1. 1
ENZYMES
Outline
Definition
Characteristics of enzymes
Types of enzymes
Factors effecting enzyme activity
The definition & Characteristics of enzymes
• A protein that acts as a catalyst, speeding the rate at
which a biochemical reaction proceeds. a type of
protein
Characters:
• a catalyst
• Effective in smaller quantities
• efficient and specific
• reaction can be reversed
• activities affected by surroundings
• need helpers – cofactors/prostethic grps
• involve in multiple steps of biochemical pathways
Classification of enzymes
 6 main classes according to International
Union of Biochemistry and Molecular
Biology (IUBMB):
1. oxidoreductase
2. transferase
3. hydrolase
4. lyase
5. isomerase
6. ligase
• Function: catalyzes oxidation-reduction reactions
• e.g. alcohol dehydrogenase
• Other e.g. Biliverdin reductase; Glucose oxidase

2. 2
• Function: catalyzes reactions involving
transfer of functional groups
• e.g. Hexokinase
• Other e.g. Glycoaldehyde transferase; DNA
nucleotidylexotransferase
• Function: catalyzes hydrolytic reactions
involving use of water mol.
• e.g. Triacylglycerol lipase
• Other e.g. -amino acid esterase;
Oxaloacetase
H2O
• Function: catalyzes cleavage of C-C, C-O, C-
N and other bonds by other means than by
hydrolysis or oxidation
• e.g. Lysine decarboxylase
• other e.g.: threonine aldolase [EC 4.1.2.5];
cystine lyase
• Function: catalyzes intramolecular arrangement
• e.g. Maleate isomerase
• Other e.g. Inositol-3-phosphate synthase;
Maltose epimerase]
• Function: catalyzes the joining of two molecules with
concomitant hydrolysis of the diphosphate bond in
ATP or a similar triphosphate
• e.g. Pyruvate carboxylase
• Other e.g. GMP synthase; DNA ligase
Enzyme as protein
• exhibits characteristics like other proteins
• primary structure
 amino acid sequence
e.g.: human pancreatic lipase (467 amino acids)
N-Met1-…-Ser171-...-Asp194-...-His281-…-Cys467-C
human trypsin (247 amino acids)
N-Met1-…-His63-…-Asp107-…-Ser200-…-Ser247-C

3. 3
Lysozyme’s tertiary structure
Anti-parallel
-sheet
(3)
-helix
(5)
Aspartate carbamoyltransferase’s quartenary structure
 2 catalytic
trimers
 3 regulatory
dimers
How enzyme reacts
E + S ES E + P
Enzyme reaction…hypothesis
• lock and key hypothesis:
 proposed by Emil Fischer (1894)
 perfect match like specific key & lock

4. 4
• induced-fit hypothesis:
 proposed by Daniel Koshland
(1958)
 shows elasticity/flexibility
at active site
 well-accepted
e.g.: hexokinase reaction
Factors affecting enzyme activity
• enzyme concentration
• substrate concentration
• pH
• temperature
• inhibitors

5. 5
A
B
Change in pH effects the pull/push force of polar/nonpolar
intramolecules which change the enzyme shape as well as active
site – in acidic conditions, basic grps are protonized while in
basic conditions, acidic grps are deprotonized.
A
C
B

6. 6
• substances which bind to enzyme &
disrupt the enzyme activity by
blocking the production of ES-complex
or E + P
• reversible & irreversible
• involves the noncovalent links between
inhibitor and enzyme
• 2 types:
 competitive inhibitor
 noncompetitive inhibitor
 uncompetitive inhibitor
Reversible inhibitors
 competitive inhibitor
 mol. similar to substrate
 compete with substrate for active site
e.g.: succinate dehydrogenase (E); succinate
(S); malonate (I)
 noncompetitive inhibitor
 mol./ion attaches to second site (other than
active site) at enzyme surface
e.g.: prostaglandin synthase (E); arachidonate
(S); aspirin (I)
 uncompetitive inhibitor
 binds to ES complex, forming an inactive ESI
complex
 e.g..: polymerase (E); nucleic acid (S);
nevirapine (I) - HIV
• covalently bonded – react with functional
grp at active site, blocking active site from
substrate rendering the enzyme inactive
• mostly are toxic substances
• e.g.: (see attached list)
Irreversible inhibitors

7. 7

8. 8

9. 9
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