Gluconeogenesis
Secondary article
John C Wallace, University of Adelaide, Adelaide, South Australia, Australia
Greg J Barritt, Flinders University School of Medicine, Adelaide, South Australia, Australia
Article Contents
. Glucose Homeostasis
. Lactic Acid and Alanine Cycles
The gluconeogenic pathway, which is found in the liver and kidney, involves the synthesis
of glucose from three-carbon precursors such as lactate, alanine and glycerol. The main
function of gluconeogenesis is to supply glucose to tissues, such as brain and red blood
cells, that depend on glucose as their main or sole energy source.
. Glucose-6-phosphatase, Fructose-1,6-bisphosphatase,
Pyruvate Carboxylase and Phosphoenolpyruvate
Carboxykinase
. Regulation of Gluconeogenesis
. Role of Fatty Acids in Gluconeogenesis
. Biochemistry of Diabetes Drugs that Inhibit Glucose
Production
Glucose Homeostasis
Glucose is a major fuel for the metabolism of skeletal and
heart muscle, brain, blood cells, adipose tissue and most
other tissues of the body. An adequate supply of glucose is
particularly important for brain and red blood cells
because, under normal conditions, glucose is the sole
substrate for these tissues. Only in starvation does the brain
metabolize ketone bodies as an additional source of
energy. In all these tissues, the main outcome of glucose
metabolism is to yield energy in the form of adenosine
triphosphate (ATP). However, some glucose is metabolized to yield precursors for biosynthetic reactions, such as
the formation of some amino acids, nucleotides and other
intermediary metabolites.
In the well-fed state (following a meal), glycogen stores
in the liver and skeletal muscle are replenished and,
together with glucose absorbed from the gut, provide the
major source of glucose for peripheral tissues for the next
few hours. However, between meals and especially during
the night, the stores of glycogen are usually depleted. As
this happens, glucose is synthesized by the gluconeogenic
pathway in the liver. Glucose moieties in muscle glycogen
can be used to provide energy for muscle cells, but cannot
be liberated as free glucose in the blood for utilization by
other tissues.
The purpose of the gluconeogenic pathway is to provide
the body with a source of glucose under physiological
conditions in which glycogen stores in the liver are
depleted and there is no glucose available from the gut. A
key feature of the gluconeogenic pathway is that it converts
three-carbon precursors such as lactate, alanine and
glycerol, formed by metabolism in peripheral tissues, to
glucose. The elements of the gluconeogenic pathway are
shown in Figure 1. As discussed in more detail below, the
pathway utilizes several reversible steps of the glycolytic
pathway but also includes steps that are unique to the
gluconeogenic pathway (Figure 1). The gluconeogenic
pathway is located principally in the liver, although some
gluconeogenesis occurs in the kidney (Haymond and
Sunehag, 1999).
There are a number of normal physiological situations in
which the demand for glucose synthesized by the gluco-
. Summary
neogenic pathway is increased. These include exercise,
pregnancy and lactation. An increased demand for
gluconeogenesis also exists in starvation and in a number
of pathological states such as traumatic injury, fever and
cachexia (severe muscle wasting) induced by cancer and
human immunodeficiency virus infection.
Hypoglycaemia (abnormally low blood glucose concentration) poses a particular problem for the body because
the brain and red blood cells depend on glucose as a source
of energy. Hypoglycaemia can be a life-threatening state.
Gluconeogenesis is the key metabolic pathway that guards
against hypoglycaemia. Examples of pathological situations that can lead to hypoglycaemia include inappropriately high insulin doses in insulin-dependent (type 1)
diabetes, severe alcoholic poisoning, some inborn errors of
metabolism, hypoxia, salicylate poisoning, and tumours
such as Wilms tumour, hepatoblastoma and Hodgkin
lymphoma (Lteif and Schwenk, 1999).
