Classification of Enzymes :

Enzymes are classified  into the following six main categories in according with  the nature of reaction the catalyze:

1.      Oxidoreductases : These serve to  transfer  hydrogen atoms  or ions  protons  H+  or electrons e-  from substrate  molecules  donors to other acceptors  molecules:

Reduced S+ oxidized s₁ → oxidized   S + reduced s₁  

These enzymes  are further classified into  three  categories

Alcohol dehydrogenases These transfer H⁺

 Alcohol + NAD→ aldehyde+ NADH + H⁺

                                      (Oxidized )( reduced)

 Cytochrome oxidases:  these transfer electrons;

 Reduced cytochrome + o₂ + 4H ⁺→ oxidized  cytochrome + 2H₂O

Cntalases : These  remove oxygen  from 2H₂O

2. Transferases: These transfer not H+  but some  functional group or radical from a substrate molecule to  another  molecule.

         S group  + S¹ → S+ S^1- group

For  example  phosphorylase catalyzes phosphorylation  of glucose  or transferring a high  energy phosphate bond from ATP to glucose.

3.Hydrolases : These  catalyze  hydrolysis  of their  substrates  i.e.  these  split  C-O-, C-H, C-C-. C-N and other similar  bonds  by addition  of water.

SS+H₂O→ H+S -OH

All digestive enzyme  and the enzymes of lysosomes are hydrolases . Lysosomes  enzyme  serve for  intracellular digestion of  materials obtained by the cells  from outside  medium  by  endocytosis.

1.      Lyases : These  also  bring  about  splitting  of C-C,C-O, C-N,C-H, and  other similar  bonds not by  hydrolysis  but by  elimination  thereby leaving  a double  bond .conversely, these may  add group  to double bonds.

2.      Insomerases : These catalyze  structural changes in the moleclules of their  substrates , resulting  into  formation of isomers.

3.      Ligases: These  catalyze linking  of molecules of different compounds  supplying energy of ATP to the reaction .For  example  ends of separate  strands  nucleic acid  are joined by  ligases.