SECTION 13.1
Glycolysis
231
F I G U R E 1 3 -4
Glyceraldehyde 3-phosphate is oxidized by NAD+, and inorganic phosphate (Pp is incorporated into the product to
form an acyl phosphate, 1,3-bisphosphoglycerate. NAD+ is reduced by transfer of a hydride ion (H_) from
thiohemiacetal to the fourth position on the nicotinamide ring of NAD+.
P h o sp h o ryla tio n o f A D P
fr o m 1 ,3 -B isp h o sp h o g lycera te
In a reversible reaction, the phosphoryl group of the acid
anhydride of glyceroyl phosphate is transferred to ADP by
phosphoglycerate kinase.
4 _
Mg2+
1.3- Bisphosphoglycerate
+ ADP
-—^
3-phosphoglycerate3“ + ATP
4
The net result of this reaction is the formation of
two molecules of ATP per molecule of glucose. This
formation of ATP is known as
su b stra te-level p h o s-
p h o ryla tio n
to distinguish it from the oxidative phos-
phorylation
coupled
to
electron
transport
that
oc-
curs in mitochondria (Chapter
14).
In erythrocytes,
1.3-
bisphosphoglycerate is converted to 2,3-bisphos-
phoglycerate (also called 2,3-diphosphoglycerate or 2,3-
DPG) by 1,3-bisphosphoglycerate mutase, which pos-
sesses 2,3-bisphosphoglycerate phosphatase activity and
is responsible for the formation of 3-phosphoglycerate
(which reenters glycolysis) and inorganic phosphate. In
this bypass pathway, the free energy associated with the
anhydride bond of 1,3-bisphosphoglycerate is dissipated
as heat. Erythrocytes contain a high level of 2,3-DPG,
which functions as an allosteric modulator of hemoglobin
and decreases affinity for oxygen (Chapter 28). 2,3-DPG
is present in trace amounts in most cells, functions as
a cofactor in the next reaction, and is synthesized by
3-phosphoglycerate kinase in the reaction
3-Phosphoglycerate2“ + ATP4-
2,3-bisphosphoglycerate4“ + ADP3“ + H+
In the next set of reactions of glycolysis, the remain-
ing phosphate group of glycerate is elevated to a high-
energy state, at which it can phosphorylate ADP in another
substrate-level phosphorylation reaction. The formation of
the high-energy intermediate occurs by way of the follow-
ing two reactions.
Iso m eriza tio n o f 3 -P h o sp h o g lycera te
to 2 -P h o sp h o g lycera te
In this reversible reaction, the phosphate group is trans-
ferred to the
2
-position by phosphoglycerate mutase:
Mg2+
3-Phosphoglycerate2“
2-phosphoglycerate2“
The reaction is similar to the interconversion catalyzed by
phosphoglucomutase (Chapter 15). The reaction is primed
by phosphorylation of a serine residue of the enzyme by
2,3-bisphosphoglycerate and occurs in the following steps
shown below.
