NOMENCLATURE &CLASSIFICATION
OF ENZYMES
UG-05
Semester-3rd
ENZYMES
• Biological catalysts which speed up the rate of reaction
without becoming part of the reaction but themselves cannot
initiate any chemical reaction.
• Enzymes :
• First name is of substrate
• second, ending in “ASE” indicating type of reaction catalyzed.
• Clarify the reaction , e.g. Malic Enzyme
• L- Malate + NAD  Pyruvate + NADH-H + CO2
• Malate NAD oxidoreductase (Decarboxylating)
Trival name
• Gives no idea of source, function or reaction
catalyzed by the enzyme.
• Example: trypsin, thrombin, pepsin.
Systematic Name
• According to the International union Of
Biochemistry an enzyme name has two parts:
-First part is the name of the substrates for
the enzyme.
-Second part is the type of reaction catalyzed
by the enzyme.This part ends with the suffix
“ase”.
Example: Lactate dehydrogenase
EC number
• Enzymes are classified into six different groups
according to the reaction being catalyzed.
• The nomenclature was determined by the Enzyme
Commission in 1961 (with the latest update having
occurred in 1992), hence all enzymes are assigned an
“EC” number.
• The classification does not take into account amino
acid sequence (ie, homology), protein structure, or
chemical mechanism.
EC numbers
• EC numbers are four digits, for example a.b.c.d, where
“a” is the class, “b” is the subclass, “c” is the subsubclass, and “d” is the sub-sub-subclass. The “b” and
“c” digits describe the reaction, while the “d” digit is
used to distinguish between different enzymes of the
same function based on the actual substrate in the
reaction.
• Example: for Alcohol:NAD+oxidoreductase EC
number is 1.1.1.1
CLASSIFICATION OF ENZYMES
Formulated by the enzyme commission of I.U.B six major
classes based on the type of reactions catalyzed
1. Oxidoreductases
•
Catalyzing oxidation reduction reactions
2. Transferases
•
Catalyzing group transfer
3. Hydrolases
•
Catalyzing hydrolytic breakdown
Classification of Enzymes
4. Lyases
• Catalysing removal of groups by mechanism other than
hydrolysis and leaving behind double bonds
5. Isomerases
• Catalysing interconversion of isomers
6. Ligases
• Catalysing formation of bonds and new compounds
1. Oxidoreductases
– Catalysing oxidation reduction reaction where one
substrate is oxidized and other is reduced
CLASSIFICATION OF ENZYMES
Oxidases. Catalyzing oxidation of the substrate and
atomic oxygen acts as recipient of hydrogen e.g.
Ascorbic acid oxidase, Cytochrome oxidase, Tyrosinase
Ascorbic acid
Oxidase
Ascorbic acid + O2
Dehydro ascorbic acid
CLASSIFICATION OF ENZYMES
Aerobic Dehydrogenases.
Catalyzing oxidation of the substrate and molecular
oxygen acts as recipients of hydrogen e.g. Glucose
oxidase, L amino acid dehydrogenase, Xanthene
dehydrogenase
glucose oxidase
Glucose
Gluconolactone
CLASSIFICATION OF ENZYMES
Anaerobic Dehydrogenases. Catalyzing oxidation of
the substrate and coenzymes act as recipients of hydrogen
e.g. Lactate Dehydrogenase with NAD and Glucose 6
phosphate dehydrogenase with NADP
Lactate
dehydrogenase
Lactic acid
+ NAD
Pyruvic acid
+ NADH – H+
CLASSIFICATION OF ENZYMES
Oxygenases:
Is an enzyme that oxidizes a substrate by transferring
the oxygen from molecular oxygen O2 (as in air) to it.
Types :
a. Monooxygenases:
Transfer one oxygen atom to the substrate, and reduce the
other oxygen atom to water.
b. Dioxygenases:
Incorporate both atoms of molecular oxygen (O2) into the
product(s) of the reaction.
Among the most important monooxygenases are
the cytochrome P450 oxidases, responsible for breaking
down numerous chemicals in the body.
