F
1. Enzyme does not affect ΔG or ΔGo between S and P (i.e., equilibrium)
2. Enzyme reduces Ea : Ea (catalyzed) Ea (uncatalyzed)
A more complete way of showing the effects of enzymes :
Enzymes bind to
substrates, so
G (ES) G (E S) .
However, if all they
did was to bind, then Ea
=ΔG (ES) for the reaction
would not be reduced.
So when they bind the
substrate, they stress
It in some way, raising
G (ES) for part of the
substrate and reducing
ΔG (ES) (=Ea) .
Quantitatively, what is the effect of reducing Ea -
For reaction A B, V = k A
k = (T/h) exp (-Ea/RT)
= Boltzman's constant; h = Plank's constant,
So k and thus V are inversely and exponentially
related to Ea and directly related to T :
A 6 kJ/mol reduction in Ea gives ca 10x increase in k
and V
k ~ exp (+6000/8.3 300) ~ 11
(reduction in Ea is an increase from -Ea)
V (catalyzed) /V (uncatalyzed) for various enzymes
varies from 104 to 1021 , meaning Ea is reduced by ca
23 to 126 kJ/mol
How do enzymes reduce Ea -
These effects raise G (ES) : cage effect, orientation,
steric straining of bonds (stress from H-,
Vanderwaal's, ionic bonds) , dislocation of bonding
electrons through +/- charges
These effects reduce G (ES) : covalent bonds, acid-
base catalysis, low-barrier hydrogen bonds, and metal
ion catalysis
Different classes of enzymes may use different
mechanisms :
1. Oxidoreductases (oxidation-reduction reactions)
2. Transferases (transfer of functional groups)
3. Hydrolases (hydrolysis reactions)
4. Lyases (addition to double bonds)
5. Isomerases (isomerization reactions)
6. Ligases (formation of bonds with ATP cleavage)
Examples :
Orientation,
Cage effect
Strain
Charge effects,
Covalent bonds,
Acid-base catalysis
An example
of an enzyme
that sterically
strains the
substrate :
Lysozyme Hydrolysis
distorts the bonds breaks the
of one of the polysaccharide
sugars in the chain and
polysaccharide weakens
of a bacterial the wall so
cell wall that the cell
lyses.
It also places a
partial charge on
the substrate,
making it react
more easily
with water
(hydrolysis) .
Example of an enzyme mechanism using covalent
bonds, acid-base catalysis, low-barrier hydrogen
bonds
Serine protease (e.g., trypsin, chymotrypsin,
acetylcholinesterase) : hydrolyzes peptide bond of
proteins (or acetylcholine) ,
substrate (A-CO-NH-B) + H2O A-COOH + H2N-B
Asp-His-Ser = DHS
e- movement
Low-barrier hydrogen bond :
e- movement
(same picture as previous)
Cleavage of the
peptide bond
Release of the
amino product
(same picture as previous)
e- movement
e- movement
ser-substrate bond breaks
#Visual