Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

IV. Metabolism of Absorbed Carbohydrates
51
FIGURE 3-5 The glycolytic or classic
Embden-Meyerhof pathway (EMP). Note that 2
moles of ATP are used and 4 moles of ATP are
generated. Abbreviations: ATP, adenosine triphosphate;
DHAP, dihydroxy acetone phosphate;
GA-3-P, glyceraldehyde-3-phosphate; NAD ,
nicotinamide adenine dinucleotide; P i , inorganic
phosphate.
(i.e., aerobic glycolysis), reduced NADH is reoxidized via
the cytochrome system:
(cytochrome)
HNADH 1 O2
NAD
2 → H 2 O
(system)
which provides a continuous source of NAD .
In the absence of O 2 (i.e., anaerobic glycolysis), NADH
is reoxidized to NAD in the reaction catalyzed by lactate
dehydrogenase (LDH) where pyruvate is reduced to lactate
and the NADH is the H donor. Therefore, by this
“ coupling ” of the LDH system to the GA-3-PD system,
anaerobic breakdown of glucose to lactate proceeds in the
absence of 02. As noted earlier, this anaerobic system generates
only 2 moles of ATP and when compared to the 36
moles of ATP generated in aerobic glycolysis, anaerobic
glycolysis is not very efficient.
d . Hexose Monophosphate Pathway
This alternate route of G-6-P oxidation has been variously
referred to as the pentose phosphate pathway (PPP), direct
oxidative pathway, Warburg-Dickens scheme, the hexose
monophosphate pathway (HMP), or the hexose monophosphate
shunt. The initial step of the shunt pathway involves
the oxidation of G-6-P at the C-1 position to form 6-
phosphogluconate (6-PG) as summarized in Figure 3-6 .
The reaction is catalyzed by glucose-6-phosphate dehydrogenase
(G-6-PD) and in this pathway, oxidized nicotinamide
adenine dinucleotide phosphate (NADP ) serves as
the hydrogen acceptor. In the second oxidative step, 6-P-G
is oxidatively decarboxylated by 6-phosphogluconate dehydrogenase
(6-P-GD) to yield a pentose phosphate, ribulose-
5-phosphate (Rib-5-P), again in the presence of NADP .
Thus, in the initial reactions, which are essentially irreversible,
2 moles of NADPH are formed. In this pathway, only
the C-1 carbon atom of the glucose molecule is evolved as
CO 2 . By contrast, glucose catabolism via the glycolytic
scheme results in the loss of both the C-1 and C-6 carbon
atoms as CO 2 when pyruvate is oxidatively decarboxylated
to form acetyl-CoA. This difference in CO 2 evolution is
used to study partitioning of glucose metabolism through
the glycolytic (EMP) pathway and the HMP shunt pathway
in domestic animals. The subsequent metabolism of the
Rib-5-P in the HMP shunt is also shown in Figure 3-6 . As
a result of the series of transformations, F-6-P and GA-3-P
are formed, which serve as recycling links into the glycolytic
pathway.
For continued functioning of the HMP shunt pathway, a
supply of NADP must be available to act as the hydrogen