Types of enzymes

 Types of enzymes             OR                             Classification of enzymes:

New Classification :

Enzymes are divide into six major groups according to the general type of chemical reactions they catalyse: 

1) Oxidoreductase 

2) Transferases 

3): Hydrolases,

4): Lyases 

5): Isomerases 

6): Ligases ( Synthetases):

These groups are as per  the enzyme Nomenclature Recommendations ( 1978) of the International Union of  Biochemistry ( IUB).

(1): Oxidoreductases: 

These enzymes catalyse the oxidation _ reduction reactions  by transfer of hydrogen or electrons from one compound to another . A compound is reduced on accepting H or e‐, and is oxidized on losing H or e-. Hence, the enzyme involved is called Oxidoreductase. They include Oxidases, dehydrogenases and reductases. 

  (i) Oxidases:Oxidases bring about oxidation by addition of oxygen to a substrate or by removal of hydrogen or one or more electrons. Term oxidase is now used for the enzyme that catalyses oxidation reaction in which roman O2 acts as an acceptor of hydrogen or of electrons; the enzyme that catalyses oxidation reaction in which hydrogen is removed from  a substrate is now termed dehydrogenase. 

(ii) Reductases:Reductases catalyse reduction by removing oxygen or by adding hydrogen or one or more electron.

Summary of Oxidation reduction Reactions  
Oxidations ( with Respect to A) 

1) Addition of Oxygen: A+ BO ➡️ AO+ B 

2):  Removal of Hydrogen: AH+ B➡️ A+ BH

3): Removal of Electrons: A➡️A++e‐
  All the above reactions released energy.

Reduction ( With Respect to A) 

1)  Removal of Oxygen: AO + B➡️A+ BO

2): Addition of Hydrogen: A + BH➡️ AH + B

3): Addition of Electrons:A+ e- ➡️A-
  All the above reactions store energy. 

Examples: (i) Cytochrome oxidase catalyses the transfer of hydrogen to oxygen, forming water in the last reaction of electron transport system___ 

( ii) 3_ phosphoglyceraldehyde dehydrogenase catalyses transfer of hydrogen from  3-phosphoglyceraldejhyde to NAD+ in glycolysis____ 

(ii) Reductases catalyse the transfer of electrons from one coenzyme to the next in the electron transport  system.

(2) Transferases :

These catalyse the transfer of specific group other than hydrogen from one substrate to another. Examples: hexokinase that transfers a phosphate group from ATP to glucose, forming  glucose  6-phosphate in glycolysis, and glutamate pyruvate transaminase which transfer amino group from glutamate to pyruvate  to form alanine.

 (3) Hydrolases: 

These catalyse splitting of larger molecules into smaller one by addition of water (hydrolysis). Examples: almost all digestive enzymes, such as amylase that hydrolyses starch and glycogen to maltose. 

(4) Lyases:

These catalyse  the breaking of specific covalent bonds and removal of group without hydrolysis. Example:  histidine decarboxylase that splits C_ C bond of histidine, forming CO2 and histamine. Aldolase splits fructose 1,6_ diphosphate into dihydrotestosterone phosphate ( DHAP) and 3_ phosphoglyceraldehyde (3_ PGAL) in glycolysis. 

 (5) Isomerases:

 These catalyse the change of a substrate into a related isomeric form by rearrangement of molecules. Examples:phosphohexose Isomerase which change glucose  6_ phosphate to fructose 6_ phosphate in glycolysis. 

(6) Ligases ( Synthetases): 

These catalyse the joining of 2 substrate molecules by getting energy from hydrolysis of ATP. Example : pyruvate carboxylysae joins pyruvate and CO2 to form oxaloacetate during CO2 fixation. Biosynthetic reactions, such as the formation of polypeptide chains from  amino acid units, require ligases.

● Old Classification :

Earlier, the enzymes were classified into 2 groups : hydrolysing enzymes ( hydrolyses), which split larger molecules into small ones, e.g., amylases, lipases, peptidases, ,etc.; and desmolysing enzymes ( desmolases) ,that catalyse reactions other than those involving hydrolysis, e.g., those involving oxidation and reduction, isomerization, etc.

Differences between Competitive Inhibition and Allosteric Inhibition  

Competitive Inhibition 

1): Inhibitor binds to active site of enzymes. 

2): Substrate fails to join enzyme as its active site is occupied by Inhibitor.

3): Inhibitor  closely resembles substrate in structure. 

4): Inhibitor is not an intermediate or a product of metabolic pathway catalysed by the enzyme.

5): It has no regulatory function.

Allosteric Inhibition 

1): Inhibitor binds to allosteric site of enzyme.

2): Substrate fails to join enzyme as its active site is deformed by Inhibitor.

3): Inhibitor does not resemble substrate in structure. 

4): Inhibitor may be an intermediate or a product of metabolic pathway catalysed by the enzyme.

5): It regulates the metabolic pathway and checks the excessive formation of the product.


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