ENZYMES

 

 Enzymes

Enzymes are biocatalyst that increases the rate of chemical reactions occurring in the living body. They make a chemical reaction millions of times faster than it would have been without it. They are protein in nature (exception – RNA acting as ribozyme), colloidal and thermolabile in character, and specific in their action. The term enzyme was coined by a German physiologist, Wilhelm Kühne, in 1878.

Nomenclature of Enzymes

In the early days, the enzymes were given names by their discoverers in an arbitrary manner. For example, the names pepsin, trypsin, and chymotrypsin convey no information about the function of the enzyme or the nature of the substrate on which they act. Some old names indicate the source but not the action, e.g., papain from Papaya, bromelain from Pineapple of the family Bromeliaceae. Sometimes, the suffix-ase was added to the substrate for naming the enzymes, e.g., lipase acts on lipids; nuclease on nucleic acids; lactase on lactose. These are known as trivial names of the enzymes, which, however, fail to give complete information about the enzyme reaction (type of reaction, cofactor requirement, etc.)

In a modern system, enzyme names are given after:

(i) Substrate acted upon, e.g., sucrase (after sucrose), lipase, proteinase, nuclease, peptidases, maltase

(ii) Chemical reaction, e.g., dehydrogenase, oxi­dase, carboxylase, decarboxylase, etc.

The second categories of names are group names. They are often qualified by the addition of the name of substrate, e.g., succinic dehydroge­nase, isocitrate dehydrogenase, glutamate-pyruvate transaminase, DNA polymerase.

Thus, DNA polymerase catalyzes the synthesis of DNA segments through polymerization of deoxyribonucleotides. Similarly, glutamate-pyruvate transaminase transfers the amino group (—NH2) from glutamate to pyruvate.

Classification of Enzymes

The International Union of Biochemistry (IUB) appointed an Enzyme Commission in 1961. This committee made a thorough study of the existing enzymes and devised some basic principles for the classification and nomenclature of enzymes. Since 1964, the IUB system of enzyme classification has been in force. Enzymes are divided into six major classes (in that order). Each class on its own represents the general type of reaction brought about by the enzymes of that class.

Oxidoreductases:
They take part in oxidation and reduction reactions or the transfer of electrons.
Lactate + NAD⁺ → Pyruvate + NADH + H⁺ (Enzyme-Lactate dehydrogenase)
Oxidoreductases are of three types— oxidases, dehydrogenases, and reductases, e.g., cytochrome oxidase (oxidises cytochrome), succinate dehydrogenase, nitrate reductase.

Transferases:
They transfer a group from one molecule to another, e.g., glutamate-pyruvate transaminase (transfers the amino group from glutamate to pyruvate during the synthesis of alanine). The chemical group transfer does not occur in the free state.
Glucose + ATP → Glucose-6-phosphate (G6P) + ADP (Enzyme- Hexokinase)

Hydrolases:
They catalyze hydrolysis of bonds like ester, ether, peptide, glycosidic, С-С, С halide, P—N, etc., which are formed by dehydration condensation. Hydrolases break up large molecules into smaller ones with the help of hydrogen and hydroxyl groups of water molecules. The phenomenon is called hydrolysis. Digestive enzymes belong to this group, e.g., amylase (hydrolysis of starch), sucrase, and lactase.
Sucrose + H₂O → Glucose + Fructose (Enzyme-Sucrase, Invertase)

d. Lyases:
The enzymes cause cleavage, removal of groups without hydrolysis, addi­tion of groups to double bonds, or removal of a group producing a double bond, e.g., histidine decarboxylase (breaks histidine to histamine and CO2), aldolase (fructose-1,6-diphosphate to dihydroxy acetone phosphate and glyceraldehyde phosphate).
Pyruvate → Acetaldehyde + CO₂ (Enzyme-pyruvate decarboxylase)

e. Isomerases:
The enzymes cause rearrangement of molecular structure to effect isomeric changes. They are of three types: isomerases (aldose to ketose group or vice-versa, like glucose 6-phosphate to fructose 6-phosphate), epimerases (change in position of one constituent or carbon group, like xylulose phosphate to ribulose phosphate), and mutases (shifting the position of a side group, like glucose-6-phosphate to glucose-1-phosphate).
Glucose-6-phosphate ⇌ Fructose-6-phosphate (Enzyme-Phosphofructokinase)

f. Ligases:
The enzymes catalyze the bonding of two chemicals with the help of energy obtained from ATP, resulting in the formation of such bonds as С-О, С-S, С-N, and P-O, e.g., pyruvate carboxylase. It combines pyruvic acid with CO2 to produce oxaloacetic acid.
Glutamate + NH₃ + ATP → Glutamine + ADP + Pi (Enzyme-Glutamine synthetase)