2.TRANSFERASES
Transaminases. Catalyzing transfer of amino group between
an amino acid and a ketoacid e.g. Aspartate transaminase (AST),
Alanine transaminase (ALT)
Aspartate
transaminase (AST)
Glutamic acid +
Oxalo acetic acid
Alanine
transaminase (ALT)
Glutamic acid +
Pyruvic acid
 ketoglutaric acid +
Aspartic acid
 ketoglutaric acid +
Alanine
2.TRANSFERASES
Transmethylases. Catalyzing transfer of methyl group
between to substrates e.g. COMT
Noradrenalin
+ CH3
Catechol-Omethyltransferase (COMT)
Adrenaline
Transpeptidases. Catalyzing transfer of amino acids to
substrates e.g. Benzyl-SCoA transpeptidase
Benzyl-SCoA
transpeptidase
Benzyl - SCoA
+ Glycine
Hippuric acid
2. TRANSFERASES
Phosphotransferases. Catalyzing transfer of phosphate
group to substrates e.g. Hexokinase, glucokinase
2.7.1.1 ATP D hexose 6 phosphotransferase [Hexokinase]
ATP + Glucose Hexokinase ADP + D-Glucose –6-P
Acetyltransferase. Catalyzing transfer of acetyl group to
substrates e.g. choline acetyltransferase
Acetyl-CoA+ Choline  CoA + Acetyl- Choline
3. HYDROLASES
Catalysing hydrolytic breakdown of different bonds. Most of the GIT
enzymes belong to this class
Enzymes hydrolyzing carbohydrates
1. Polysaccharidases
Starch
Amylase Maltose, maltotrios, dextrins
2. Oligosaccharidases
Dextrins
Dextrinase
glucose
3. Disacharidases
Maltose, Lactose, Sucrose
Disacharidases Maltase, Lactase, Sucrase
Enzymes Hydrolyzing Lipids
Triacyl glycerol
Cholesterol ester
lipase
cholesterol
esterase
monoacyl glycerol + 2 F.F.A
free cholesterol + FFA
monosaccharides
3. HYDROLASES
Phospholipids Phospholipase lysophospholipids
Lecithin
Lysolecithin
Enzymes Acting on Peptide Bonds:
exopeptidases carboxypeptidase
amino acids
endopeptidase
e. g pepsin (smaller peptides)
aminopeptidases
3. HYDROLASES
Tripeptidase : Tripeptide  A.A
Dipeptidase : Dipeptide  A.A
Phosphatases
1. Phosphomonoesterases:
Glucose – 6.P. + H2O Phosphatase
G 6. Phosphate Glucose +Pi
2. Phosphodiesterases:
Removal of phosphate Group of diesters breakdown of 3’-5’ p
linkages in cyclic AMP
4. LYASES
• Catalysing reactions in which groups are removed without
hydrolysis leaving a double bond or add groups to already existing
double bonds.
CH3. CO. COOH
(pyruvate)
COOH.CH = CH. COOH
(Fumaric acid)
pyruvate decarboxylase
Fumerase
CH3. CHO+ CO2
(acetaldehyde)
COOH-CHOH. CH2-COOH
(malic acid)
5. ISOMERASES
•
Involved in inter conversion of pair of isomeric compounds
•
Glucose 6. P
Phosphogluco mutase
•
Glucose 6.P
Phosphohexose isomerase
•
All trans retinene
Retinene
Isomerase
glucose I.P
Fructose 6.P
11- CIS retinene
6. LIGASES
• Catalyze reactions in which linking together of two
molecules occur coupled with the breakdown of a high
energy phosphate bonds like ATP, GTP
Acetate + CoA +ATP
Succinate + CoA + ATP
Pyruvate + CO2 + ATP
Fatty acid + CoA + ATP
Acetyl CoA
Synthetase
Succinyl CoA
Synthetase
Pyruvate
Carboxylase
Acyl CoA
Synthetase
Acetyl CoA+AMP
Succinyl CoA + ADP+ Pi
Oxaloacetate + ADP + Pi
Acyl CoA (Activated fatty acid) + AMP + PiPi
EC
• Every enzyme code consists of the letters "EC" followed by four
numbers separated by periods. Those numbers represent a
progressively finer classification of the enzyme.
• For example, the tripeptide aminopeptidases have the code "EC
3.4.11.4", whose components indicate the following groups of
enzymes:
• EC 3 enzymes are hydrolases (enzymes that use water to break up
some other molecule)
• EC 3.4 are hydrolases that act on peptide bonds
• EC 3.4.11 are those hydrolases that cleave off the aminoterminal amino acid from a polypeptide
• EC 3.4.11.4 are those that cleave off the amino-terminal end from
a tripeptide