Lactic Acid and Alanine Cycles
The anaerobic metabolism of glucose by red blood cells,
skeletal muscle and other peripheral tissues leads to the
formation of lactate. The amount of lactate formed
depends on the balance between aerobic and anaerobic
metabolism. Lactate released from the tissues moves
through the blood where it is taken up by the liver,
converted to pyruvate and, through the gluconeogenic
pathway, converted to glucose. This cycling of glucose
from the liver to skeletal muscle and of lactate from skeletal
muscle back to the liver, where it is resynthesized to
glucose, is called the glucose–lactate or Cori cycle (red
arrows in Figure 2). The cycle was discovered by Carl and
Gertrude Cori.
The glucose–lactate cycle is particularly important in
overnight fasting because, under these conditions, liver
glycogen stores become depleted and the only source of
glucose for red blood cells and brain is the gluconeogenic
pathway. This functions in collaboration with the elements
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1
Gluconeogenesis
Glucose
Endoplasmic reticulum
Glucose
Mitochondrial membrane
G2
G6Pase
Malate
G-6-P
6PF-1K
GK
Malate
+
G-6-P
F-6-P
OAA
B
F-2,6-BP
F-1,6-BP
PEPCK-M
PEPCK-C
PEP
OAA
+
–
Plasma membrane
F1,6BPase
AcCoA
PC
PEP
+
PK
Pyr
Pyr
–
Ala
Lactate
Figure 1 Schematic representation of the pathway of gluconeogenesis. The reactions catalysed by four key enzymes of gluconeogenesis – pyruvate
carboxylase (PC), cytoplasmic phosphoenolpyruvate carboxykinase (PEPCK-C) or mitochondrial phosphoenolpyruvate carboxykinase (PEPCK-M),
fructose-1,6-bisphosphatase (F1,6BPase) and glucose-6-phosphatase (G6Pase) (circled) – are indicated by red arrows; the opposing reactions of glycolysis
catalysed by pyruvate kinase (PK), 6-phosphofructo-1-kinase (6PF-1K) and glucokinase (GK) (circled) are shown by blue arrows. The bifunctional enzyme
6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (6PF2K/F2,6BPase) is indicated by a ‘B’ (circled). The allosteric inhibition of F1,6BPase by fructose
2,6-bisphosphate (F-2,6-BP) and PK by alanine are shown by dashed green arrows with a negative sign. The allosteric activation of 6PF-1K by F-2,6-BP, of PK
by F-1,6-BP, and of PC by acetylcoenzyme A (AcCoA) is indicated by dashed green arrows with a positive sign. In the interests of clarity and simplicity, other
reactions and membrane transporters are shown by thin black arrows. Only the main substrates, alanine (Ala) and pyruvate (Pyr), as well as the key
intermediates phosphoenolpyruvate (PEP), fructose 1,6-bisphosphate (F-1,6-BP), fructose 6-phosphate (F-6-P) and glucose 6-phosphate (G-6-P) are
shown. The plasma membrane, endoplasmic reticulum and mitochondrial membrane are shown schematically by thin parallel lines.
of the glucose–lactate cycle that deliver lactate, the major
substrate for gluconeogenesis, to the liver. The function of
this cycle is particularly important in fasting rodents as,
under these conditions, glucose is not available from
glycogen, and hence the Cori cycle and gluconeogenesis are
the only means of providing glucose. However, larger
animals, including humans, appear to mobilize their
glycogen reserves less urgently, and indeed coordinate
glycogenolysis and gluconeogenesis in a more complementary manner (Bergman and Ader, 2000).
During starvation, considerable amounts of skeletal
muscle protein are degraded to yield ammonia and
glutamate. The transamination of pyruvate with
glutamate yields alanine which, in turn, is transported
through the blood to the liver. Here another transamination reaction reforms pyruvate and glutamate. The
pyruvate is a substrate for gluconeogenesis. The resulting
glucose can be transported back to the skeletal muscle and
used in glycolysis to yield ATP (blue arrows in Figure 2).
This cycle, called the glucose–alanine cycle, was first
identified by Philip Felig. There are two main purposes of
2
the glucose–alanine cycle. One is to provide carbon
as a precursor of glucose synthesis in the liver. The other
is to transport nitrogen atoms to the liver for excretion as
urea.
