Transcript Document

Provide Free Energy
Transfer phosphoryl groups
ATP is a waystation for free energy, not a storage
molecule
Free energy transmission
GroEL-ES: use it to fold proteins
Heat generation
Phosphocreatine acts as a free energy reservoir
ATP + creatine
Phosphocreatine + ADP
∆Gº’ = + 12.6kJ/mol
However
∆G = 0 kJ/mol
How do enzymes catalyze reactions?
• Acid-base catalysis
• Covalent catalysis
• Metal ion/electrostatic catalysis
Acid-base catalysis
General acid/base catalysis is a process in
which proton transfer to or from a
Brønsted acid or base lowers the
activation energy of a reaction’s transition
state
Mechanisms of keto–enol
tautomerization.
(a) Uncatalyzed.
R
R
R
C
O
C
H2C
H
H2C


O
C
O
H
CH2
H

General acid catalyzed keto–enol
tautomerization.
General base catalyzed keto–enol
tautomerization.
Acid/Base Catalyzed Mutorotation
H
H
A
CH2OH
A
CH2OH
O
O
H
OH
OH
OH
O
OH

H
H
OH
B-
OH

A-
CH2OH
H
O
H
O
OH
H
OH
OH
B
OH
B-
Covalent Catalysis
Rate acceleration through
transient formation of a
catalyst-substrate
covalent bond
Schiff Base Formation
H
H
N+
R
H
C
H
H
A
H+
N
O
N+
R
B-
C
R
OH
C
H2O
H
H
A
B-
BH
H
H
R
H
N+
H
C
O
R
N+
H
A
H
H+
N
R
C
O
C
O
H
H
H
A
B-
The decarboxylation of acetoacetate.
Metal ion/electrostatic catalysis
Lewis Acids
Superacids
Mn+
-O
C
O
Mn+
O
CH3
C
C
CH3
O
-O
C
C
O-
O
Mn+
O-
CH3
O
C
C
C
CH3
O
H+
-O
C
O
O
CH3
C
CH
CH3
Nucleophilic Catalysis
H 2O
HO- + H+
pKa > 15
(NH3)5Co3+(H2O)
(NH3)5Co3+(HO-) + H+
pKa = 6.6
H
N
N
H
O
H
H
H
H
H
O
O
Zn2+
Im
Im
Im
H
N
N+
H
O
H
H
H
H
O
O-
H
Im
Zn2+
Im
Im
H
O
C
O
OIm
Zn2+
Im
-O
H
O
H
O-
H2 O
C
O-
Im
Im
Zn2+
Im
Im
Zn2+
Im
Im
+ H+
O
H
O
C
O-
Im
Electrostatic Catalysis
Binding of substrate generally
excludes water from active site
therefore active site dielectric
constant is significantly different
from the standard state. ∆G
changes!
Increases electrostatic interactions
often causing the stabilization of
the transition state
Charge Shielding
NH2
N
N
O
-O
P
O-
O
O
P
N
O
O
P
O
O
H
O-
-O
H
H
Mg2+
H
OH
OH
N
Zn2+
Zn2+
O
O
C
CH
Carbanion
C
CH
Zn2+-stabilized
enolate
H
B
O
C
H
B
O
CH
Hydrogen bonded
carbonyl
C
CH
Hydrogen bonded
enolate
Enzyme Classification According to
Reaction Type.
Oxidoreductases: transfer of
electrons in the form of hydride ions
(H-) or H atom radicals (H•)
Alkene
Oxidoreductases
B
O
H
C
-O
C
H
O
O-
C
O-
H
H
C
C
C
C
O
C
-O
H
H
O
+
+
O
O
N
H
N
NH
NH
N
O
N
N
R
H+
R
Giving and taking electrons
N
H
O
Oxidation reduction reactions
Oxidation and Reduction Reactions
Redox/Oxidoreduction reactions
Oxidant + Reductant
Reduction Oxidation
Fe3+ + eCu+
Fe3+ + Cu+
Fe2+
e- + Cu2+
Fe2+ + Cu2+
Aoxn+ + Bred
Ared + Boxn+
∆G = ∆Gº + RT ln
∆G = - wel
[Ared][Boxn+]
[Bred][Aoxn+]
w = electrical work
wel = -nFe
∆G = -nFe
∆e = ∆eº - RT/nF ln
[Ared][Boxn+]
[Bred][Aoxn+]
Every redox reaction can be broken into two half reactions
Aoxn+ + ne-
Ared
Boxn+ + ne-
Bred
∆e = ∆eº - RT/nF ln
∆e = ∆eº - RT/nF ln
[Ared]
[Aoxn+]
[Bred]
[Boxn+]
Can couple half-reactions to make reactions spontaneous
∆e’º = e’º(e- acceptor) - e’º(e- donor)
∆G’º = -nF∆e’º
If you subtract the e’º and
the number is positive
then the ∆G’º is negative
and the reaction is
spontaneous.
Don’t forget! Equations
must balance!
NAD+ is kept high to be used as a cellular oxidant
NADPH is kept high to be used as a cellular reductant
These molecules are stored ∆G for redox
reactions. They are much like ATP in this
respect
Transferases: group transfer
from one molecule to another
Acyl group transfer.
Phosphoryl group transfer.
Glycosyl group transfer.
Transferases
NH2
O-
O-
N
N
O
P
O
P
O-
O
N
O
O
P
O
O
O-
B
H
H
OH
OH
H
HOCH2
N
NH2
H
O-
N
N
O
-O
P
O
N
O
OH
OH
OH
O
P
O-
OH
O
O
O-
H
H
OH
OH
H
O
P
O
+
O
O-
OH
OH
OH
OH
H
N
Hydrolases are a catalyze hydrolysis
special class of transferase that catalyze
the transfer of water
O
H
B
H2N
O
R
+
H
H
H
R
H
N
ON
H
N
H
R
O
O
R
O
R
R
R
R
H
H
Lyases: Group elimination
to elicit double bond
formation. Or addition of
groups to double bonds.
Enzymes that transfer hydrides (oxidoreductases)
can be placed in this class technically
Lyases
O
H
C
-O
O
C
C
H
H
H
OH
OPO32-
H+
C
C
-O
C
OPO32-
B
Group elimination to elicit
Double bond formation
H
+ H2O
Elimination reaction mechanisms
Isomerases: transfer of groups
within molecules to yield
isomeric forms
Isomerases
-OOC
-OOC
OH
COO-
-OOC
CH2
COO-
-OOC
C
H2
C
H
H
H
C
HO
H
Aldose–ketose isomerization.
Ligases: Formation of C-C, C-S, C-O
and C-N bonds by condensation
reactions coupled to ATP cleavage
Ligases
O
O
H2C
H
C
C
C
-O
O
O
O-
C
C
O
CH2
OC
O
ATP
B
ADP
O