Life is possible due to the co-ordination of numerous metabolic reactions inside the cells. Proteins can be hydrolyzed with hydrochloric acid by boiling for a very long time; but inside the body, with the help of enzymes, proteolysis takes place within a short time at body temperature.
Enzyme catalysis is very rapid; usually 1 molecule of an enzyme can act upon about 1000 molecules of the substrate per minute. Lack of enzymes will lead to block in metabolic pathways causing inborn errors of metabolism. The substance upon which an enzyme acts, is called the substrate. The enzyme will convert the substrate into the product or products.
Characteristics of Enzymes
i. Almost all enzymes are proteins. Enzymes follow the physical and chemical reactions of proteins. ii. They are heat labile. iii. They are water-soluble. iv. They can be precipitated by protein precipitating reagents (ammonium sulfate ortrichloroacetic acid). v. They contain 16% weight as nitrogen.
Classifications of enzymes
When early workers isolated certain enzymes, whimsical names were given. Some of these, such as Pepsin, Trypsin, Chymotrypsin, etc. are still used. Later, it was agreed to call the enzymes by adding the suffix “-ase” to the substrate. Thus, enzyme Lactase acts on the substrate lactose, and the products glucose and galactose are formed.
Enzymes tha thydrolyse starch (amylose) are termed as amylases; those that dehydrogenate the substrates are called dehydrogenases. These are known as the trivial names of the enzymes.
IUBMB System of Classification International Union of Biochemistry and Molecular Biology (IUBMB) in 1964, (modified in 1972 and 1978), suggested the IUBMB system of nomenclature of enzymes. It is complex and cumbersome; but unambiguous. As per this system, the name starts with EC (enzyme class) followed by 4 digits.
First digit represents the class Second digit stands for the subclass Third digit is the sub-subclass or subgroup Fourth digit gives the number of the particular enzyme in the list.
The enzymes are grouped into following six major classes
• Class 1: Oxidoreductases
This group of enzymes will catalyse oxidation of one substrate with simultaneous reduction ofanother substrate or co-enzyme. This may be represented as AH2 + B ————-→ A + BH2 for example, Alcohol + NAD+ —-→ Aldehyde + NADH + H+ The enzyme is Alcohol dehydrogenase; IUB name is Alcohol-NAD-oxidoreductase; Code number is EC.220.127.116.11. Oxidoreductases may alsooxidise substrates by adding oxygen, e.g.oxidases, oxygenases and dehydrogenases.
• Class 2: Transferases
This class of enzymes transfers one group (other than hydrogen) from the substrate to another substrate. This may be represented as A-R + B → A + B-R , For example, Hexose + ATP → Hexose-6-phosphate + ADP The name of enzyme is Hexokinase and systematic name is ATP-Hexose–6-phosphatetransferase.
• Class 3: Hydrolases
This class of enzymes can hydrolyse ester, ether, peptide or glycosidic bonds by adding water and then breaking the bond. Acetyl choline + H2O ——–→ Choline + acetate The enzyme is Acetyl choline esterase or Acetyl choline hydrolase (systematic). All digestive enzymes are hydrolases.
• Class 4: Lyases
These enzymes can remove groups from substrates or break bonds by mechanisms other than hydrolysis. For example, Fructose-1,6-bisphosphate ——-→ Glyceraldehyde-3-phosphate +dihydroxy acetone phosphateThe enzyme is Aldolase (see Chapter 9 for details).
• Class 5: Isomerases
These enzymes can produce optical, geometric or positional isomers of substrates. Racemases, epimerases, cis-trans isomerases are examples. Glyceraldehyde-3-phosphate —–→ Di-hydroxy-acetone-phosphate. Enzyme is Triose phosphate isomerase.
• Class 6: Ligases
These enzymes link two substrates together, usually with the simultaneous hydrolysis of ATP, (Latin, Ligare = to bind). For example, Acetyl CoA + CO2 + ATP → Malonyl CoA + ADP + Pi. Enzyme is Acetyl CoA carboxylase