The use of glucose labelled isotopically with 3H or 14C at
different positions, and 13C and 1H nuclear magnetic
resonance, has allowed the study of glucose homeostasis,
the glucose–lactate and glucose–alanine cycles in the
whole animal and in humans (Shulman, 1999). These
experiments have helped to gain a better understanding of
the cycles and have shown that the glucose–alanine cycle
also functions during and after prolonged exercise. It may
also deliver alanine to skeletal muscle during recovery after
exercise.
The activity of the glucose–lactate and glucose–alanine
cycles is regulated by hormones. Insulin and glucagon are
particularly important in regulating the glucose–lactate
cycle during the transition from the fed to the fasted state.
Adrenaline, cortisol and insulin play major roles in
regulating both cycles in starvation and prolonged
exercise.
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Gluconeogenesis
Blood
Liver
Glycogen
Skeletal muscle
Glucose
Glycogen
Urea
Urea
cycle
NH3
Protein
Glucose
6-phosphate
Gluconeogenesis
Glutamate
NH3
Glycolysis
Pyruvate
Pyruvate
NADH
NAD
Glutamate
NADH
NAD
Lactate
α-Ketoglutarate
Amino acids
Glucose
6-phosphate
Lactate
Alanine
Alanine
α-Ketoglutarate
Lactate
Alanine
Figure 2 The Cori (glucose–lactate) (red arrows) and the glucose–alanine (blue arrows) cycles. In the glucose–lactate cycle, pyruvate formed in
skeletal muscle (and in a number of other tissues) is reduced to lactate, which is released into the blood, taken up by the liver, used to form glucose,
then released to the blood and taken up by skeletal muscle and other peripheral tissues. Alanine is formed from pyruvate and glutamate in skeletal muscle
and undergoes a similar cycling.
Glucose-6-phosphatase, Fructose-1,
6-bisphosphatase, Pyruvate
Carboxylase and Phosphoenolpyruvate
Carboxykinase
The pathway of gluconeogenesis (see Figure 1), which
occurs in the periportal cells of the liver and in the kidney
cortex, utilizes most of the enzymes of the glycolytic
pathway except those catalysing the steps between (a)
phosphoenolpyruvate and pyruvate, (b) fructose 6-phosphate and fructose 1,6-bisphosphate, and (c) glucose and
glucose 6-phosphate.
To circumvent the large free energy changes in these
glycolytic reactions catalysed respectively by (a) pyruvate
kinase (PK), (b) 6-phosphofructo-1-kinase (6PF-1K) and
(c) hexokinase/glucokinase (GK), the gluconeogenic pathway employs (1) a tandem combination of pyruvate
carboxylase (PC) and phosphoenolpyruvate carboxykinase (PEPCK), (2) fructose-1,6-bisphosphatase (FBPase),
and (3) glucose-6-phosphatase (G6Pase).
While these two sets of enzymes catalysing opposing
reactions would appear to represent potentially ‘futile
cycles’, they are in fact known to be targets of short-term
and long-term regulation, as discussed below, and are also
known as ‘substrate cycles’.
The bifunctional enzyme 6-phosphofructo-2-kinase/
fructose-2,6-bisphosphatase (6PF2K/F2,6BPase) is responsible both for the ATP-dependent formation from
fructose 6-phosphate of fructose 2,6-bisphosphate, an
important allosteric effector (see below), and for its
reconversion to fructose 6-phosphate by dephosphorylation. Fructose 2,6-bisphosphate is a very powerful
allosteric activator of 6-phosphofructo-1-kinase, and also
an allosteric inhibitor of fructose-1,6-bisphosphatase.
Thus, although not a catalytic component of the gluconeogenic pathway, 6PF2K/F2,6BPase nevertheless plays such
an integral role in influencing the net activities of one of the
substrate cycles that its own regulation in liver also
deserves a mention.
Pyruvate carboxylase
Pyruvate carboxylase
reaction [I].
ATP2− + HCO3− + Pyruvate
(PC)
[EC
6.4.1.1]
catalyses
2+
(Mg , acetyl-CoA)
Oxaloacetate + ADP + Pi
[I]
Where ADP is adenosine diphosphate and Pi is inorganic
phosphate.
