Enzyme Classification

1. Enzyme
Classification

2. Classification of Enzymes
According to the International
Union of Biochemistry or
Biochemists Enzymes are
classified into SIX major
classes

3. Class I – Oxidoreductases,
catalyze oxidation reductions
Class II – Transferases
Catalyzing group transfer
Class III– Hydrolases
Catalyzing hydrolysis
Class IV– Lyases
Catalyzing addition of groups to
═ bonds
Class V -- Isomerases
Catalyzing isomerization
Class VI – Ligases
Catalyzing condensations

4. I Oxidoreductases
They catalyze oxidation
and reduction reaction.
These enzymes are
divided into three groups.

5. (a) Oxidases: Those which use
oxygen as hydrogen as acceptor
eg:- L amino acid oxidases.
L a.a + H2O + O2  2 oxoacids +
NH3+H2O2
Eg: D-amino acid oxidases,
Tyrosinase,Cyt-oxidases, Uricases.

6. (b) Anaerobic Dehydrogenases
Those which use some other
substances as Hydrogen acceptor ,
eg: NAD,NADP
Enz: 1) Malate Dehydrogenase
2) Succinate Dehydrogenase
3) Lactate Dehydrogenase.

7. These anaerobic dehydrogenases can
further be sub-classified as
(i) Those acting on CH-OH of donors
eg: Glycero-P-dehydrogenase
Gly-3-P + NAD ═ Di Hydroxy Acetone P
+ NADH
Other eg:-- lactate Dehydrogenase
Malate Dehydrogenase
Isocitrate Dehydrogenase

8. (ii)Those acting on CH-CH group of donors
eg: Dihydrouracil dehydrogenase
4,5 dihydrouracil + NAD+
═ Uracil + NADH
(iii) Those acting on CH-NH2 group of donor
with NAD or NADP as acceptor.
eg: Glutamate Dehydrogenase
Glutamate + H2O + NAD+
═ 2 oxoglutarate
NH3 + NADH

9. (C) Hydroperoxidases
Acting on H2O2 as acceptor, or those which
use hydrogen peroxide as substrate.
eg:Catalase , Peroxidase
H2O2 + H2O2 ══ O2 +2H2O

10. II Transferases
 They catalyze the transfer of some groups from
one molecule to the other.
(1) Methyl Transfer: Methyl transferases
a. S-adenosyl Methionine + Guanidino acetate
║Guanidino acetate Methyl transferase
S-adenosyl homocysteine + Creatine
b. Serine hydroxy methyl transferase
Serine  Glycine

11. (2)Carboxl transfer-- eg: Aspartate
Transcarbamylase
(3) Aldehyde, Keto Residue Transfer--eg:
Transaldolase, Transketolase
(4) Glucosyl Transfer--eg: Transglucosylase
(5)Amino Grp Transfer--eg: Transaminases
(6) Phosphorus Transfer eg:
Glucokinases,Hexokinases
(7) Transfering Sulphur-- eg: Trans-sulfurase
(8) Acyl Transferases—eg:Transacylases

12. III Hydrolases
They catalyze the
hydrolysis of substrate
by addition of water
molecule across the
bond which they split

13. (I) Acting on Ester bonds-Esterases
• (a) Acetylcholine-esterase
Acetylcholine +H2O  Choline + Acetic Acid
• (b) Alkaline Phosphatase
Orthophosphoric mono ester+H2O  an Alcohol
+H3PO4
Eg: G-6-P + H2O D glucose + H3PO4

14. (ii) Acting on Glycosyl compound
• Amylases β galactosidases
(iii) Acting on Peptide bonds
• Peptidases 
aminopeptidases,carboxypeptidases
(iv)Acting on C-N bonds
• Deamidases Urease (Urea Amido Hydrolase)
Eg: Urea + H2O = CO2 + 2 NH3

15. (v) Acting on Acid Anhydride bonds
(a)Pyrophosphate Phosphorylase (Synthetic
Name)
• This enzyme is also called Inorganic pyro
phosphatase (trivial name)
• Pyro PO4 + H2O == 2 ortho phosphate
(b)ATP Phospho Hydrolase (synthetic name)
• ATP ase (trivial name)
• ATP + H2O == ADP + orthophosphate

16. IV .Lyases
•They catalyze the
splitting of group from
substrate without
hydrolysis or
oxidation,reduction.

17. (i) C-C Lyases : Pyruvate Decarboxylase
2 oxo acids == an Acid + CO2
(ii) Keto Acid Lyase : Isocitrate Lyase
Citrate + CoA = Acetyl CoA + H2O +oxalo
acetate
(iii) C-oxylyase : Serine Hydrolyase or
Serine Hydratase
Serine + H2O == Pyruvic + NH3 + H2O
Acid

18. V Isomerases
• They catalyze isomeric
conversions or epimeric
conversions.
• Eg: Isomerases,Racemases,
Epimerases

19. VI- Ligases
(i) Tyrosine S RNA Ligase or Tyrosyl RNA
Synthetase.
ATP + Tyr. + RNA ═ AMP + Pyr PO4 +
Tyrosyl S RNA
(ii) CTP Synthetase
(iii) GMP Synthetase.

20. Ligases can join or form
• C-O bonds 
Tyrosyl S. RNA Synthetase
Leucyl S.RNA Synthetase
Alanyl S.RNA Synthetase
• C-N bonds 
GMP Synthetase
CTP Synthetase

21. • C-C bonds 
(i) Pyruvate CO2 Ligase or Pyruvate
Carboxylase
ATP +Pyruvate +CO2 + H2O = ADP +Oxaloacetate +
Ortho P
(ii) Acetyl CoA CO2 Ligase or Acetyl CoA
Carboxylase
• ATP + Acetyl CoA + CO2 + H2O
║
ADP + Ortho P + Malonyl CoA