adaptation of PFK to specific conditions is found
in the production of antifreeze substances in overwintering larvae of the gallfly Eurosta solidaginis.
As the ambient temperature gradually decreases,
the haemolymph accumulates first glycerol and
then sorbitol. This switch in polyol synthesis is
due to the inactivation of PFK at low temperatures. The specific activity of PFK declines rapidly between + 10 and 0 °c, the QlO having the
rather high value of 3.64. At the same time, the
substrate affinity and AMP sensitivity decrease,
and the sensitivity to inhibitors increases [240]. At
the beginning of flight, the concentration of Fru2,6-P2 in the flight muscles of the migratory locust
Locusta migratoria sinks dramatically. The consequently low PFK activity facilitates the conservation of carbohydrate reserves for prolonged
flight. Deterioration of the energy status of the
flight muscles results in an increase in AMP and Pi
and activation of PFK [272]. The PFK of the
honey-bee flight muscle is rather unusual in that it
is inhibited by Glc-1,6-P2 and Fru-1,6-P2. This
inhibition is relieved by Fru-6-P and, in particular, by Fru-2,6-P2, but not by AMP. The minor
regulatory significance of AMP coincides with the
relatively low activity of adenylate kinase in these
muscles [271].
The opposing reactions of PFK-1 and fructose1,6-bisphosphatase give rise to a substrate cycle
that allows for fine regulation of Fru-1,6-P2 synthesis. AMP and Fru-2,6-P2 activate PFK-1 and
simultaneously inhibit the phosphatase; this regulation of the substrate cycle between Fru-6-P and
Fru-1,6-P2 is probably the most important biological function of Fru-2,6-P2. Two extreme situations can be distinguished in the cells of the liver.
In the hungry animal, the Fru-2,6-P2 concentration is low, PFK-1 is inactive, and only gluconeogenesis occurs. In the well-fed animal, the Fru2,6-P2 concentration is high, PFK-1 is active, the
phosphatase is more-or-less inhibited, and thus
glycolysis predominates. Because the fructose1,6-bisphosphatase in the presence of Fru-2,6-P2
shows sigmoidal substrate kinetics, it can function
simultaneously as an emergency brake that prevents the occurrence of excessive Fru-1,6-P2 concentrations [100]. In the bumble bee Bombus affinis, ATP catabolism resulting from the Fru-6-P/
Fru-1,6-P2 substrate cycle serves to increase the
temperature and, consequently, the capacity of
the flight muscles. Accordingly, the cycle rate
increases with decreasing temperature in the
resting muscle and reaches lOA /!mol/g per min at
5°C, whereas the rate of glycolysis is only
5.8 /!mol/g per min. At the onset of flight, the
14.5 Aldolases
527
cycling is stopped and the rate of glycolysis
increases to 2004 /!mol/g per min [248]. Fru-2,6-P2
and Ca 2 + are involved in the regulation of the
substrate cycle in the flight muscles [82, 136].
14.5 Aldolases
The aldolases cleave Fru-1,6-P2, and most of them
also cleave Fru-1-P, to the corresponding trioses.
In the vertebrates, the invertebrate metazoans
and many unicellular organisms, aldolases are tetramers of 40-kDa subunits [62, 63, 163]. At least
three different types of subunit are found in the
vertebrates, and each is encoded by a different
gene. Electrophoresis at pH 8.6 of mammalian
aldolase preparations separates a weakly cathodic
band A from the strongly cathodic band Band
the anodic band C. A fourth subunit, D, is
encoded in the tetraploid genome of the salmonids and lies between A and C on electropherograms. The subunits form heterotetramers both in
vivo and in vitro. Type A4 from skeletal muscle
metabolizes Fru-1,6 P2 many times more efficiently than it metabolizes Fru-1-P, and is therefore
especially well adapted for glycolytic sugar catabolism. Type A is also the predominant form in
mammalian embryos. Type B4 from the liver,
kidney cortex and small intestine shows high
activity with Fru-1-P and is therefore especially
well adapted to fructose metabolism and gluconeogenesis. Type B predominates in avian embryos.
Type C4 is found in addition to heterotetramers of
C, A and B in the brain and the spinal cord; the
catalytic properties of this type are intermediate
between those of A4 and B4 [74].
The aldolases of many mammals, birds, amphibians and fish have been isolated and characterized [1], as have several from nematodes, insects
and the edible snail [22,63, 148,239]. Most
invertebrates appear to possess only one aldolase; an exception is the fly Phormia regina, in
which different isoenzymes have been detected in
the muscle and the fat body [63]. These enzymes
are all very similar in their catalytic properties;
the turnover rate of Fru-1,6-P2 and the affinity for
this substrate is usually much higher than for Fru1-P; an exception is aldolase B from vertebrate
liver (Table 14.3). In general, the aldolases are
not considered to be important for the regulation
of glycolytic flux. However, the enzyme from the
flight muscles of the locust Schistocerca gregaria
is specifically inhibited by citrate and acylcarnitine; this inhibition of aldolase may be important
in the production of antifreeze substances in overwintering larvae of the gallfly Eurosta solidaginis.