PC, a member of the biotin-dependent carboxylase
family, catalyses the ATP-dependent carboxylation of
pyruvate to form oxaloacetate, which is used both in
gluconeogenesis by liver and kidney, and in lipogenesis by
liver, adipose tissue and lactating mammary gland, and
neurotransmitter synthesis by the brain. PC is a homotetrameric enzyme (subunit Mr approximately 130 kDa),
encoded by a single nuclear gene. It occurs exclusively in
the mitochondria of mammalian tissues where its activity is
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Gluconeogenesis
very dependent on the concentration of its allosteric
activator, acetylcoenzyme AcCoA (Jitrapakdee and Wallace, 1999).
Phosphoenolpyruvate carboxykinase
Phosphoenolpyruvate carboxykinase
4.1.1.32] catalyses reaction [II].
Glucose-6-phosphatase (G6Pase) [EC 3.1.3.9] catalyses
reaction [IV].
Glucose-6-P + H2O !Glucose + Pi
(PEPCK)
[EC
Oxaloacetate + GTP!Phosphoenolpyruvate
+GDP +CO2
[II]
Where GTP is guanosine triphosphate and GDP is
guanosine diphosphate.
PEPCK, a monomeric enzyme (Mr 69 kDa), occurs to
varying extents, depending on species, in both the
mitochondria and cytosol of liver, kidney cortex, white
and brown adipose tissue, lactating mammary gland and
small intestine. For example, in liver the cytosolic
component of PEPCK activity (PEPCK–C) in rat is 80–
90%, in humans 30–50% and in guinea-pig 15–20%,
whereas in chickens about 95% is mitochondrial. In
kidney, PEPCK–C activity is 75% in rat, 20% in guineapig and 40% in chicken. The mitochondrial and cytoplasmic isoforms have similar kinetic properties and approximately similar molecular weights but are immunologically
distinct, each being encoded by a separate nuclear gene
(Hanson and Reshef, 1997). Whereas the role of PEPCK–
C in liver and kidney is clearly related to the body’s need for
gluconeogenesis, its role in the other tissues appears to be
related to a high demand for glycerol 3-phosphate
synthesis in lipogenesis.
Fructose-1,6-bisphosphatase
Fructose-1,6-bisphosphatase (FBPase) [EC 3.1.3.11] catalyses reaction [III].
Fructose 1,6-(P)2 + H2O !Fructose 6-P + Pi [III]
In mammals, FBPase, a homotetramer (subunit Mr
37 kDa), is encoded by two distinct genes which are
expressed with significant tissue specificity. The FBP1encoded isoform occurs in the cytoplasm principally of
liver, kidney and monocytes. Monocytes, therefore,
represent a useful alternative source of messenger ribonucleic acid (mRNA) of the liver isoform for diagnosis of this
enzyme’s deficiency as a cause of childhood hypoglycaemia. FBPase deficiency may be one of the inherited
metabolic diseases responsible for up to 25% of cases of
sudden infant death syndrome. FBPase activity has also
been reported in brain, adipose tissue, lung, some muscles
and intestine. At least the last two tissue isoforms are
encoded by distinct transcripts from a separate gene,
FBP2. FBPase is inhibited synergistically by fructose 2,6bisphosphate and adenosine monophosphate (AMP)
(Pilkis and Granner, 1992).
4
Glucose-6-phosphatase
[IV]
G6Pase is a multisubunit microsomal enzyme which
catalyses the hydrolysis of glucose 6-phosphate to release
glucose for transport by the bloodstream (Nordlie et al.,
1999). The catalytic subunit (Mr 36 kDa) of G6Pase occurs
in liver, kidney cortex, small intestine and the b cells of the
endocrine pancreas. It is located on the luminal side of the
endoplasmic reticulum in association with at least four
membrane-spanning translocases which allow substrates
access to the active site. The best characterized translocase,
albeit incompletely as yet, is the 46-kDa putative glucose 6phosphate transporter, T1, which occurs in multiple
isoforms and may well have a wider range of functions
(van de Werve et al., 2000).
