Chemistry Reference
In-Depth Information
Hydrolysis of ATP to ADP is rationalized simply as
nucleophilic attack of water on to the terminal P
=
O
double bond, followed by cleavage of the anhydride
bond and expulsion of ADP as the leaving group (see
Box 7.25).
anhydride
ester
Ad = adenosyl
nucleophilic attack
onto P=O of
anhydride
O
P
O
O
O
P
O
O
O
P
O
P
O
H
2
O
HO
- H
+
P
P
Ad
P
P
Ad
+ H
+
P
Ad
HO
O
O
O
HO
O
O
O
HO
OH
HO
O
O
OH
OH
OH
OH
OH
OH
OH
OH
OH
ATP
phosphate
adenosine
diphosphate; ADP
also possible:
ester
anhydride
O
P
O
O
O
P
O
O
O
P
O
O
P
HO
P
P
Ad
- H
+
P
P
Ad
P
+ H
+
Ad
HO
O
O
O
HO
O
O
O
HO
O
OH
HO
O
OH
OH
OH
OH
OH
OH
OH
OH
OH
H
2
O
ATP
diphosphate
adenosine
monophosphate; AMP
There are two anhydride linkages in ATP, but
nucleophilic attack in the enzyme-controlled reaction
usually occurs on the terminal P
water. Overall, the oxidation of a substrate AH
2
could
be represented by the equation
O (hydrolysis of
ATP to ADP), and only occasionally do we encounter
attack on the central P
=
O (hydrolysis of ATP to
adenosine
=
AH
2
+
½ O
2
A
+
H
2
O
large negative
Δ
G
and this reaction has the potential to liberate energy,
i.e. it has a large negative
G
.
Now this reaction is not possible directly. We
are not accustomed to seeing our food sponta-
neously reacting with atmospheric oxygen and ignit-
ing because of the energy released! However, food
such as carbohydrate, fat and protein is oxidized after
we have eaten it, and energy is released and utilized
by our bodies. The secret is to react AH
2
through
the involvement of a suitable
coenzyme
, not directly
with oxygen. This reaction can be considered as
monophosphate,
AMP).
Both
reactions
34 kJ mol
−
1
.
This is not surprising, since in each case the same
type of bond is being hydrolysed. The further
hydrolysis of AMP to adenosine breaks an ester
linkage and would liberate only a fraction of the
energy,
G
−
yield the same amount of energy,
G
−
9 kJ mol
−
1
, and this reaction is not
biochemically important.
Oxidation reactions
are the main providers of
energy for ATP synthesis. Whilst oxidation usually
involves incorporation of one or more oxygen atoms,
in its simplest form it can be thought of as a loss of
electrons. Thus, the transformation of ferrous ion to
ferric ion is an oxidation reaction and involves loss
of one electron. Such electrons can be considered
as carrying the energy released from the oxidation
reactions.
+
AH
2
+
X (oxidized)
A
X (reduced)
coenzyme
coenzyme
where X is the coenzyme. The reaction is catalysed
by an enzyme termed a dehydrogenase, which
removes two hydrogen atoms from the substrate. The
coenzyme system involved can generally be related to
the functional group being oxidized in the substrate.
If the oxidation process is
oxidation
Fe
2+
Fe
3+
+
packets of
energy
e
CH
OH
C
O
In biochemical reactions, these electrons are even-
tually passed to oxygen, which becomes reduced to
