Mechanism of enzyme action - An enzyme attracts substrates to its active site, catalyzes the chemical reaction by which products are formed, and then allows the products to dissociate (separate from the enzyme surface). The combination formed by an enzyme and its substrates is called the enzyme–substrate complex.

1. Mechanism of
Enzyme Action

2. Enzymes
• Biocatalyst
• Protein in nature
• Active Sites
• Efficiency
• Specificity
• Holoenzymes
• Regulation
• Location
3D Model of an Enzyme

3. Mechanism of Enzyme Action
• An enzyme allows a reaction to proceed rapidly under conditions
prevailing in the cell by providing an alternate reaction pathway
with a lower free energy of activation.
• The enzyme does not change the free energies of
the reactants or products and, therefore, does not change the
equilibrium of the reaction.
• It does accelerate the rate with which equilibrium is reached.
• The rate enhancement (ratio of the rates of the catalyzed and
uncatalyzed reactions) is given by eΔΔG ‡ cat/RT

4. Catalytic Mechanisms
1. Acid–base catalysis
2. Covalent catalysis
3. Metal ion catalysis
4. Proximity and orientation effects
5. Preferential binding of the transition state complex

5. Acid–Base Catalysis

6. Bovine Pancreatic RNase A
• Secreted by pancreas into
small intestine
• Hydrolyzes RNA to
component nucleotides
X-Ray structure of bovine pancreatic RNase A

9. Covalent Catalysis

10. Covalent Catalysis Examples

11. Metal Ion Catalysis
Carbonic Anhydrase
Mechanism

12. Catalysis Through Proximity and Orientation Effects
Intermolecular Reaction
Intramolecular Reaction is 24 times faster

13. Orientation of SN2 reaction
Any deviation from this optimal geometry would increase the free energy
of the transition state and reduces the rate of the reaction.

14. Catalysis Through Preferential Binding of the
Transition State Complex
• An enzyme may bind the transition state of the reaction it catalyzes with
greater affinity than its substrates or products!
• The more tightly an enzyme binds its reaction’s transition state relative to
the substrate, the greater is the rate of the catalyzed reaction relative to
that of the uncatalyzed reaction.
• Where transition state stabilization makes only a minor contribution to rate
enhancement the enzyme promotes the reaction by instead stabilizing the
so-called near attack conformation, a step along the reaction coordinate in
which the reactants are properly oriented and in van der Waals contact but
have not yet reached the transition state.

15. Proof!
Proline Analogs
Proline racemase is inhibited by the planar analogs of proline, pyrrole- 2-carboxylate and
Δ-1-pyrroline-2-carboxylate, both of which bind to the enzyme with 160-fold greater
affinity than does proline. These compounds are therefore thought to be analogs of the
transition state in the proline racemase reaction.

16. References
• Voet, D.; Voet, J.; Pratt, C.W.; Fundamentals of Biochemistry Life at the
Molecular Level, 5th Edition, Wiley
• Berg, J.M.; Tymoczko, J.L.; Stryer, L; Biochemistry, 7th Edition, W.H.
Freeman and Company New York
• Harvey, R.A.; Ferrier, D.R.; Lippincott’s Illustrated Reviews:
Biochemistry, 5th Edition, Wolters Kluwer
• Nelson, D.L.; Cox, M.; Lehninger Principles of Biochemistry, 7th Edition,
W.H. Freeman

17. Thank You.