Regulation of Gluconeogenesis
As with all metabolic pathways, the regulation of
gluconeogenesis can be achieved at three levels: (1) the
supply of substrate(s); (2) the short-term (minute to
minute) control of the activities of the existing enzyme or
transporter molecules by allosteric effectors or by covalent
modifications (e.g. phosphorylation and dephosphorylation); and (3) the long-term (hours to days) control of the
number and distribution (intracellular, cellular and tissue)
of enzyme or transporter molecules. This last means can be
effected by increased or decreased rates of transcription
and/or translation of specific mRNA species, and by
control over the the rates of degradation of the resulting
mRNA or protein molecules respectively. The hormones
insulin and glucagon are the major opposing endocrine
controllers of glucose production and utilization, with
glucocorticoids playing a permissive role in support of
glucagon. Starvation, a low carbohydrate diet, and
exercise reduce insulin release from the b cells of the
pancreas while increasing the secretion of glucagon by the
pancreatic a cells.
Substrate supply
Even in a fed human, the liver is required to reconvert into
glucose approximately 40 g of lactate produced per day by
essentially anaerobic tissues such as erythrocytes, kidney
medulla and retina. Approximately twice this amount is
produced daily by other tissues, depending on their level of
activity, with skeletal muscle being capable of producing
much more than this if its aerobic capacity for ATP
production is exceeded during vigorous exercise. Low
plasma insulin levels favour lipolysis with the release of free
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Gluconeogenesis
fatty acids and glycerol, as well as the breakdown of
(predominantly) muscle protein with the release of amino
acids. Thus, in fasting humans at rest, about 19 g of
glycerol are released per day from adipose tissue and most
is converted to glucose, but this figure is greatly increased
by exercise or stress. Similarly, in each of the first few days
of starvation around 75 g of muscle protein is broken down
to release amino acids, which are converted by the liver and
kidney to glucose required by the brain. However, this
debilitating and unsustainable loss of muscle is reduced to
20 g per day after 3 days as the plasma concentrations of
ketone bodies rise and meet about one-third of the brain’s
energy needs, thereby reducing its demand for glucose.
Short-term regulation of enzyme activity
As is evident in Figure 1, there are three places where the
opposing pathways of gluconeogenesis and glycolysis
bifurcate. Control of these ‘substrate cycles’ is crucial to
determining the net flux in glucose production or utilization by the liver and kidney. In situations requiring an
increased rate of gluconeogenesis this is achieved by several
means:
1. Glucagon, via its intracellular messenger cyclic AMP
(cAMP), activates protein kinase A to phosphorylate
liver pyruvate kinase, thereby decreasing its activity.
The phosphorylated pyruvate kinase is less sensitive to
activation by fructose 1,6-bisphosphate and more
sensitive to inhibition by ATP and alanine.
2. Phosphorylation of a single serine residue in each
subunit of the dimeric, bifunctional 6PF2K/
F2,6BPase by glucagon-activated protein kinase A
results in an increase of F2,6BPase activity and a
concomitant loss of kinase activity. This dual effect
explains the very low levels of fructose 2,6-bisphosphate found in the livers of starved or diabetic rats.
3. A decrease in the concentration of fructose 2,6bisphosphate results simultaneously in inhibition of
6-phosphofructo-1-kinase and activation of fructose1,6-bisphosphatase.
4. The decline in plasma levels of insulin leads to
increased levels of plasma free fatty acids that undergo
b-oxidation, thereby increasing the level of mitochondrial acetyl-CoA that allosterically activates pyruvate
carboxylase activity.
5. Increased b oxidation of fatty acids also leads to
ketone body formation, which induces a mild metabolic acidosis that increases renal gluconeogenesis.
6. The major hepatic isoform of hexokinase, glucokinase
(also known as hexokinase IV), has been shown to be
inhibited by long-chain acyl–CoA compounds, by
association with a ‘glucokinase regulatory protein’
and by sequestration to the nucleus (Nordlie et al.,
1999). Whereas no allosteric effector of either the
cytosolic or mitochondrial isoform of PEPCK has
been described (Hanson and Reshef, 1997), the acute
regulation of G6Pase has many candidate effectors
whose relative importance has yet to be determined
(Nordlie et al., 1999).
