Biochemistry and Molecular Biology - Belyasova N.A. 2002

Structure and Function of Cellular Components
Proteins: Structural Organization and Enzyme Functions
Properties of Enzymes

When characterizing the METABOLISM/8.html">Properties of Enzymes, THE CONCEPT OF “activity” is primarily used. Enzyme activity is defined as The amount of enzyme that catalyzes The conversion of a specific amount of substrate per unit of time. Two alternative units are used to express The activity of enzyme preparations: the international unit (IU) and the katal (kat). The international unit of enzyme activity is defined as the amount of enzyme that catalyzes the conversion of 1 μmol of substrate into product per 1 min under standard (typically optimal) conditions. One katal represents the amount of enzyme that catalyzes the conversion of 1 mol of substrate per 1 s. 1 kat = 6∙10⁷ IU.

Enzyme preparations are frequently characterized by their specific activity, which reflects the degree of Enzyme Purification. Specific activity is the number of units of enzyme activity per 1 mg of protein.

Enzyme activity depends heavily on environmental conditions, among which Temperature and ambient pH are of paramount importance. An increase in temperature within the 0–50°C range typically leads to a gradual rise in enzymatic activity, which is associated with the acceleration of substrate-enzyme complex formation and all subsequent catalytic steps. However, further temperature increases are generally accompanied by a rise in the amount of inactivated enzyme due to the Denaturation of its protein moiety, resulting in a decline in activity. Each enzyme is characterized by an optimum temperature, which is the temperature at which its highest activity is recorded. Most commonly, the temperature optimum for plant-derived enzymes lies within 50–60°C, whereas for animal enzymes it falls between 40 and 50°C. Enzymes from thermophilic Bacteria exhibit very high temperature optima.

The dependence of enzyme activity on ambient pH also has a complex character. Each enzyme has an optimum pH at which it exhibits maximum activity. Deviating from this optimum in either direction leads to a decrease in enzymatic activity. This is explained by alterations in the state of the enzyme's Active Site (decreased or increased ionization of functional groups) as well as the Tertiary Structure of the entire protein molecule, which depends on The ratio of cationic and anionic centers within it. Most enzymes have a pH optimum in the neutral range. However, some enzymes exhibit maximum activity at pH 1.5 (Pepsin) or 9.5 (arginase).

Enzyme activity is subject to significant fluctuations depending on The Influence of inhibitors (substances that decrease activity) and activators (substances that increase activity). Metal cations, certain anions, Phosphate Group Carriers, reduction equivalents, specific Proteins, metabolic intermediates and end products, among others, can act as inhibitors and activators. These substances may enter The Cell from the outside or be synthesized within it. In the latter case, we speak of the Introduction/15.html">Regulation of enzyme Activity, an essential component of overall Metabolic Regulation.

Substances affecting enzyme activity can bind to the active and allosteric sites of the enzyme, as well as outside these sites. Specific Examples of such phenomena will be discussed in Chapters 7–19. To generalize certain patterns of Enzyme Inhibition, it should be noted that these phenomena generally fall into two types: reversible and irreversible. During reversible inhibition, no permanent changes are introduced into the enzyme molecule after its dissociation from the inhibitor. An example is the action of substrate analogs, which can bind to the Active Site of the enzyme, preventing the enzyme from interacting with the true substrate. However, increasing the Substrate Concentration leads to the “displacement” of the inhibitor from the active site, restoring The rate of the catalyzed reaction (competitive inhibition). Another case of reversible inhibition is the binding of an inhibitor to the prosthetic group of the enzyme, or apoenzyme, outside the active site. Examples include the interaction of enzymes with heavy Metal Ions that attach to The sulfhydryl groups of amino acid residues of the enzyme, Protein-Protein Interactions, or Covalent Modification of the enzyme. This type of activity inhibition is called non-competitive.

Irreversible inhibition is in most cases based on the binding of so-called “suicide substrates” to the active sites of enzymes. In this process, covalent bonds are formed between the substrate and the enzyme, which cleave very slowly, leaving the enzyme unable to perform its function for a prolonged period. An example of a “suicide substrate” is the antibiotic penicillin (Chapter 18, Fig. 18.1).

Because enzymes are characterized by reaction Specificity, they are classified According to the type of reaction they catalyze. According to the currently accepted Classification, enzymes are grouped into 6 classes:

1. Oxidoreductases (oxidation-reduction reactions).

2. Transferases (Reactions Involving the transfer of functional groups between substrates).

3. Hydrolases (Hydrolysis reactions, where a Water molecule serves as the acceptor of the transferred group).

4. Lyases (reactions involving the non-hydrolytic Cleavage of groups).

5. Isomerases (isomerization reactions).

6. Ligases, or synthetases (synthesis reactions driven by The energy released from the cleavage of nucleoside triphosphates, most commonly ATP).

The number of the corresponding enzyme Class is embedded in its numerical code (systematic name). The enzyme code consists of four numbers separated by dots, denoting the enzyme class, subclass, sub-subclass, and serial number within the sub-subclass.