280
chapter 15
Carbohydrate Metabolism II: Gluconeogenesis, Glycogen Synthesis and Breakdown, and Alternative Pathways
activity is indirectly stimulated by the catabolic hormones
glucagon (via cAMP) and epinephrine (independent of
cAMP), in four ways:
1. By increasing substrate availability through
stimulation of mitochondrial respiration. This
decreases the intramitochondrial concentration of H+
and increases the rate of pyruvate transport. The
[ATP]/[ADP] ratio also rises.
2. By increasing the rate of fatty acid oxidation, which
results in an increase in the mitochondrial
concentration of acetyl-CoA, an allosteric activator of
pyruvate carboxylase.
3. By decreasing the mitochondrial concentration of
glutamate, an inhibitor of pyruvate carboxylase,
through stimulation of the TCA cycle (secondary to
the increase in mitochondrial acetyl-CoA) and the
aspartate shuttle (secondary to the increase in
cytosolic PEPCK induced by glucagon).
4. By decreasing the activity of glycolytic enzymes
competing for the same substrate; in this case, by
inactivating pyruvate kinase in the cytosol and
pyruvate dehydrogenase in the mitochondria (via
cAMP stimulation of protein kinase).
C on version o f O x a lo a ceta te to P h o sp h o en o lp yru va te
Short-term regulation of this reaction is accomplished
by changes in the relative proportions of substrates and
products. Increased concentrations of oxaloacetate and
GTP (or ITP) increase the rate, and accumulation of phos-
phoenolpyruvate and GDP (or IDP) decreases it. The
cytosolic PEPCK is also under long-term regulation by
hormones. Its synthesis is increased by corticosteroids.
Starvation and diabetes mellitus increase the synthesis of
cytosolic PEPCK, whereas refeeding and insulin have the
opposite effect.
The net effect of increasing the ratio of cytosolic PEPCK
to mitochondrial PEPCK is to maintain gluconeogenesis
even during periods of increased fatty acid metabolism.
When gluconeogenesis is strongly stimulated, lipoly-
sis and intramitochondrial fatty acid oxidation increase
with the activity of cytosolic PEPCK. The increase in
fatty acid oxidation elevates the [NADH]/[NAD+] ra-
tio within the mitochondria, inhibiting phosphoenolpyru-
vate formation by channeling oxaloacetate into formation
of malate or aspartate. At higher [NADH]/[NAD+] ra-
tios, aspartate synthesis is favored because of increased
formation
of glutamate
by
mitochondrial
glutamate
hydrogenase:
a-Ketoglutarate + NHj" + NADH ^ glutamate + NAD+ + H20
The increase in glutamate favors transamination of ox-
aloacetate and limits oxaloacetate availability for phos-
phoenolpyruvate synthesis. When the [NADH]/[NAD+]
ratio is low, malate formation occurs more readily. The
cytosolic PEPCK is relatively unaffected by the mito-
chondrial [NADH]/[NAD+] ratio. Once malate and as-
partate are transported to the cytosol and they are recon-
verted to oxaloacetate, cytosolic PEPCK can convert it to
phosphoenolpyruvate.
C on version o f F ru cto se-1 ,6 -B isp h o sp h a te
to F ru cto se-6 -P h o sp h a te
This reaction is catalyzed by fructose-1,
6
-bispho-
sphatase (FBPase-1). This enzyme is a tetramer (M.W.
164,000) of identical subunits. It is inhibited by AMP,
inorganic phosphate, and fructose-
2
,
6
-bisphosphate, all
of which are allosteric activators of
6
-phosphofructo-
1-kinase (PFK-1), the competing glycolytic enzyme
(Figure 15-5). This inhibition prevents the simultaneous
activation of the two enzymes and helps prevent a futile
ATP cycle. However, this futile cycle apparently does oper-
ate at a low rate in mammalian muscle, and in the bumble-
bee it is essential for maintaining the flight muscle at 30°C.
Activation of the futile cycle occurs after exposure
to certain anesthetic agents such as suxamethonium
and/or
volatile
halogenated
compounds.
In
suscep-
tible persons,
m a lig n a n t h yp erth erm ia
is character-
ized by a hypermetabolic state and is accompanied
by a catastrophic rise in body temperature (to 42°C
or
higher),
massive
increase
in
oxygen
consump-
tion,
rhabdomyolysis,
(disintegration
of muscle,
as-
sociated with excretion of myoglobin in the urine)
and metabolic acidosis.
R h a b d o m yo lysis
is assessed by
measurement of serum creatine kinase levels (Chap-
ter
8
).
In
susceptible
persons,
the
hypermetabolic
events
can
also
be
triggered
by
excessive
exer-
cise under hot conditions, infections, and neuroleptic
drugs.
Fructose 6-phosphate
0
©AMP]
F 2,6-BPj
1*4
Fructose-1,6-bisphosphatasel
H P - 0
^ATP
PFK-1
^-ADP
©ATP
©Citrate
Fructose 1,6-bisphosphate
©AMP
© F 2,6-BP
FIGURE 15-5
Regulation of liver
6
-phosphofructokinase and fructose-1,
6
-
bisphosphatase. These multimodulated enzymes catalyze nonequilibrium
reactions, the former in glycolysis and the latter in gluconeogenesis. Note
the dual action of fructose-2,
6
-bisphosphate (F-2,6-BP), which activates
phosphofructokinase (PFK-1) and inactivates fructose-1,
6
-bisphosphatase.
The activity of F-2,6-BP is under hormonal and substrate regulation
(Figure 15-6). © = positive effectors; © = negative effectors.