Long-term regulation of gluconeogenesis
All seven key enzymes catalysing the three substrate cycles
(Figure 1), as well as the bifunctional 6PF2K/F2,6BPase,
are regulated coordinately, principally by insulin and
glucagon, by transcriptional and in some cases also by
posttranscriptional means. Expression of the key enzymes
of gluconeogenesis (PC, PEPCK, FBPase and G6Pase) is
inhibited by insulin but stimulated by glucagon, whereas
expression of their glycolytic counterparts (PK, 6PF–1K
and GK) is stimulated by insulin and inhibited by glucagon
and cAMP.
Phosphenolpyruvate carboxykinase and pyruvate
carboxylase versus pyruvate kinase
PEPCK is by far the most comprehensively characterized
of the gluconeogenic enzymes at the level of gene
expression (Hanson and Reshef, 1997). Synthesis of the
cytoplasmic isoform of the enzyme (PEPCK-C) is induced
in liver by fasting, low carbohydrate diet and diabetes,
whereas it is repressed by a high carbohydrate diet in a
normal animal or by insulin administration to a diabetic
animal. In the kidney it is also regulated by the animal’s
acid–base status. Expression of the gene encoding
PEPCK-C in the periportal region of the liver is rapidly
upregulated (10-fold in 20 min) by glucagon (via cAMP),
glucocorticoids and thyroid hormone, but is decreased by
insulin. Glucagon also stabilizes the usually short-lived
mRNA by 5–8-fold. The transcriptional regulatory
elements of the PEPCK-C gene promoter have been
investigated very intensively (Hanson and Reshef, 1997),
and these studies have gone a long way towards explaining
its tissue-specific regulation by multiple hormones. The
mitochondrial PEPCK is expressed constitutively, and is
not induced by glucagon.
PC is also upregulated by the same stimuli as PEPCK-C,
although usually less rapidly and to a lesser extent. The
human and rat genes encoding PC have only recently been
isolated and sequenced, and hence the characterization of
their promoter regions is as yet much less well developed.
As the anaplerotic functions of PC serve pathways other
than gluconeogenesis, we can anticipate that the interactions of the various regulatory elements in their promoters
will be even more complex. Thus it is not surprising that
two human and five rat mRNA isoforms with distinct 5’
untranslated regions have been identified as being alternative transcripts expressed in a tissue-specific manner
(Jitrapakdee and Wallace, 1999).
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Gluconeogenesis
As the potential catalytic activity of the PK expressed in
liver (PK–L) (approximately 50 units g 2 1 in rat liver) far
exceeds the levels of both PEPCK-C and PC activities (about
7 units g 2 1), it is essential, if pyruvate or its precursors are to
be converted to glucose, that the activity of PK be controlled
efficiently. This is achieved in starvation and diabetes by
glucagon acting via cAMP to inhibit transcription of the PKL gene and to accelerate the degradation of PK-L mRNA.
Conversely, upon refeeding or administration of insulin, PKL catalytic activity is regained as mRNA levels are restored
by increased transcription and stabilization (Yamada and
Noguchi, 1999).
Fructose-1,6-bisphosphatase versus 6-phosphofructo-1kinase
The liver isoform of fructose-1,6–bisphosphatase is
regulated in a manner similar to PEPCK-C: starvation
and diabetes increase its activity as a result of cAMP
increasing the level of the FBP1 mRNA, whereas insulin
can repress this effect. However, in keeping with the
absence of consensus glucocorticoid response elements in
the 5’ flanking region of this gene, glucocorticoids have no
effect on the expression of FBP1.
Conversely, the activity of 6-phosphofructo-1-kinase in
liver is decreased by starvation and diabetes, but is
regained upon refeeding or treatment with insulin. The
control of expression of the three genes encoding 6PF-1K
in liver during development and during different nutritional regimens appears to involve transcript-specific
alterations in the rates of transcription and translation,
as well as in mRNA stability, under the reciprocal control
of insulin and cAMP (Pilkis and Granner, 1992).
