In this article we will discuss about the classification of enzymes.
Some enzymes are often designated by common names based on usage (pepsin, trypsin, chymotrypsin, papain etc); but these names contain no information on the substrate and the reaction catalyzed. In certain cases, enzymes catalyzing hydrolysis reactions are designated by the name of the substrate followed by the suffix “ase” (peptidase, phosphatase, arginase, etc.).
A slightly more precise denomination uses the name of the substrate and then that of the reaction catalyzed, with the suffix “ase”, for example, violate dehydrogenase.
In 1961, the Commission on Enzymes of the International Union of Biochemistry established a systematic and much more rigorous classification and nomenclature comprising 6 classes divided into sub-classes; the latter are themselves divided into sub-sub-classes which are numbered.
In this new denomination, malate dehydrogenase is called malate-NAD-oxidoreductase; this name reflects not only the type of reaction catalyzed, but also the name of the substrate and that of the hydrogen acceptor.
The following are a few examples illustrating this classification:
1. Oxidoreductases:
This class comprises the enzymes which were earlier called dehydrogenases, oxidases, peroxidases, hydroxylases, oxigenases, etc.
1.1 Acting on a 
1.1.1 With NAD+ or NADP+ as hydrogen acceptor
Ex. L-Malate : NAD-oxidoreductase (1.1.1.37), see fig. 4-38.
L-lactate: NAD-oxidoreductase (1.1.1.27), see fig. 4-30.
1.1.2 With a cytochrome as acceptor
Ex. L-lactate: ferricytochrome c-oxidoreductase (1.1.2.3).
1.1.3 With O2 as hydrogen acceptor
Ex. glucose oxidase or β-D-glucose: oxygen-oxidoreductase (1.1.3.4).
1.2 Acting on a 
1.2.1 With NAD+ or NADP+ as acceptor
Ex. D-glyceraldehyde-3-phosphate: NAD oxidoreductase (1.2.1.12) see fig. 4-27.
1.2.3 With O2 as acceptor
Ex. xanthine; oxygen oxidoreductase (1.2.3.2).
1.2.4 With lipoic acid as acceptor.
1.3 Acting on a 
1.3.1 With NAD+ or NADP+ as acceptor
Ex. 4.5 dihydrouracil: NAD oxidoreductase (1.3.1.1.1).
1.3.2 With a cytochrome as acceptor.
1.3.3 With O2 as acceptor
Ex. 4.5-dihydro-orotate: oxygen oxidoreductase (1.3.1.1.1), see fig. 6-22.
1.4 Acting on a 
1.4.1 With NAD+ or NADP+ as acceptor
Ex. L-Glutamate: NAD oxidoreductase (1.4.1.2) see fig. 7-3. etc.
2. Transferases:
2.1 Transferring a monocarbon group (C1):
2.1.1 Methyl transferases
Ex. S-adenosyl-methionine: L-homocysteine S-methyl transferase (2.1.1.10).
2.1.2 Hydroxymethyl transferases and formyl transferases
Ex. L-serine : tetrahydrofolate 5,10 hydroxymethyl transferase (2.1.2.1) see fig. 7-9.
2.1.3 Carboxyl transferases and carbamoyl transferases
Ex. carbamylphosphate: L-aspartate carbamyl transferase
2.3 Acyl transferases.
2.4 Glycosyl transferases:
2.4.1 Hexosyl transferases
Ex. UDPG-glucose: D-fructose glucosyl transferase (2.4.1.13).
2.4.2 Pentosyl transferases
Ex. Uridine: Orthophosphate ribosyltransferase (2.4.2.3).
2.5 Alkyl transferases.
2.6 Transferring nitrogen groups:
2.6.1 Amino transferases
Ex. L-aspartate: ketoglutarate amino transferase (2.6.1.1)
2.7 Phosphoryl transferases:
2.7.1 With an alcohol group as acceptor
Ex. ATP: D-hexose-6-phosphotransferase (2.7.1.1) see. fig. 4-20.
2.7.2 With a carboxylic group as acceptor
Ex. ATP: 3 Phosphoglycerate 1-phosphotransferase (2.7.2.3) see fig. 4-28
2.7.3 With a nitrogen group as acceptor
Ex. ATP: creatine phosphotransferase (2.7.3.2) see fig. 7-13. etc.
3. Hydrolases:
3.1 Splitting the Ester Bonds:
3.1.1 Carboxylester-hydrolases
Ex. Lipase or glycerol-ester hydrolase (3.1.1.3).
3.1.3 Phosphomonoesterases
Ex. alkaline phosphatase (3.1.3.1) see fig. 6-13.
3.1.4 Phosphodiesterases
Ex. ribonucleases, deoxyribonucleases, see figs. 6-10 to 6-12 deoxyribonucleate 3′ nucleotido hydrolase (3.1.4.6).
3.2 Splitting oside bonds
3.2.1 Glucosidases
Ex. β-glucosidase or β-D-glucoside glucohydrolase (3.2.1.21)
3.4 Splitting peptide bonds
3.4.1 α-aminopeptido-amino acid hydrolases
Ex. aminopeptidase or aminoacyl-peptide hydrolase (3.4.1.2).
3.4.2 α-carboxypeptido-amino acid hydrolases
Ex. carboxypeptidase A or peptidyl-L amino acid hydrolase (4.3.2.1).
3.4.4 Peptido-peptide hydrolases (endopeptidases).
Ex. trypsin (3.4.4.4)
4. Lyases:
Catalyzing the removal of a group by a process other than hydrolysis (often, with formation of a double bond) or on the contrary, catalyzing the addition of a group.
4.1 C-C lyases
4.1.1 Carboxylases (Carboxylases or Decarboxylases)
Ex. aspartate decarboxylase or L-aspartate 4 carboxylase (4.1.1.12).
4.1.2 Aldehyde-lyases
Ex. fructose-bisphosphate aldolase or fructose 1-6 bisphosphate:
D-glyceraldehyde-3-phosphate lyase (4.1.2.13) see fig. 4-26.
4.2 C-O lyases
4.3 C-N lyases
4.3.1 Ammonia lyases
Ex. L-aspartate-ammonium lyase (4.3.1.1) see fig. 7-5 etc.
5. Isomerases:
5.1 Racemases and epimerases
5.1.1 Acting on amino acids
Ex. alanine racemase (5.1.1.1).
5.1.3 Acting on oses
Ex. D-ribulose-5-phosphate-3-epimerase (5.1.3.1) see fig. 4-40.
5.2 Cis-trans isomerases
Ex. 4 maleyl-aceto acetate cis-trans isomerase (5.2.1.2) see fig. 7-24.
5.3 Intramolecular oxidorcductases
5.3.1 Catalyzing the interconversion aldose-ketose.
Ex. D-glyceraldehyde 3 phosphate keto-isomcrase or triosephosphate isomerase (5.3.1.1) see fig. 4-26.
5.4 Intramolecular transferases
Ex. L-methylmalonyl-coA-coA-carbonyl mutase (5.4.99.2) see fig. 5-13. etc.
