lipases, which cleave ester bonds in various molecular structures. Stereochemical substrate specificity represents the highest degree of enzymatic specificity, wherein enzymes act on only one of several optical isomers of a substrate. For example, specific enzymes catalyze the conversion exclusively of the D-isoform of glucose and have no effect on its L-isoform. Group specificity is characteristic of enzymes acting on substrates sharing a common bond type and similar molecular structure. Thus, cholinesterases cleave the ester bond in numerous substrates containing a Choline moiety.

The Introduction/15.html">Regulation of enzyme Activity provides finer control over the rate and direction of metabolic processes. This ensures the constancy of the body's internal environment and the adaptation of METABOLISM to environmental conditions. Regulatory mechanisms operate both at the level of already functioning cellular enzymes and at the level of their synthesis. The activity of existing Enzymes can be altered through various modifications of their molecules—such as phosphorylation and dephosphorylation (the attachment and detachment of a phosphoric acid residue), proteolysis (the cleavage of a protein segment by specific agents), and feedback regulation, wherein the end product of a metabolic pathway inhibits the activity of The enzyme catalyzing its initial step (these topics are discussed in greater detail in Chapter 13).

The regulation of enzyme synthesis occurs at the Gene level and at the level of Transcription (mRNA synthesis). Such alterations are observed under the long-term influence of various environmental factors, physical exertion, and Steroid Hormones.

Processes of bodily adaptation to physical exertion are closely intertwined with the refinement of various mechanisms regulating enzyme activity. During immediate adaptation to Physical Exercise, the activity of pre-existing enzymes changes. In long-term adaptation, Protein Synthesis is enhanced within the organism, leading to an increased quantity of enzymes. Such modifications improve the adaptive capacity of metabolism. Because enzyme activity is adjustable, it is possible to modulate specific links in the body's metabolism, which represents a crucial challenge in modern medicine and sports science.