6-Phosphofructo-2-kinase/fructose-2,
6-bisphosphatase
The level of this bifunctional enzyme is decreased by
starvation, diabetes and adrenalectomy but restored by
refeeding, insulin administration and treatment with
glucocorticoids, respectively. These latter effects are the
result of increased mRNA synthesis, which in the case of
insulin and glucocorticoids also requires the presence of
glucose and is blocked by cAMP.
Glucose-6-phosphatase versus glucokinase
Fasting increases liver G6Pase activity, and both glucocorticoids and cAMP increase the level of its mRNA.
However, both its mRNA and activity are low in fed and
refed animals in which insulin levels are raised. Conversely,
both the enzyme’s activity and its mRNA levels are
increased in diabetes, but insulin administered to diabetic
rats or to rats treated with glucocorticoids and cAMP
reduces G6Pase expression to normal levels.
GK, which is expressed only in liver and pancreatic b
cells, responds to fasting and to streptozotocin-induced
diabetes by its mRNA level being depressed. Conversely,
6
its mRNA level is markedly increased by refeeding and
insulin treatment.
Role of Fatty Acids in Gluconeogenesis
Only odd-chain free fatty acids (FFAs) can contribute
carbon to glucose synthesis via the production of
propionate in the b-oxidation pathway. Propionate is
metabolized via propionyl-CoA, methylmalonyl-CoA,
succinyl-CoA, succinate and fumarate to malate, which
can exit the mitochondrion and give rise to cytoplasmic
oxaloacetate the substrate of PEPCK. This pathway is well
developed in ruminants whose starch and cellulose-rich
diet is fermented largely in the rumen to a mixture of shortchain fatty acids, thereby leaving little if any carbohydrate
to be absorbed from the gut. Hence, ruminants must meet
all their glucose needs, including the prodigious amounts
needed for lactose synthesis by milking cows, from
gluconeogenesis (Herdt, 2000).
Plasma FFAs can influence the concentration of glucose
in the blood in several ways. First, FFAs can suppress
glucose uptake and utilization by the allosteric inhibition
of pyruvate dehydrogenase by acetyl-CoA and reduced
nicotinamide–adenine dinucleotide (NADH) produced by
b oxidation of FFAs, and the allosteric inhibition of 6PF1K by citrate formed from that acetyl-CoA. However,
from the timing of the effect of FFAs on glucose uptake in
humans, it appears this may involve translational or
posttranslational events (Bergman and Ader, 2000).
Raised levels of FFAs are often associated with hyperlipidaemia, which itself can reduce the effect of insulin on
blood flow in insulin-sensitive tissues. In rodents there is
evidence that chronic exposure to increased levels of
plasma FFAs will impair the insulin secretory function of
the pancreas. In humans there are some supportive data
from longitudinal studies of Pima Native Americans and of
Parisian police officers that raised plasma FFA levels are
predictive of a transition from normal glucose tolerance to
type 2 diabetes (Bergman and Ader, 2000). These endocrine effects of FFAs could, therefore, be superimposed on
the direct stimulatory effects that FFAs can have on
hepatic gluconeogenesis. Raised plasma levels of FFAs can
lead to an increase in hepatic acetyl-CoA concentration
and hence to an increase in pyruvate carboxylase activity.
b-Oxidation of FFAs also provides a ready source of
NADH with which to reduce oxaloacetate to malate for
exiting the mitochondrion.
Biochemistry of Diabetes Drugs that
Inhibit Glucose Production
Diabetic subjects exhibit abnormally high blood glucose
concentrations after a meal. This is principally due to
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Gluconeogenesis
enhanced gluconeogenesis in the liver, and decreased
disposal of glucose by peripheral tissues. Diabetes is
usually classified as insulin dependent (type 1) or insulin
independent (type 2). The former is characterized by
insulin insufficiency in the pancreas and the latter by insulin
resistance in the liver and peripheral tissues. The likelihood
of a person developing diabetes is probably determined by
mutations or polymorphisms in a number of ‘susceptibility’ genes. In this sense, diabetes is a ‘complex’ disease.
