2. Mechanisms of Enzyme Action

3. 
Activation Energy (AE) – The energy require
to reach transition state from ground state.

AE barrier must be exceeded for rxn to
proceed.

Lower AE barrier, the more stable the
transition state (TS)

The higher [TS], the move likely the rxn will
proceed.

5. S  Ts  P

7. 
Transition state = unstable high-energy
intermediate

Rate of rxn depends on the frequency at which
reactants collide and form the TS

Reactants must be in the correct orientation and
collide with sufficient energy to form TS

Bonds are in the process of being formed and
broken in TS

Short lived (10 –14 to 10 -13 secs)

8. • Intermediates are stable.
• In rxns w/ intermediates, 2 TS’s are involved.
• The slowest step (rate determining) has the
highest AE barrier.
• Formation of intermediate is the slowest
step.

10. 
Enzyme binding of substrates decrease
activation energy by increasing the initial
ground state (brings reactants into correct
orientation, decrease entropy)

Need to stabilize TS to lower activation energy
barrier.

12. ES complex must not be too stable
Raising the energy of ES will increase the
catalyzed rate
•This is accomplished by loss of entropy due to
formation of ES and destabilization of ES by
•strain
•distortion
•desolvation

15. • Equilibrium between ES <-> TS, enzyme
drives equilibrium towards TS
• Enzyme binds more tightly to TS than
substrate

16. Transition
state analog

20. 
Substitutions rxns

Bond cleavage rxns

Redox rxns

Acid base catalysis

Covalent catalysis

21. Nucleophillic Substitution–
Nucleophillic = e- rich
Electrophillic = e- poor

22. Direct Substitution
Transition state

23. 
Loose e - = oxidation (LEO)

Gain e - = reduction (GER)

Central to energy production

If something oxidized something must be
reduced (reducing agent donates e - to
oxidizing agent)

24. 
Oxidations = removal of hydrogen or
addition of oxygen or removal of e -

In biological systems reducing agent is
usually a co-factor (NADH of NADPH)

25. 
Heterolytic vs homolytic cleavage

Carbanion formation (retains both e - )
R 3 -C-H  R 3 -C: - + H +

Carbocation formation (lose both e - )
R 3 -C-H  R 3 -C + + H: -

Hydride ion
Free radical formation (lose single e - )
R 1 -O-O-R 2  R 1 -O* + *O-R 2

26. 
Accelerates rxn by catalytic transfer of a
proton

Involves AA residues that can accept a
proton

Can remove proton from –OH, -NH, -CH,

27. Carbanion intermediate

28. 
20% of all enzymes employ covalent catalysis
A-X + B + E <-> BX + E + A

A group from a substrate binds covalently to
enzyme
(A-X + E <-> A + X-E)

The intermediate enzyme substrate complex (AX) then donates the group (X) to a second
substrate (B)
(B + X-E <-> B-X + E)

29. Protein Kinases
ATP + E + Protein <-> ADP + E + Protein-P
1)
A-P-P-P(ATP) + E-OH <-> A-P-P (ADP) + EO-PO 4 -
2)
E-O-PO 4 - + Protein-OH <-> E + Protein-OPO 4 -

30. The Serine Proteases
Trypsin, chymotrypsin, elastase, thrombin,
subtilisin, plasmin, TPA
• All involve a serine in catalysis - thus the
name
• Ser is part of a "catalytic triad" of Ser, His,
Asp (show over head)

31. 
Serine proteases are homologous, but
locations of the three crucial residues
differ somewhat

Substrate specificity determined by
binding pocket

33. Chymotrpsin
Trypsin
Elastase