CLINICAL CASES
193
Glucose
GTP
GDP
OAA - ^ H p EPCK|< ^
PEP
ADP
NADH
NAD
CO.
Malate
F1.6BP ©
Malate
n a d :
ADP + P/
ATP
NADH
OAA
Pyruvate
ATP
co2
NADH
LDH - Lactate dehydrogenase
NAD
PEPCK - PEP carboxykinase
AcCoA ©
PK - pyruvate kinase
Lactate
PC - pyruvate carboxylase
MITOCHONDRION
Figure 21-1. Interconversion of phosphoenolpyruvate (PEP) and pyruvate.
The conversion of PEP to pyruvate is thermodynamically irreversible in the
cell. To convert pyruvate back to PEP for gluconeogenesis, pyruvate must
enter the mitochondrion, be carboxylated to oxaloacetate (OAA), and reduced
to malate. After exiting the mitochondrion, malate is oxidized back to OAA
and converted to PEP by the action of phosphoenolpyruvate carboxykinase.
Glucose
G lucose 6-phosphatase
h
yz?
Glucokinase
G 6 P '
Î
F6P,
Fructose 1,6-bisphosphatase
T
F-1 ,6 -B P
Î
t
t
Phosphofructokinase
ADP
GDP
GTP
PEP
U ADP
0AA">4?
J^ATP
iD P v
pyruvate
P + ADP
ATP
Figure 21-2. The reactions that make up the thermodynamically irreversible
steps in glycolysis and gluconeogenesis. These steps make up potential
“futile” cycles that must be carefully regulated.
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