Insulin-dependent diabetes is treated by insulin injection. The injected hormone enhances glucose oxidation
and glycogen synthesis in skeletal muscle and other
peripheral tissues, and inhibits gluconeogenesis in the
liver. Insulin replacement therapy by injection is effective
provided that blood glucose levels are tightly controlled
within the normal physiological levels. This prevents
complications due to high blood glucose concentrations.
Insulin-independent diabetes is characterized by high
blood glucose and insulin concentrations. Despite the high
insulin concentration, the liver and peripheral tissues are
relatively insensitive to insulin action. Frequently subjects
also exhibit some impairment of insulin release from the
pancreas. As a result of insensitivity to insulin, high rates of
gluconeogenesis and glucose release from the liver are
maintained. This is the major contributor to the high blood
glucose level.
In addition to changes to the patient’s dietary and
exercise habits, several pharmacological interventions are
employed to treat noninsulin-dependent diabetes. These
include insulin injection, which increases plasma insulin
concentrations, and the use of several oral hypoglycaemic
drugs. The latter include (i) the biguanide metformin,
which inhibits gluconeogenesis; (ii) sulphonylureas, which
enhance insulin secretion from the pancreas; (iii) acarbose,
an inhibitor of the enzyme a-glucosidase which inhibits
carbohydrate digestion in the gut, and (iv) thiazolidenediones, which facilitate insulin action on skeletal muscle.
While these oral hypoglycaemic agents are used to treat
diabetes in the clinic, they cannot completely mimic the
normal physiology of insulin secretion and action, so that
some patients may have poorly controlled blood glucose
concentrations.
The action of metformin (dimethylbiguanide) on the
liver requires the presence of insulin. Metformin principally inhibits the gluconeogenic pathway by enhancing the
activity of PK. This presumably increases the cycling of
carbon around the potential futile cycle created by PK,
PEPCK and PC (Figure 1). It is hypothesized that
metformin potentiates the activation of PK by fructose
1,6-bisphosphate. Metformin also acts at other sites, and
these actions also contribute to the inhibition of gluconeogenesis. Thus it reduces lipolysis, lowers the concentration of FFAs in the blood, and inhibits FFA oxidation.
This indirectly inhibits the gluconeogenic pathway. There
is evidence that metformin also enhances insulin-receptor
tyrosine kinase activity, inhibits glycogenolysis and
inhibits G6Pase activity. The drug also acts at other
tissues, including skeletal muscle where there is evidence
that it enhances glucose uptake by increasing glucose
transport across the plasma membrane. Metformin also
suppresses the action of glucagon. An important aspect of
metformin action is that it does not cause the onset of
hypoglycaemia. Treatment with metformin is effective in
lowering blood glucose concentrations, reducing the risk of
microangiopathy, and reducing the mortality rate from
cardiovascular disease (Wiernsperger and Bailey, 1999).
Summary
The pathway of gluconeogenesis is found in the liver and
kidney where it converts three-carbon precursors, such as
lactate, alanine and glycerol, into glucose.
The main function of gluconeogenesis is to supply
glucose to tissues, such as brain, red blood cells, white
blood cells, kidney medulla and the eyes, that depend on
glucose as their main or sole energy source.
The gluconeogenic pathway utilizes most of the enzymes
of the glycolytic pathway except glucokinase, 6PF-1K and
PK. To circumvent the large free energy changes in these
particular glycolytic reactions, the gluconeogenic pathway
employs four different enzymes: PC, PEPCK, fructose-1,6bisphosphatase and G6Pase to catalyse bypass reactions.
The hormones insulin and glucagon are the major
opposing endocrine controllers of glucose utilization and
production, respectively, with glucocorticoids playing a
permissive role in support of glucagon. Starvation, a low
carbohydrate diet and exercise reduce insulin release from
the b cells of the pancreas, while increasing the secretion of
glucagon by the pancreatic a cells.
A high glucagon : insulin ratio in the blood stimulates
gluconeogenesis at all levels of control. Conversely, upon
refeeding, especially on a high carbohydrate diet, insulin
secretion is stimulated and has the effect of inhibiting
gluconeogenesis while also stimulating glucose utilization
for energy production, for the repletion of liver and muscle
glycogen, and for lipid synthesis.
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