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Classification of Enzymes|Enzyme classification|Enzyme

Posted by Mumtaz khan Sunday, 1 January 2012

The six major classes of enzymes are:

This class includes enzymes which were earlier called as Dehydrogenase.They bring about oxidation reduction reaction between two substrates S,S'.
       More precisely, these enzymes Catalyse electron transfer reactions. In this class are included enzymes catalyzing oxido-reductions of CH-OH,C=O,CH-CH,CH-NH2,CH-NH groups.Accordingly they are numbered as 1.1,1.2,1.3,1.4,1.5.

1.1 : Acting on CH-OH group(Alcohol):
1.1.1-With NAd or NADP as hydrogen acceptor.
Ex. L-Malate : NAD-oxido reductase

1.1.2-With a cytochrome as acceptor.
Ex. L-Lactate : ferricytochrome C -oxido reductase.

1.1.3-With Oxygen as hydrogen acceptor.
Ex. glucose oxidase or beta-D-glucose : oxygen-oxido-reductase.

1.2 :Acting on a C=O group (aldehyde or ketone):
1.2.1- With NAD or NADP as acceptor.
Ex.  D-glyceraldehyde-3-phosphate : NAD oxido-reductase.

1.2.3-With oxygen as aceptor
Ex. Xanthine oxygen oxido reductase.

1.3 : Acting on CH-CH group:
1.3.1 -With NAD or NADP as acceptor.
Ex. 4.5 dihydrouracil : NAD oxido reductase.

1.3.2-With a cytochrome as acceptor.

1.3.3-With Oxygen as acceptor
Ex. 4.5-dihydro-orotate : Oxygen oxidi reductase.

1.4 : Acting on CH-NH group(amidos):
1.4.1-With NAD or NADP as acceptor.
Ex. L-glutamate : NAD oxido-reductase.

Enzymes which catalyse the transfer of a group G other than hydrogen between a pair of substrates S and S' are called transferases.
         In these are included the enzymes catalisng the transfer of one carbon group aldehydic or ketonic residue,acyl,glycocyl,alkyl,phosphorous or sulphur containing groups.They are numbered as 2.1,2.2,2.3,2.4,2.5,2.6,2.7,..

2.1 : Transferring a monocarbon group.
2.1.1- Methyl transferase.
Ex . S- adenosyl-methionine : L-homocysteine S-methyl transferase.

2.1.2- Hydroxymethyl transferases and formyl transferases.
Ex. L-serine : tetrahydrofoliate 5,10 hydroxymethyl transferase.

2.1.3: Carboxyl transferases and Carbamyl transferases.
Ex. Carbamyl phosphate : L aspartate Carbamyl transferase.

2.3 : Acyl transferases

2.4 : Glycosyl transferases

2.4.1-Hexocyl transferases
Ex. UDPG-glucose : D-fructose glucocyl transferase.

2.4.2-Pentosyl transferases
Ex. Uridine : Orthophosphate ribosyl transferase.

2.5 : Alkyl transferases

2.6 : Transferring nitrogen groups.
2.6.1- Amino transferases
Ex. L-aspartest : Ketoglutarate aminotransferase.

2.7 : Phosphoryl transferases
2.7.1- With an alcohol group as acceptor
Ex. ATP : D-hexose-6-phosphotransferase.

2.7.2-With a Carboxylic group as acceptor.
Ex. ATP : 3 phosphoglycerate 1-phosphotransferase

2.7.3-With a nitrogen group as acceptor.
Ex.Atp: creatine phosphotransferase.

These enzymes catalyze the hydrolysis of the substrates  by adding constituents of water across the bond they spilt.The substrate include esters,glycosides: glycosyl compounds,ethers,peptides,acid anhydrides, C-C halides and P-N bonds.
        They are correspondingly numbered as 3.1,3.2,3.3,3.4,3.5,3.6,3.7,3.8,...

3.1 : Splitting the ester bonds
3.1.1- Carboxylester-hydrolases.
Ex. Lipase or glycerol-ester hydrolase.

Ex. alkaline phosphate.

3.1.4-phospho diesterases
Ex. Alkaline phosphatases

3.2 : Splitting oside bonds
Ex. beta-glucosidase or beta-D-glucoside glucohydrolase.

3.4 : Splitting peptide bonds.
3.4.1-alpha-aminopeptido-amino acid hydrolases.
Ex.aminopeptidase or aminoacyl-peptide hydrolase.

3.4.2- alpha-carboxypeptido-amino acid hydrolases.
Ex. carboxypeptidase A or peptidyl-L-aminoacid hydrolase.

3.4.4-Peptido-peptide hydrolases(endopeptidases).
Ex. Trypsin.

These are enzymes which catalyze the remol of groups from substrates by mechanism other than hydrolysis leaving double bonds.
        In these,the enzymes included are those which act on C-C,C=O,C-N,C-S,C-halide bonds.
Accordingly they are numbered as 4.1,4.2,4.3,4.4,4.5,..

4.1 : C-C Lyases.
4.1.1 -Carboxylases(Carboxylases or decarboxylases).
Ex .aspartate decarboxylase or L-aspartate 4,Carboxylase.

Ex. fructose-biphosphate aldolase or fructose1-6 biphosphate : D-glyceraldehyde-3-phosphate lyase.

4.2 : C-O lyases.

4.3 : C-N Lyases

4.3.1 Ammonia lyases.
Ex. L-aspartate-ammonium lyase.

They catalyze interconversion of optical-geometic or positional isomers by intra-molecular rearrangement of atoms or groups.
Ex. Racemases and Epimerases.
Trans-Isomerase,intramolecular oxidase etc.

5.1 : Racemases and Epimerases
5.1.1-Acting on amino acids
Ex. alanine racemase

5.1.3-Acting on oses
Ex. D-ribulose-5-phosphate-3-epimerase

5.2 : Cis-trans isomerases
Ex. 4 malelyl-aceto acetate cis-trans isomerase.

5.3 : Intramolecular Oxidoreductases.
5.3.1 - Catalizing the interconversion aldose-ketose.
Ex. D-glyceraldehyde 3-phosphate keto-isomerase or triosephosphate isomerase.

5.4 : Intramolecular transferases.
Ex. L-methylmalonyl-CoA-carbonyl mutase.

Ligases or synthetases:
They catalyze linking of two compounds using energy from ATP or a similar energy compound.This category includes catalyzing reactions forming C-O,C-S,C-N Nd C-C bonds.
Accordingly they are numbered as 6.1,6.2,6.3,6.4,... 

6.1 :Formation of C-O bonds.
6.1.1-Amino acyl-tRNA synthetase(or amino acid tRNA ligases)
Ex.Alanyl-tRNA synthetase.

6.2 : Formation of C-N bonds.
6.3 : Formation of C-N bonds.
6.3.1-Acid-ammonia ligases.
Ex. L-glutamate : ammonia ligase or glutamine synthetase.

6.3.2-Acid amino acid ligases.
Ex. Y-L-glutamyl-L-cysteine : glycine ligase or glutathione synthetase.

6.4 : Formation of C-C bonds.


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