Biochemistry and Molecular Biology - Belyasova N.A. 2002

Structure and Functions of Cellular Components
Proteins. Organizational Features and Enzyme Functions
Properties of Enzymes

When characterizing the METABOLISM/8.html">Properties of Enzymes, THE CONCEPT OF «activity» is primarily employed. Enzyme activity is defined as the quantity 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 (U) and the katal (kat). The international unit of enzyme activity is defined as the amount 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 U.

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

Enzyme activity is heavily dependent 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 steady rise in enzymatic activity, driven by the acceleration of substrate-enzyme complex formation and all subsequent catalytic steps. However, a further increase in temperature is generally accompanied by an increase in inactivated enzyme due to the Denaturation of its protein moiety, which manifests as a decline in activity. Each enzyme is characterized by a temperature optimum—the temperature at which its highest activity is recorded. For plant-derived enzymes, the temperature optimum most commonly lies between 50–60° C, whereas for animal-derived enzymes it is between 40 and 50° C. Enzymes of thermophilic Bacteria exhibit very high temperature optima.

The dependence of enzyme activity on ambient pH values also exhibits a complex nature. Each enzyme has an optimum pH at which it displays maximal activity. Moving away from this optimum in either direction results in decreased 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. Nevertheless, some enzymes exhibit maximal 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, intermediate and final products of metabolism, and Other Compounds can all act as inhibitors and activators. These substances may enter The Cell from the outside or be synthesized within it. In the latter case, this refers to the Introduction/15.html">Regulation of enzyme Activity, an essential link in overall Metabolic Regulation.

Substances that affect 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 summarize certain regularities 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 following its dissociation from the inhibitor. An example is the action of substrate analogs that can bind to the Active Site of the enzyme, preventing the enzyme from interacting with the true substrate. However, an increase in Substrate Concentration leads to the «displacement» of the inhibitor from the active site, and the velocity of the catalyzed reaction is restored (competitive inhibition). Another case of reversible inhibition involves the binding of an inhibitor to the prosthetic group of the enzyme, or the apoenzyme, outside the active site. Examples include the interaction of enzymes with heavy Metal Ions that attach to The sulfhydryl groups of the enzyme's amino acid residues, Protein-Protein Interactions, or Covalent Modification of the enzyme. This type of activity inhibition is termed noncompetitive.

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

Since 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 the 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 derived from the cleavage of nucleoside triphosphates, most commonly ATP).

The number corresponding to a given enzyme Class is embedded in its numerical code (cipher). The enzyme code consists of four numbers separated by periods, designating the enzyme class, subclass, sub-subclass, and serial number within the sub-subclass.