Enzymes are proteins that act as catalysts to speed up biochemical reactions. They have several key characteristics - they are effective in small quantities, efficient, specific, and their activity is affected by their surroundings. There are six main classes of enzymes classified by their catalytic activity. Factors like enzyme concentration, substrate concentration, pH, temperature, and inhibitors can affect an enzyme's activity. Inhibitors bind to enzymes to disrupt their activity, and can be competitive, noncompetitive, or irreversible.

2. ENZYMES
Outline
Definition
Characteristics of enzymes
Types of enzymes
Factors effecting enzyme activity

3. 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

4. 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

6. • Function: catalyzes oxidation-reduction reactions
• e.g. alcohol dehydrogenase
• Other e.g. Biliverdin reductase; Glucose oxidase

7. • Function: catalyzes reactions involving
transfer of functional groups
• e.g. Hexokinase
• Other e.g. Glycoaldehyde transferase; DNA
nucleotidylexotransferase

8. • Function: catalyzes hydrolytic reactions
involving use of water mol.
• e.g. Triacylglycerol lipase
• Other e.g. -amino acid esterase;
Oxaloacetase
H2O

9. • 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

10. • Function: catalyzes intramolecular arrangement
• e.g. Maleate isomerase
• Other e.g. Inositol-3-phosphate synthase;
Maltose epimerase]

11. • 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

12. 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

13. Lysozyme’s tertiary structure
Anti-parallel
-sheet
(3)
-helix
(5)

14. Aspartate carbamoyltransferase’s quartenary structure
 2 catalytic
trimers
 3 regulatory
dimers

16. How enzyme reacts
E + S ES E + P

17. Enzyme reaction…hypothesis
• lock and key hypothesis:
 proposed by Emil Fischer (1894)
 perfect match like specific key & lock

20. • induced-fit hypothesis:
 proposed by Daniel Koshland
(1958)
 shows elasticity/flexibility
at active site
 well-accepted

22. e.g.: hexokinase reaction

23. Factors affecting enzyme activity
• enzyme concentration
• substrate concentration
• pH
• temperature
• inhibitors

25. A
B

26. 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.

30. A
C
B

31. • substances which bind to enzyme &
disrupt the enzyme activity by
blocking the production of ES-complex
or E + P
• reversible & irreversible

32. • involves the noncovalent links between
inhibitor and enzyme
• 2 types:
 competitive inhibitor
 noncompetitive inhibitor
 uncompetitive inhibitor
Reversible inhibitors

33.  competitive inhibitor
 mol. similar to substrate
 compete with substrate for active site
e.g.: succinate dehydrogenase (E); succinate
(S); malonate (I)

34.  noncompetitive inhibitor
 mol./ion attaches to second site (other than
active site) at enzyme surface
e.g.: prostaglandin synthase (E); arachidonate
(S); aspirin (I)

35.  uncompetitive inhibitor
 binds to ES complex, forming an inactive ESI
complex
 e.g..: polymerase (E); nucleic acid (S);
nevirapine (I) - HIV

36. • 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

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