Biochemical Energetics
1
Free energy of a reaction
The free energy change (DG) of a reaction determines
its spontaneity. A reaction is spontaneous if DG is
negative (if the free energy of products is less than that
of reactants).
For a reaction A + B  C + D
[C] [D]
DG = DG ' + RT ln
[A] [B]
o
DGo' = standard free energy change (at pH 7, 1M
reactants & products); R = gas constant; T2 = temp.
For a reaction A + B  C + D
[C] [D]
DG = DGº' + RT ln
[A] [B]
DGo' of a reaction may be positive, & DG negative,
depending on cellular concentrations of reactants and
products.
Many reactions for which DGo' is positive are
spontaneous because other reactions cause depletion
of products or maintenance of high substrate
concentration.
3
At equilibrium DG = 0.
[C] [D]
DG = DGº' + RT ln
[A] [B]
[C] [D]
ratio  = DGº' + RT ln [A] [B]
K'eq,
the
[C][D]/[A][B]
at
equilibrium,
is
the
equilibrium constant.
An equilibrium constant
(K'eq) greater than one
indicates a spontaneous
reaction (negative DG').
[C] [D]
DGº' = - RTln
[A] [B]
[C] [D]
defining K'eq =
[A] [B]
DGº' = - RT ln K'eq
4
DGo' = -RT ln K'eq
Variation of equilibrium constant with DGo‘ (25 oC)
K'eq
DG º'
kJ/mol
Starting with 1 M reactants &
products, the reaction:
10
4
- 23
proceeds forward (spontaneous)
10
2
- 11
proceeds forward (spontaneous)
100 = 1
10
10
0
is at equilibrium
-2
+ 11
reverses to form “reactants”
-4
+ 23
reverses to form “reactants”
5
“High energy” bonds
NH2
ATP
adenosine triphosphate
O
-O
P
O-
O
O
P
O-
N
N
O
O
P
O
phosphoanhydride
bonds (~)
N
adenine
CH2
O-
N
O
H
H
OH
H
OH
H
ribose
Phosphoanhydride bonds (formed by splitting out H2O
between 2 phosphoric acids or between carboxylic &
phosphoric acids) have a large negative DG 6of hydrolysis.
NH2
ATP
adenosine triphosphate
O
-O
P
O-
O
O
P
adenine
N
N
O
O
O-
phosphoanhydride
bonds (~)
P
N
O
CH2
O-
O
H
H
OH
H
OH
H
N
ribose
Phosphoanhydride linkages are said to be "high energy"
bonds. Bond energy is not high, just DG of hydrolysis.
"High energy" bonds are represented by the "~" symbol.
~P represents a phosphate group with a large negative DG
of hydrolysis.
7
Potentially, 2 ~P bonds can be cleaved, as 2 phosphates
are released by hydrolysis from ATP.
AMP~P~P  AMP~P + Pi
(ATP  ADP + Pi)
AMP~P  AMP + Pi
(ADP  AMP + Pi)
Alternatively:
AMP~P~P  AMP + P~P
P~P  2 Pi
8
(ATP  AMP + PPi)
(PPi  2Pi)
9
10
11