The student-teacher relationship may be a good example to understand how a catalyst works. The students often find it difficult to learn from a textbook on their own. The teacher explains the subject to the students and increases their understanding capability. It is no wonder that certain difficult things which the students take days together to understand, and sometimes do not understand at all – are easily learnt under the guidance of the teacher. Here, the teacher acts like a catalyst in enhancing the understanding ability of students. A good teacher is always a good catalyst in students’ life!
Enzymes may be defined as biocatalysts synthesised by living cells. They are protein in nature (exception – RNA acting as ribozyme), colloidal and thermolabile in character, and specific in their action.
In the laboratory, hydrolysis of proteins by a strong acid at 100°C takes at least a couple of days. The same protein is fully digested by the enzymes in gastrointestinal tract at body temperature (37°C) within a couple of hours. This remarkable difference in the chemical reactions taking place in the living system is exclusively due to enzymes. The very existence of life is unimaginable without the presence of enzymes.
HISTORICAL BACKGROUND
Berzelius in 1836 coined the term catalysis (Greek : to dissolve). In 1878, Kuhne used the word enzyme (Greek : in yeast) to indicate the catalysis taking place in the biological systems. Isolation of enzyme system from cell-free extract of yeast was achieved in 1883 by Buchner. He named the active principle as zymase (later found to contain a mixture of enzymes), which could convert sugar to alcohol. In 1926, James Sumner first achieved the isolation and crystallization of the enzyme urease from jack bean and identified it as a protein.NOMENCLATURE AND CLASSIFICATION
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. 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 of enzyme reaction (type of reaction, cofactor requirement etc.)Enzymes are sometimes considered under two broad categories :
(a) Intracellular enzymes– They are functional within cells where they are synthesized.
(b) Extracellular enzymes–These enzymes are active outside the cell; all the digestive enzymes belong to this group.
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.
1. Oxidoreductases : Enzymes involved in oxidation-reduction reactions.
2. Transferases : Enzymes that catalyse the transfer of functional groups.
3. Hydrolases : Enzymes that bring about hydrolysis of various compounds.
4. Lyases : Enzymes specialised in the addition or removal of water, ammonia, CO2 etc.
5. Isomerases : Enzymes involved in all the isomerization reactions.
6. Ligases : Enzymes catalysing the synthetic reactions (Greek : ligate—to bind) where two molecules are joined together and ATP is used.
[The word OTHLIL (first letter in each class) may be memorised to remember the six classes of enzymes in the correct order].
Each class in turn is subdivided into many sub-classes which are further divided. A four digit Enzyme Commission (E.C.) number is assigned to each enzyme representing the class (first digit), sub-class (second digit), sub-sub class (third digit) and the individual enzyme (fourth digit). Each enzyme is given a specific name indicating the substrate, coenzyme (if any) and the type of the reaction catalysed by the enzyme.
Although the IUB names for the enzymes are specific and unambiguous, they have not been accepted for general use as they are complex and cumbersome to remember. Therefore, the trivial names, along with the E.C. numbers as and when needed, are commonly used and widely accepted.