As the ambient temperature gradually decreases,
the haemolymph accumulates first glycerol and
then sorbitol. This switch in polyol synthesis is
due to the inactivation of PFK at low temperatures. The specific activity of PFK declines rapidly between + 10 and 0 °c, the QlO having the
rather high value of 3.64. At the same time, the
substrate affinity and AMP sensitivity decrease,
and the sensitivity to inhibitors increases [240]. At
the beginning of flight, the concentration of Fru2,6-P2 in the flight muscles of the migratory locust
Locusta migratoria sinks dramatically. The consequently low PFK activity facilitates the conservation of carbohydrate reserves for prolonged
flight. Deterioration of the energy status of the
flight muscles results in an increase in AMP and Pi
and activation of PFK [272]. The PFK of the
honey-bee flight muscle is rather unusual in that it
is inhibited by Glc-1,6-P2 and Fru-1,6-P2. This
inhibition is relieved by Fru-6-P and, in particular, by Fru-2,6-P2, but not by AMP. The minor
regulatory significance of AMP coincides with the
relatively low activity of adenylate kinase in these
muscles [271].
The opposing reactions of PFK-1 and fructose1,6-bisphosphatase give rise to a substrate cycle
that allows for fine regulation of Fru-1,6-P2 synthesis. AMP and Fru-2,6-P2 activate PFK-1 and
simultaneously inhibit the phosphatase; this regulation of the substrate cycle between Fru-6-P and
Fru-1,6-P2 is probably the most important biological function of Fru-2,6-P2. Two extreme situations can be distinguished in the cells of the liver.
In the hungry animal, the Fru-2,6-P2 concentration is low, PFK-1 is inactive, and only gluconeogenesis occurs. In the well-fed animal, the Fru2,6-P2 concentration is high, PFK-1 is active, the
phosphatase is more-or-less inhibited, and thus
glycolysis predominates. Because the fructose1,6-bisphosphatase in the presence of Fru-2,6-P2
shows sigmoidal substrate kinetics, it can function
simultaneously as an emergency brake that prevents the occurrence of excessive Fru-1,6-P2 concentrations [100]. In the bumble bee Bombus affinis, ATP catabolism resulting from the Fru-6-P/
Fru-1,6-P2 substrate cycle serves to increase the
temperature and, consequently, the capacity of
the flight muscles. Accordingly, the cycle rate
increases with decreasing temperature in the
resting muscle and reaches lOA /!mol/g per min at
5°C, whereas the rate of glycolysis is only
5.8 /!mol/g per min. At the onset of flight, the
14.5 Aldolases
527
cycling is stopped and the rate of glycolysis
increases to 2004 /!mol/g per min [248]. Fru-2,6-P2
and Ca 2 + are involved in the regulation of the
substrate cycle in the flight muscles [82, 136].
14.5 Aldolases
The aldolases cleave Fru-1,6-P2, and most of them
also cleave Fru-1-P, to the corresponding trioses.
In the vertebrates, the invertebrate metazoans
and many unicellular organisms, aldolases are tetramers of 40-kDa subunits [62, 63, 163]. At least
three different types of subunit are found in the
vertebrates, and each is encoded by a different
gene. Electrophoresis at pH 8.6 of mammalian
aldolase preparations separates a weakly cathodic
band A from the strongly cathodic band Band
the anodic band C. A fourth subunit, D, is
encoded in the tetraploid genome of the salmonids and lies between A and C on electropherograms. The subunits form heterotetramers both in
vivo and in vitro. Type A4 from skeletal muscle
metabolizes Fru-1,6 P2 many times more efficiently than it metabolizes Fru-1-P, and is therefore
especially well adapted for glycolytic sugar catabolism. Type A is also the predominant form in
mammalian embryos. Type B4 from the liver,
kidney cortex and small intestine shows high
activity with Fru-1-P and is therefore especially
well adapted to fructose metabolism and gluconeogenesis. Type B predominates in avian embryos.
Type C4 is found in addition to heterotetramers of
C, A and B in the brain and the spinal cord; the
catalytic properties of this type are intermediate
between those of A4 and B4 [74].
The aldolases of many mammals, birds, amphibians and fish have been isolated and characterized [1], as have several from nematodes, insects
and the edible snail [22,63, 148,239]. Most
invertebrates appear to possess only one aldolase; an exception is the fly Phormia regina, in
which different isoenzymes have been detected in
the muscle and the fat body [63]. These enzymes
are all very similar in their catalytic properties;
the turnover rate of Fru-1,6-P2 and the affinity for
this substrate is usually much higher than for Fru1-P; an exception is aldolase B from vertebrate
liver (Table 14.3). In general, the aldolases are
not considered to be important for the regulation
of glycolytic flux. However, the enzyme from the
flight muscles of the locust Schistocerca gregaria
is specifically inhibited by citrate and acylcarnitine; this inhibition of aldolase may be important