Essential Cell Biology 5th edition

The Breakdown and Utilization of Sugars and Fats
431
CH 2 OH
O
O O –
C
C O
HO
OH
OH
OH
ATP
ATP
NADH
NADH
ATP
ATP
ATP
ATP
CH 3
O O –
C
C O
glucose NET RESULT: GLUCOSE 2 PYRUVATE + 2 ATP + 2 NADH
CH 3
two molecules
of pyruvate
chemical re-arrangements that ultimately generate pyruvate release
energy because the electrons in a molecule of pyruvate are, overall, at
a lower energy state than those in a molecule of glucose. Nevertheless,
ECB5 e13.04/13.04
for each molecule of glucose that enters glycolysis, two molecules of ATP
are initially consumed to provide the energy needed to prepare the sugar
to be split. This investment of energy is more than recouped in the later
steps of glycolysis, when four molecules of ATP are produced. During this
“payoff phase,” energy is also captured in the form of NADH. Thus, at the
end of glycolysis, there is a net gain of two molecules of ATP and two
molecules of NADH for each molecule of glucose that is oxidized (Figure
13–4).
Figure 13–4 Glycolysis splits a molecule
of glucose to form two molecules of
pyruvate. The process requires an input of
energy, in the form of ATP, at the start. This
energy investment is later recouped by the
production of two NADHs and four ATPs.
Glycolysis Produces both ATP and NADH
Piecing together the complete glycolytic pathway in the 1930s was a
major triumph of biochemistry, as the pathway consists of a sequence
of 10 separate reactions, each producing a different sugar intermediate
and each catalyzed by a different enzyme. These reactions are presented
in outline in Figure 13–5 and in detail in Panel 13–1 (pp. 436–437). The
different enzymes participating in the reactions of glycolysis, like most
enzymes, all have names ending in -ase—like isomerase and dehydrogenase—which
specify the type of reaction they catalyze (Table 13–1).
Much of the energy released by the breakdown of glucose is used to
drive the synthesis of ATP molecules from ADP and P i . This form of ATP
TABLE 13–1 SOME TYPES OF ENZYMES INVOLVED IN GLYCOLYSIS
Enzyme Type General Function Role in Glycolysis
Kinase
Isomerase
Dehydrogenase
Mutase
catalyzes the addition of
a phosphate group to
molecules
catalyzes the rearrangement
of bonds within a single
molecule
catalyzes the oxidation of
a molecule by removing
a hydrogen atom plus an
electron (a hydride ion, H – )
catalyzes the shifting of a
chemical group from one
position to another within a
molecule
a kinase transfers a phosphate
group from ATP to a substrate
in steps 1 and 3; other kinases
transfer a phosphate to ADP to
form ATP in steps 7 and 10
isomerases in steps 2 and 5
prepare molecules for the
chemical alterations to come
the enzyme glyceraldehyde
3-phosphate dehydrogenase
generates NADH in step 6
the movement of a phosphate
by phosphoglycerate mutase
in step 8 helps prepare the
substrate to transfer this group
to ADP to make ATP in step 10