Characteristics of enzymes

Highly specific- active site of an enzyme shows a strong affinity for a specific substrate
Extremely efficient- transforming about 100 to 10,000 substrate molecules into product per second and proceeding from 103 to 108 times faster than the uncatalyzed reaction.
Stable structure- Enzymes themselves remain unchanged during the reaction.
Require activators or coenzymes- Sometimes, activators and coenzymes are necessary for enzymatic catalysis.
Optimum temperature- The effectiveness of an enzyme catalyst could be maximized at its optimum temperature.
Dependent on pH- An enzyme exerts its full potential at an optimum pH ranging between 5-7 pH values.
Repressed by inhibitors-The catalytic activity of enzymes can be inhibited by competitive inhibitors, noncompetitive inhibitors, or irreversible inhibitors.
Beneficial enzymes amount increase-Increase in the concentration of the reactants could increase the reaction rate until the enzyme becomes saturated with the substrate, while an increase in the amount of enzyme will continuously enhance the rate of the reaction.
Function reversely- Enzymes could function reversely, signifying that the enzyme does not determine the direction of reaction.


Factors affecting the enzyme activity

The factors affecting the enzyme activity are as follows:

(1) Concentration of Enzyme

(2) Concentration of Substrate

(3) Effect of Temperature

(4) Effect of pH

(5) Effect of Product Concentration and

(6) Effect of Activators.(Cofactors and Coenzymes)

(1) Concentration of Enzyme - As the concentration of the enzyme is increased, the velocity of the reaction proportionately increases. More enzymes mean more active sites of enzymes to react with substrate to form products, so the rate of reaction increases.

(2) Concentration of Substrate -An increase in the substrate concentration gradually increases the velocity of the enzyme reaction within the limited range of substrate levels. A rectangular hyperbola is obtained when velocity is plotted against the substrate concentration. As substrate concentration increases, the rate increases because substrate binds to the active site of enzymes, forming a product. At a certain point, however, increasing the substrate concentration will have no further effect on the rate of reaction. This is because all of the enzyme’s active sites are now occupied by substrate molecules, so increasing the substrate concentration further will have no effect.

(3) Effect of Temperature-The velocity of an enzyme reaction increases with an increase in temperature, goes up to a maximum and then declines. A bell-shaped curve is usually observed. The optimum temperature for most of the enzymes is between 40°C-45°C, where the rate of reaction is maximum. However, a few enzymes (e.g. venom phosphokinases) are active even at 100°C. In general, when the enzymes are exposed to a temperature above 50°C, denaturation leading to derangement in the native (tertiary) structure of the protein and active site are seen. The majority of the enzymes become inactive at higher temperature (above 70°C).

(4) Effect of pH- Increase in the hydrogen ion concentration (pH) considerably influences the enzyme activity and a bell-shaped curve is normally obtained. Each enzyme has an optimum pH at which the velocity is maximum. Most of the enzymes of higher organisms show optimum activity around neutral pH (6-8). There are, however, many exceptions like pepsin (1-2), acid phosphatase (4-5) and alkaline phosphatase (10-11) for optimum pH.

(5) Effect of Product Concentration - The accumulation of reaction products generally decreases the enzyme velocity. For certain enzymes, the products combine with the active site of the enzyme and form a loose complex, thus inhibiting the enzyme activity. In the living system, this type of inhibition is generally prevented by a quick removal of products formed.

(6) Effect of cofactors- Cofactors are inorganic substrates. Some of the enzymes require certain inorganic metallic cations like Mg2+, Mn2+, Zn2+, Ca2+, Co2+, Cu2+, Na+, K+, etc., for their optimum activity. So the presence of cofactors will enhance the rate of enzymatic reactions.

(7) Effect of coenzymes-Coenzymes are organic molecules. Certain enzymes need coenzymes to bind to the substrate and cause a reaction. Actually, they loosely bind to enzymes to help them complete their activities. Coenzymes are nonprotein, organic molecules that facilitate the catalysis, or reaction, of their enzyme.