Models Classification

1. ENZYMES (PART II)
 Models of active site
 Classification of enzymes

2. Template or Lock-and-Key Model
(Fisher model)
 The active site already
exists in proper
conformation even in
absence of substrate.
 Substrate fits into
active site of an enzyme
as the key fits into the
lock

3. Induced-Fit Model
(Koshland Model, 1963)
 The important
feature of this model
is the flexibility of
the active site.
 The substrate during
its binding induces
conformational
changes in the
active site and
determine the final
catalytic shape and
form.

4. Classification of enzymes
 Enzymes are generally named after adding
the suffix – “ase’ to the name of the
substrate, e.g. enzymes acting on nucleic
acid are known as nucleases, enzymes
hydrolyzing dipeptides are called
dipeptidases.
 Classification of enzymes based on
chemical reaction type. According to this
system, enzymes are grouped in six classes.

6. Class A
Oxidoreductases
 Catalyze Oxidation –reduction reaction
 Sub-classes
 Oxidases. Only oxygen is used as hydrogen
acceptor. Water is formed as a product.
CH
H
OH OH
O
O
CH2OH
HO CH
H
O O
O
O
CH2OH
HOASC acid oxidase
1/2 O2
H2O
ascorbic acid (ASC acid) Dehydroascorbic acid

7. Aerobic Dehydrogenases
 May be two types:
 Aerobic dehydrogenases. Can use
oxygen or any other substance as hydrogen
acceptor. H2O2 is formed as product.
CHO
OHH
HHO
OHH
OHH
CH2OH
O
OH
CH2OH
O
OH
HO
O2
H2O2
glucose oxidase

8. Anaerobic dehydrogenases
Unable to use oxygen as hydrogen acceptor.
Uses special hydrogen acceptors-coenzymes:
NAD+, NADP+, FAD etc.
CH3 CH COOH
OH
CH3 C COOH
O
LDH
NAD+
NADH
lactate
pyruvate

9. Hydroxyperoxidases
 Uses H2O2 or organic peroxide as a
substrate
 Catalases (found in plants as well as
animals)
 Peroxidases (found in milk, plant, platelets,
thyroid gland)
 Examples: 2H2O2 = 2H2O + O2
 H2O2 + H+ + 2e = 2H2O

10. Reductases
 Convert double Carbon-carbon bond to single
 E.g.
 biliverdin ----------bilirubin
testosterone ------dehydrotestosterone(DHT)

11. CLASS B
Transferases
 Exchange groups (except hydrogen) between
two compounds
 Phosphotransferases (kinases)
O
OH
OH
OH
CH2OH
O
OH
OH
OH
CH2O-PO3H2
OH + ATP
OH + ADP
glucokinase
glucose
glucose-6-P

12. Transferases
 Transmethylases
 Catalyse the transfer of methyl groups
CH-CH2
OH NH2
HO
HO
CH-CH2
OH NH(CH3)
HO
HO
Noradrenaline
(norepinephrine)
Donor of -CH3 group
(e.g. SAM: S-adenosylmethionine)
CH3-donor
Adrenaline
(epinephrine)

13. Transferases
 Transacylases
 Catalyze the transfer of acyl- group (R-
CO-).
 CH3-CO- acetyl group. Important donor of
acetyl group is Acetyl CoA.
CH3 C
O
SCoA + CH2
OH
CH2
N(CH3)3
CH2
O
CH2
N(CH3)3C
O
H3C
HSCoA
Acetyl CoA
Choline
transacetylase
acetylcholine

14. CLASS C
Hydrolases
 Catalyse the hydrolysis ( decomposition of
substrate with addition of water)
 Sub-classes
 Proteolytic enzymes ( protein hydrolyzing
enzyme)
 Amylolytic enzymes (carbohydrates
hydrolyzing enzyme)
 Lipases (lipids hydrolyzing enzymes)

15. CLASS E
Isomerases
 Catalyses the formation of isomers of
substrate.
O
OH
OH
OH
CH2-O-PO3H2
OH
CH2OH
CH2-O-PO3H2
OH
OH
O
OH
phosphoxexose
isomerase
GLUCOSE-6-P FRUCTOSE-6-P

16. CLASS D
Lyases
 These enzymes catalyze the addition of NH3,
CO2, and H2O and formation of new bond C-
N, C-O, C-C
 or the removal of these form double bonds
(without hydrolysis).

17. CLASS F
Ligases
 Catalyses the reaction of the joining of two
molecules by forming С - O, С — S,
С — N, and С— С bonds. Similar to liases but
requires energy (ATP)
CH3
C O
SCoA
+ CO2 CH2
C O
SCoA
COOH
ATP
ADP
AcetylCoA carboxylase
Acetyl CoA
Malonyl CoA