MECHANISM OF ENZYME ACTION
M.SC. CHEMISTRY (SEMESTER-3)
DEPARTMENT OF CHEMISTRY S.P.C.G.C. AJMER
1. INTRODUCTION OF ENZYMES
2. TRANSITION-STATE THEORY
3. COVALENT CATALYSIS
4. ACID BASE CATALYSIS
5. INTRAMOLECULAR CATALYSIS
6. EXAMPLE OF CHYMOTRYPSIN ENZYME
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
MECHANISM OF ENZYME ACTION
The catalystic efficiency of enzyme is explained
by 2 perspectives:
2.Processes at the active site
• What is transition-state theory?
• All chemical reaction have eneygy bariers between reactant and
• Activational Barrier:- Differnce in energy level of Transitional state
• 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
• 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
• 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
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
• 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
(ORIENTATION AND STERIC EFFECT)
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
Therefore it has 3 chains (red, blue, green) but these are covalently linked by
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.
• Serine protease
• Catalystic mechanism involves Ser residue.
• Utilizes catalytic triad
• Ser provides nucleophile(o atom)
• His acts as base catalyst to activate Ser
• Asp stabilizes protonated His
• 2-step reaction covalent catalysis
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