SECTION 15.1
Gluconeogenesis
281
The primary biochemical abnormality of malignant
hyperthermia is a membrane defect that leads to a rapid
and sustained rise in Ca2+ levels in the cytosol of skeletal
muscle cells. Sarcoplasmic reticulum is the main storage
site of intracellular Ca2+ and it is released into the my-
oplasm by excitation-contraction coupling (Chapter 21).
Dantrolene, a drug useful in the treatment of malignant
hyperthermia, inhibits excitation-contraction coupling.
However, dantrolene does not affect neuromuscular trans-
mission or the electrical properties of skeletal muscle. In
the animal model, pigs susceptible for malignant hyper-
thermia exhibit a genetic defect in a Ca2+ release channel
receptor located in the sarcoplasmic reticulum. This recep-
tor protein is named
ryanodine receptor (RYR)
because
it binds to a plant alkaloid ryanodine and upon binding
causes Ca2+ release. The mutation that alters the receptor
protein causes a substitution of cysteine for arginine at
position 615.
The concentration of fructose-2,
6
-bisphosphate is con-
trolled by two competing enzyme activities in a single
insulin
G lucagon
FIGURE 15-6
Regulation o f fructose-2,6-bisphosphate (F-2.6-BP) concentration in
liver. F-2.6-B P is a m ajor factor controlling the relative activities of
fructose-1,6-bisphosphatase (FBPase-1) and 6-phosphofructo-l-kinase
(PFK-1), key enzym es in gluconeogenesis and glycolysis, respectively.
6-Phosphofructo-2-kinase (PFK -2) and fructose-2,6-bisphosphatase
(FBPase-2) are separate enzym es expressed by a single m ultifunctional
protein. Insulin can antagonize the effect o f glucagon on cAM P, but
the m echanism is not known. D otted lines = regulatory factors;
Q = inhibitory; © = stimulatory. F6P = fructose-6-phosphate;
PEP = phosphoenolpyruvate; a -G P = or-glycerol phosphate;
Pi = inorganic phosphate; F-6-P = fructose-6-phosphate.
multifunctional protein (M.W. 110,000) with two identi-
cal subunits (Figure 15-6). Phosphorylation by cAMP-
dependent protein kinase produces one phosphoserine
per subunit, inhibiting 6-phosphofructo-2-kinase (PFK-2)
activity
and
stimulating
fructose-
2
,
6
-bisphosphatase
(FBPase-2) activity. The phosphotransferase inactivation
results from an increased
Km
for fructose-
6
-phosphate and
a small decrease in Vmax. An increase in
V max
and a de-
crease in
Km
for fructose-2,
6
-bisphosphate increases the
phosphohydrolase activity. The protein phosphatase that
reverses the cAMP-dependent phosphorylation has not yet
been characterized. The nomenclature of these enzyme
activities (PFK-1, PFK-2, FBPase-1, and FBPase-2) re-
flects both the order of their discovery and the carbon
atom of fructose that is phosphorylated or dephosphory-
lated.
Phosphorylation is regulated by insulin and glucagon.
Diabetes mellitus (in which the ratio of glucagon to in-
sulin is increased) and glucagon therapy reduce the hep-
atic activity of PFK-2 and increase that of FBPase-2.
The concentration of fructose-2,
6
-bisphosphate is thus re-
duced, thereby stimulating gluconeogenesis.
The concentration of fructose-2,
6
-bisphosphate is also
influenced by nonhormonal factors. PFK-2 is alloster-
ically stimulated by inorganic phosphate, AMP, and
fructose-
6
-phosphate and inhibited by citrate, fructose-
2
,
6
-bisphosphate, phosphoenolpyruvate, and a-glycerol
phosphate. FBPase-2, on the other hand, is stimulated by
phosphoenolpyruvate and a-glycerol phosphate and inhib-
ited by phosphate and fructose-
6
-phosphate. Glucose in-
creases the concentration of fructose-
2
,
6
-bisphosphate
in
vivo,
probably by increasing the availability of fructose-
6
-phosphate, thereby stimulating PFK-2, the kinase for
which this is a substrate and inhibiting the phosphatase,
FBPase-2. The effect is to increase glycolysis and in-
hibit gluconeogenesis. The concentration of a-glycerol
phosphate
in vivo
can be altered by ischemia con-
sumption of ethanol, and feeding of various sugars; it
may be an important regulatory molecule in determin-
ing the levels of fructose-2,
6
-bisphosphate. A rise in a-
glycerol phosphate decreases fructose-
2
,
6
-bisphosphate
concentration,
decreasing
glycolysis
and
increasing
gluconeogenesis.
Conversion o f Glucose-6-Phosphate to Glucose
Glucose-
6
-phosphatase and the corresponding gly-
colytic
enzyme,
glucokinase,
are
not controlled by
the
metabolites
that
affect
PFK-1
and
FBPase-1.
Glucose-
6
-phosphatase seems to be regulated only by
the
concentration
of glucose-
6
-phosphate,
which
is
