At the end of this lecture, student will be able to

• Explain definition and nomenclature

• Classify enzymes

• Describe properties of enzymes

• Explain the therapeutic applications of coenzyme


• Enzymes are biocatalysts - the catalysts of life

• Catalyst is defined as a substance that increases the velocity or rate of a chemical reaction without itself undergoing any change in the overall process

• Enzymes may be defined as biocatalysts synthesized by living cells, protein in nature, colloidal and thermolabile in character and specific in their action

• In the laboratory, hydrolysis of proteins by a strong acid at 100oC takes at least a couple of days & same protein is fully digested by the enzymes in gastrointestinal tract at body temperature (37oC) within a couple of hours due to enzymes

Nomenclature and classification of enzymes

• In the early days, the enzymes were given names by their discoverers in an arbitrary manner e.g. pepsin, trypsin, chymotrypsin (convey no information about the function of the enzyme or the nature of the substrate on which they act

• Sometimes 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

• 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 (lUB) 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

• Since 1964 the IUB system of enzyme classification has been in force

• Enzymes are divided into six major classes

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. lsomerases: Enzymes involved isomerization reactions

6. Ligases: Enzymes catalysing the synthetic reactions. Where two molecules are joined together and ATP is used

• Each class in turn is subdivided into many sub-classes which are further divided

• A four digit Enzyme Commission (EC) 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 but 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

Properties of enzymes

• All the enzymes are invariably proteins, however, few RNA molecules have been shown to function as enzvmes

• Each enzyme has its own tertiary structure and specific conformation which is very essential for its catalytic activity

• The functional unit of the enzyme is known as holoenzyme which is often  made  up  of  apoenzyme  (the  protein  part)  and  a coenzyme (non-protein organic part)

Holoenzyme → Apoenzyme + Coenzyme

(Active enzyme) → (protein part) + (non-protein part)

• The term prosthetic group is used when the non-protein moiety tightly binds with the apoenzyme & coenzyme can be separated by dialysis from the enzyme while the prosthetic group cannot be

• The term monomeric enzyme is used if it is made up of a single polypeptide e.g. ribonuclease, trypsin

• Some of the enzymes possess more than one polypeptide chain are known as oligomeric enzymes e.g. lactate dehydrogenase

• Enzymes form colloidal solutions instead of true solution in water

• Enzymes vary in their molecular weight

• Enzymes can be precipitated by dehydrogenation or neutralization

• Based on enzyme complex, wide variations in the enzyme shape

• By biotechnology it is possible to modify enzyme with desirable characters to improve catalytic activity under unusual condition

• The enzymes exhibit all the general properties of proteins

• Enzymes possess enormous potential use in medicine and industry in genetic manipulations and industrial applications

Application of enzymes

• Decreased in plasma enzymes in certain diseases


• Enzymes are biocatalysts - the catalysts of life

• Enzymes are divided into six major classes; oxidoreductases, Transferases, Hydrolases, Lyases, lsomerases and ligases

• The functional unit of the enzyme is known as holoenzyme which is often made up of apoenzyme and a coenzyme

• Enzymes are used as therapeutic agents, as analytical reagents, in genetic manipulations, in industrial applications and in diagnosis of diseases

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