SECTION 15.1
Gluconeogenesis
277
INNER MITOCHONDRIAL
MEMBRANE
Oxaloacetate
Glutamate—^
a-Ketoglutarate-
Aspartate-
Oxaloacetate
- Glutamate
4
a-Keto-
glutarate
-Aspartate
A s p a rta te s h u ttle
Cytoplasm
Glucose-
e-phosphatase
Glucose 6-phosphatase
Regulatory protein (SP)
Endoplasmic
reticulum
membrane
FIGURE 15-2
Shuttle pathways for transporting oxaloacetate from mitochondria
into the cytosol. The shuttles are named for the molecule that
actually moves across the mitochondrial membrane. 1 and 3 =
malate dehydrogenase; 2 = malate translocase; 4 and 7 = aspartate
aminotransferase; 5 = glutamate dehydrogenase;
6
= aspartate
translocase.
The proportion of oxaloacetate carried by each shuttle
probably depends on the redox state of the cytosol. If most
of the pyruvate is derived from lactate, the NADH / NAD+
ratio in the cytosol is elevated. In this situation, there is
no need to transport reducing equivalents out of the mito-
chondria, and the asparate shuttle predominates. However,
if alanine is the principal source of pyruvate, no cytoso-
lic reduction occurs, and the glyceraldehyde-phosphate
dehydrogenase reaction (which requires NADH) requires
transport of reducing equivalents via the malate shuttle. In
species in which oxaloacetate is converted in mitochon-
dria to phosphoenolpyruvate (which is readily transported
to the cytosol, perhaps via its own carrier system), no trans-
port of oxaloacetate or reducing equivalents is required. In
pigeons, which have virtually no cytosolic PEPCK, the rate
of gluconeogenesis from pyruvate is much slower than it is
from lactate, since there is no source of cytosolic reducing
equivalents.
Phosphoenolpyruvate is converted to fructose-1,6-
bisphosphate by reversal of glycolysis in the cytosol
via reactions that are at near-equilibrium and whose di-
rection is dictated by substrate concentration. Conver-
sion of fructose-
1
,
6
-bisphosphate to fructose-
6
-phosphate
is a nonequilibrium step, catalyzed by fructose-
1
,
6
-
bisphosphatase:
Mg
t
Fructose-1,6-bisphosphate
+ H
2
0 —>
fructose-
6
-phosphate2” + P
2
Fructose-
6
-phosphate is then converted to glucose-
6
-
phosphate by reversal of another near-equilibrium reac-
tion of glycolysis. In the last reaction in gluconeoge-
FIGURE 15-3
Schematic representation of the hepatic microsomal glucose-
6
-phosphatase
system. The hydrolysis of glucose phosphate to glucose and inorganic
phosphate (P;) consists of (1) glucose-
6
-phosphate transport protein (Tx ),
(
2
) glucose-
6
-phosphatase with its catalytic site located on the luminal side
of the membrane, (3) glucose-
6
-phosphatase Ca2+-binding stabilizing
protein (SP) also known as regulatory protein, (4) phosphate transport
protein (T
2
/J), (5) phosphate, pyrophosphate, and carbamoyl phosphate
transport protein (T
2
/I), and (
6
) glucose transport protein (GLUT7).
nesis, glucose-
6
-phosphate is converted to glucose by
glucose-
6
-phosphatase:
Glucose-
6
-phosphate2' + H20 —>• glucose + P2-
Glucose-
6
-phosphatase is part of a multi-component in-
tegral membrane protein system (Figure 15-3) located in
the endoplasmic reticulum of liver, kidney, and intestine
but not of muscle or adipose tissue.
This system consists of six different proteins. These are
1. Glucose-
6
-phosphate transport protein (Tl), which
transports glucose-
6
-phosphate into the lumen of the
endoplasmic reticulum;
2. Catalytic subunit of glucose-
6
-phosphatase (M.W.
36,500), which hydrolyzes glucose-
6
-phosphate into
glucose and phosphate at the luminal surface;
3. Glucose-
6
-phosphatase regulating protein (M.W.
2 1
,
0 0 0
), which stabilizes the activity of
glucose-
6
-phosphatase;
4. Microsomal phosphate transport protein (T
2
a) which
mediates the efflux of P;, an inhibitor of the
glucose-
6
-phosphatase, from the lumen of the
endoplasmic reticulum to the cytosol;
5. Microsomal phosphate/pyrophosphate transport
protein (T
2
/
6
, M.W. 37,000), which transports
phosphate, pyrophosphate, and carbamoyl phosphate,
which are substrates for glucose-
6
-phosphatase; and
6
. Microsomal glucose transport protein (GFUT7),
which is a member of the family of facilitative
glucose transport proteins (Chapter 13) and which
transports glucose into the cytosol.
