Sharda Tarad semester 3

1. MECHANISM OF ENZYME ACTION
2021-2022
SHARDA TARAD
M.SC. CHEMISTRY (SEMESTER-3)
DEPARTMENT OF CHEMISTRY S.P.C.G.C. AJMER

2. CONTENT
1. INTRODUCTION OF ENZYMES
2. TRANSITION-STATE THEORY
3. COVALENT CATALYSIS
4. ACID BASE CATALYSIS
5. INTRAMOLECULAR CATALYSIS
6. EXAMPLE OF CHYMOTRYPSIN ENZYME

3. INTRODUCTION OF ENZYMES
• Biological catalysts
• Neither consumed nor permanently altered
• All enzymes are proteins in nature except ribozymes which are RNA in nature
• Highly efficient
• Act as selective catalysts
• Site where actual Reaction occurs
• Substrate-bound by weak interaction

4. MECHANISM OF ENZYME ACTION
The catalystic efficiency of enzyme is explained
by 2 perspectives:
1.Thermodynamic change
2.Processes at the active site

5. • What is transition-state theory?
• All chemical reaction have eneygy bariers between reactant and
products.
• Activational Barrier:- Differnce in energy level of Transitional state
and Substrate.
SUBSTRATE PRODUCTS

6. • Activation Energy:- of a chemical reaction is the minimum energy that is needed to
make the reaction happen.
• Only a few Substrate can cross this barrier to be converted to Products.
• That is why rate of uncatalyzed reaction is much slow.
• When enzyme is present it provide an alternate pathway for conversion of Substrate
into product.
• Enzymes accelerate reaction rate by providing Transtion state with low activational
energy for formation of products.
• The total energy of the system remains the same equilibrium state is not disturbed

8. PROCESS AT THE ACTIVE SITE

9. • Enzyme from covalent linkages with substrate forming transient enz-subs complex
with very low activational barrier.
• Enzyme is then released unchanged and unconsumed and substrate is converted into
products.

10. ACID BASE CATALYSIS
• Mostly undertaken by oxido-reductases.
• Mostly at the active site, either histidine is present which act both as a proton donor and a
proton acceptor.

11. INTRAMOLECULAR CATALYSIS
• This mechanism is mostly undertaken by ligases.
• The rate of reaction is by bringing substrate closer to each other at the a.site.
• A region of high substrate conc. is produced at the active site..
• The substrates molecule is placed at bond forming distances.
• Since substrate is placed at optimal distances.
• The probability of collision and substrate is eventually converted into
products.
(ORIENTATION AND STERIC EFFECT)

12. Structure of Chymotrypsin
Globular single-domain protein.
Originally synthesized as a 245 residue protein, chymotrypsinogen.
Dipeptides 14-15 and 147-148 are clipped out, transforming the protein into active
chymotrypsin.
Therefore it has 3 chains (red, blue, green) but these are covalently linked by
disulfide bridges.
The reactive serine 195 is located in a cleft on the molecule, the active site.
Ser195 is adjacent to His57 and Asp102 which are responsible for its reactivity.

13. CHYMOTRYPSIN
• Serine protease
• Catalystic mechanism involves Ser residue.
• Utilizes catalytic triad
• Asp102-His57-Ser195
• Ser provides nucleophile(o atom)
• His acts as base catalyst to activate Ser
• Asp stabilizes protonated His
• 2-step reaction covalent catalysis

14. MECHANISM OF CHYMOTRYPSIN
STEP 1: H+ Shift generates Ser-O¯(substrate binding)
STEP 2: Ser-O¯binds to C꓿O(Nucleophilic attack)
STEP 3: Transition state ‫׀‬
STEP 4: Peptide bond breaks

15. MECHANISM OF CHYMOTRYPSIN
STEP 5: C-Terminal Peptide leaves
STEP 6: Ionization of water
STEP 7: Transition State ‫װ‬
STEP 8: N-Terminal peptide leaves

16. REFERENCES : -
• Harper’sIllustrated Biochemistry
• Bio-OrganicChemistryby
Dr.VinayPrabhaSharma