Chemistry Reference
In-Depth Information
with NAD
+
, since it contains an oxidizable CH-OH
function. The product is then a thioester. The
thioester is attacked by a phosphate nucleophile, and
since it contains a good leaving group, EnzS
−
,the
reaction product is released from the enzyme as
1,3-
diphosphoglycerate
.
stepwise sequence involving thiol-containing enzyme
O
P
NAD
+
NADH
H
H
H
O
O
H
O
EnzS
OH
EnzS
O
OH
Enz
SH
C
O
phosphate
acting as
nucleophile
oxidizable
function
hemithioacetal
thioester
O
O
O
P
EnzS
O
P
If we look at the structure of 1,3-diphosphoglycer-
ate, we can see that it is actually an anhydride, albeit a
mixed anhydride of a carboxylic acid and phosphoric
acid (see Box 7.27). Accordingly, we expect it to
be fairly reactive towards nucleophiles, and indeed it
is. It is sufficiently reactive that hydrolysis liberates
enough energy to synthesize ATP from ADP.
1,3-diphosphoglycerate is a mixed anhydride of carboxylic and phosphoric acid
H
anhydride
O
P
OH
OH
O
P
OR
O
O
O
O
OH
O
OH
HOR
P
OR
OH
(ADP)
(ATP)
H
OH
CH
2
O
P
H
OH
CH
2
O
P
H
OH
CH
2
O
P
3-phosphoglycerate
ester
hydrolysis liberates sufficient energy to synthesize ATP from ADP
substrate-level phosphorylation
ATP synthesis is achieved by ADP acting as the
nucleophile towards this mixed anhydride, attacking
the P
=
O bond, with the carboxylate being the leav-
ing group. Note that this reaction is favoured, whereas
the alternative possibility involving hydrolysis of the
phosphate ester does not occur. This is precisely
what we would predict knowing the different reac-
tivities of anhydrides and esters (see Section 7.8).
This direct synthesis of ATP by a process in which
ADP acquires an additional phosphate from a suit-
able donor molecule is often termed
substrate-level
phosphorylation
, differentiating it from ATP synthe-
sis that is achieved through oxidative phosphoryla-
tion.
After donating its phosphate group to ADP, 1,3-
diphosphoglycerate
is
converted
into
3-phospho-
glycerate
.
This
reaction
is
followed
by
enzymic
modification to
2-phosphoglycerate
.
CO
2
H
CO
2
H
H
O
P
CH
2
OH
H
OH
CH
2
O
P
phosphoglycerate
mutase
2-phosphoglycerate
3-phosphoglycerate
