Explained briefly about on Reversible inhibition

1. Types of enzyme inhibition
Mentor: Dr. Gaurav Shah
By: Kashvi N. Jadia
Densi M. Patel
Sem 7 Msc Biotechnology

2. TYPES OF ENZYME INHIBITION
Types of enzyme inhibition
Reversible
inhibition
Irreversible
inhibition
Suicidal
inhibition
Group
specific
inhibition
Non
competitive
inhibition
Competitive
inhibition

3. Reversible inhibition:
● Reversible inhibitors bind to an enzyme in a reversible fashion and
can be removed by dialysis (or simply dilution) to restore full
enzymatic activity.
● Reversible inhibitors usually rapidly form an equilibrium system
with an enzyme to show a definite degree of inhibition.

4. Competitive inhibition:
● The substrate and the inhibitor exhibit structural similarity,
due to which they compare for same active site.
● Also called structural analogue.

6. ● Function by decreasing the proportion of enzyme
available for substrate binding.
● Can be relieved by increasing the substrate
concentration.
● Le chatelier's principle
● Degree of inhibition can be calculated by following
equation
Degree of inhibition= I = 1- Vi/V0 x 100

8. MM plot and LB plot for competitive inhibition

9. Examples of competitive inhibitors with enzymes:
ENZYME INHIBITOR
Succinate dehydrogenase Malonate
Folate synthesising enzyme Sulphonamide
Dihydrofolate reductase Methotrexate
Xanthine oxidase Allopurinol

10. Malonate/Oxaloacetate inhibits succinate dehydrogenase in TCA
cycle.

11. Methotrexate inhibits
dihydrofolate (DHF)
reductase
Tetra hydro folate
Allopurinol inhibits Xanthine
oxidase in purine metabolic
pathway
THF - Tetrahydrofolate

12. ● Metal ions also acts as competitive inhibitor
● Sulfonamide inhibits folate biosynthesis

13. Application of competitive inhibition:
■ Competitive inhibitors are used to treat intoxication caused by the
product of a particular enzyme. For example, methanol poisoning and
ethyl glycol poisoning can be treated by inhibiting alcohol
dehydrogenase using ethanol as an inhibitor.
■ Sulfonamide is used to inhibit bacterial growth by preventing folate
synthesis, thereby preventing folate-catalyzed one carbon transfer
reactions.

14. ■ Allopurinol is used to treat gout. Gout is a disease characterized by
high serum levels of uric acid, the final product of purine
degradation. It is formed by xanthine oxidase activity. Allopurinol
is used to treat gout by impairing the xanthine-oxidase catalyzed
synthesis of uric acid
■ Methyl pyrazole is used to treat metabolic acidosis, depression,
and renal stone formation by inhibiting the formation of glycolic
acid and oxalic acid.

15. Non competitive inhibitor:
● The substrate and inhibitor have no structural similarity, hence do
not compete for the same active site.
● Non-competitive inhibitors bind the allosteric site (regulatory
site) of the enzyme and alters the three-dimensional structure of
the active site.
● A noncompetitive inhibitor binds either to the free enzyme or to
the enzyme substrate complex.

17. ● Function by decreasing the turnover number of the enzyme.
● Unlike competitive inhibition, non-competitive inhibition cannot be
relieved at a higher substrate concentration, so the V cannot be attained.
● Binding of non competitive inhibitor induce change in the shape of Active
site, so the enzyme cannot bind to the substrate.
● The enzyme will remain inhibited until the non-competitive inhibitor
leaves the regulatory site.
● The inhibitor effectively lowers the concentration of active enzyme and
hence lowers Vmax. There is almost no effect on Km, however.

18. MM plot and LB plot for noncompetitive inhibition:

19. Examples:
● Heavy metal ions
● EDTA
● Fluoride
● Hydrogen sulphide
● Cyanide
● Pepstatin

20. Heavy metal
● Heavy metals Enzymes containing active sulphur, oxygen, and nitrogen are non
competitively inhibited by heavy metal ions like Pb, Hg, and Ag.
● Interact with the thiol group of the enzyme forming mercaptide complex which is
inactive.
E - SH + Hg+
● Pb is known to inhibit porphobilinogen synthase and ferrochelatase by
binding to the active thiol group.
● This enzyme is also called as 8-aminolevulinate dehydratase, Ferrochelatase
catalyzes the synthesis of heme from protoporphyrin IX by inserting ferrous
ion to the substrate (last step of the heme biosynthetic pathway).
E - S -H + H+ (Inactive mercaptide complex)

21. Fluoride
● Fluoride Aldolase is an enzyme that converts fructose-1,6-bisphosphate to
dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate
(G3P) during the fourth step of glycolytic pathway.
● Fluoride ion inhibits aldolase by binding to active Mg or Mn. Thus, fluoride
is used as preservative for blood glucose analysis.

22. Hydrogen sulphide and cyanide
● Hydrogen sulphide (H2S) and cyanide (CN) inhibition-containing
enzymes like superoxide dismutase (SOD) and catalase (CAT).
● Super oxide dismutase catalyzes the dismutation of superoxide
anion radical forming hydrogen peroxide. Catalase catalyzes the
detoxification of hydrogen peroxide to water and oxygen.

23. Pepstatin
Pepstatin non-competitively inhibits renin.
EDTA
EDTA (chelating agent) functions as a non-competitive inhibitor of
metalloenzymes by removing the metal ion cofactors like Mg+2 and Ca+2.

24. Applications of non-competitive inhibitors
● To relieve heavy metal poisoning, e.g. lead poisoning and mercury
poisoning
● To relieve cyanide poisoning
● Fluoride is used as blood preservative for subsequent analysis of glucose.
● In chronic disease like type 2 diabetes (diabetes mellitus).
(https://www.ncbi.nlm.nih.gov/books/NBK545242/#:~:text=In%20noncompetitive%20inhibition%2C%20the%20inhibitor%20binds%20at%20an%20al
losteric%20site,presence%20of%20a%20bound%20substrate.)

25. Uncompetitive inhibition
Uncompetitive inhibitors have no affinity for substrates. It binds to the
allosteric site of the enzyme-substrate complex only, but not to the free
enzyme.

27. MM plot and LB plot for uncompetitive inhibition

28. Examples for uncompetitive inhibition
● Uncompetitive inhibition is rare in mono substrate
reactions, but it is common in bisubstrate reactions showing
double displacement mechanism
● L-Phenylalanine L-phenylalanine is an uncompetitive
inhibitor for intestinal alkaline phosphatase.

30. Refrences
➢ Reference books:1) Enzymology by T.Devasena
2)Biochemistry by U. Satyanarayana and U. Chakrapani
➢ https://www.ncbi.nlm.nih.gov/books/NBK545242/#:~:text=In%20noncomp
etitive%20inhibition%2C%20the%20inhibitor%20binds%20at%20an%20all
osteric%20site,presence%20of%20a%20bound%20substrate.