Gluconeogenesis is a metabolic pathway that results in the generation of glucose from
non-carbohydrate carbon substrates such as pyruvate, lactate, glycerol, glucogenic amino
acids.
While also odd-chain fatty acids can be converted into glucose, it is possible for at least
some even-chain fatty acids.
It is one of the two main mechanisms humans and many other animals use to keep blood glucose
levels from dropping too low.
The other means of maintaining blood glucose levels is through the degradation of glycogen.
Gluconeogenesis is a ubiquitous process, present in plants, animals, fungi, bacteria, and other
microorganisms.
In vertebrates, gluconeogenesis takes place mainly in the liver and, to a lesser extent,
in the cortex of kidneys.
In ruminants, this tends to be a continuous process.
In many other animals, the process occurs during periods of fasting, starvation, low-carbohydrate
diets, or intense exercise.
The process is highly exergonic until ATP or GTP are utilized, effectively making the
process endergonic.
For example, the pathway leading from pyruvate to glucose-6-phosphate requires 4 molecules
of ATP and 2 molecules of GTP.
Gluconeogenesis is often associated with ketosis.
Gluconeogenesis is also a target of therapy for type II diabetes, such as metformin, which
inhibits glucose formation and stimulates glucose uptake by cells.
In ruminants, because metabolizable dietary carbohydrates tend to be metabolized by rumen
organisms, gluconeogenesis occurs regardless of fasting, low-carbohydrate diets, exercise,
etc.
Precursors
In humans the main gluconeogenic precursors are lactate, glycerol, alanine and glutamine.
Altogether, they account for over 90% of the overall gluconeogenesis.
Other glucogenic amino acid as well as all citric acid cycle intermediates, the latter
through conversion to oxaloacetate, can also function as substrates for gluconeogenesis.
In ruminants, propionate is the principal gluconeogenic substrate.
Lactate is transported back to the liver where it is converted into pyruvate by the Cori
cycle using the enzyme lactate dehydrogenase.
Pyruvate, the first designated substrate of the gluconeogenic pathway, can then be used
to generate glucose.
Transamination or deamination of amino acids facilitates entering of their carbon skeleton
into the cycle directly, or indirectly via the citric acid cycle.
Whether even-chain fatty acids can be converted into glucose in animals has been a longstanding
question in biochemistry.
It is known that odd-chain fatty acids can be oxidized to yield propionyl CoA, a precursor
for succinyl CoA, which can be converted to pyruvate and enter into gluconeogenesis.
In plants, specifically seedlings, the glyoxylate cycle can be used to convert fatty acids into
the primary carbon source of the organism.
The glyoxylate cycle produces four-carbon dicarboxylic acids that can enter gluconeogenesis.
In 1995, researchers identified the glyoxylate cycle in nematodes.
In addition, the glyoxylate enzymes malate synthase and isocitrate lyase have been found
in animal tissues.
Genes coding for malate synthase gene have been identified in other [metazoans] including
arthropods, echinoderms, and even some vertebrates.
Mammals found to possess these genes include monotremes and marsupials but not placental
mammals.
Genes for isocitrate lyase are found only in nematodes, in which, it is apparent, they
originated in horizontal gene transfer from bacteria.
The existence of glyoxylate cycles in humans has not been established, and it is widely
held that fatty acids cannot be converted to glucose in humans directly.
However, carbon-14 has been shown to end up in glucose when it is supplied in fatty acids.
Despite these findings, it is considered unlikely that the 2-carbon acetyl-CoA derived from
the oxidation of fatty acids would produce a net yield of glucose via the citric acid
cycle - however, acetyl-CoA can be converted into pyruvate and lactate through the ketogenic
pathway.
Put simply, acetic acid is used to partially produce glucose; acetyl groups can only form
part of the glucose molecules and require extra substrates in order to form the rest
of the glucose molecule.
But a roundabout pathway does lead from acetyl-coA to pyruvate, via acetoacetate, acetone, acetol
and then either propylene glycol or methylglyoxal.
Location In mammals, gluconeogenesis is restricted
to the liver, the kidney and possibly the intestine.
However these organs use somewhat different gluconeogenic precursors.
The liver uses primarily lactate, alanine and glycerol while the kidney uses lactate,
glutamine and glycerol.
Propionate is the principal substrate for gluconeogenesis in the ruminant liver, and
the ruminant liver may make increased use of gluconeogenic amino acids, e.g. alanine,
when glucose demand is increased.
The capacity of liver cells to use lactate for gluconeogenesis declines from the preruminant
stage to the ruminant stage in calves and lambs.
In sheep kidney tissue, very high rates of gluconeogenesis from propionate have been
observed.
The intestine uses mostly glutamine and glycerol.
In all species, the formation of oxaloacetate from pyruvate and TCA cycle intermediates
is restricted to the mitochondrion, and the enzymes that convert Phosphoenolpyruvic acid
to glucose are found in the cytosol.
The location of the enzyme that links these two parts of gluconeogenesis by converting
oxaloacetate to PEP, PEP carboxykinase, is variable by species: it can be found entirely
within the mitochondria, entirely within the cytosol, or dispersed evenly between the two,
as it is in humans.
Transport of PEP across the mitochondrial membrane is accomplished by dedicated transport
proteins; however no such proteins exist for oxaloacetate.
Therefore, in species that lack intra-mitochondrial PEP carboxykinase, oxaloacetate must be converted
into malate or aspartate, exported from the mitochondrion, and converted back into oxaloacetate
in order to allow gluconeogenesis to continue.
Pathway Gluconeogenesis is a pathway consisting of
a series of eleven enzyme-catalyzed reactions.
The pathway may begin in the mitochondria or cytoplasm, this being dependent on the
substrate being used.
Many of the reactions are the reversible steps found in glycolysis.
Gluconeogenesis begins in the mitochondria with the formation of oxaloacetate by the
carboxylation of pyruvate.
This reaction also requires one molecule of ATP, and is catalyzed by pyruvate carboxylase
. This enzyme is stimulated by high levels of acetyl-CoA and inhibited by high levels
of ADP and glucose.
Oxaloacetate is reduced to malate using NADH, a step required for its transportation out
of the mitochondria.
Malate is oxidized to oxaloacetate using NAD+ in the cytosol, where the remaining steps
of gluconeogenesis take place.
Oxaloacetate is decarboxylated and then phosphorylated to form phosphoenolpyruvate using the enzyme
phosphoenolpyruvate carboxykinase.
A molecule of GTP is hydrolyzed to GDP during this reaction.
The next steps in the reaction are the same as reversed glycolysis.
However, fructose-1,6-bisphosphatase converts fructose-1,6-bisphosphate to fructose 6-phosphate,
using one water molecule and releasing one phosphate.
This is also the rate-limiting step of gluconeogenesis.
Glucose-6-phosphate is formed from fructose 6-phosphate by phosphoglucoisomerase.
Glucose-6-phosphate can be used in other metabolic pathways or dephosphorylated to free glucose.
Whereas free glucose can easily diffuse in and out of the cell, the phosphorylated form
is locked in the cell, a mechanism by which intracellular glucose levels are controlled
by cells.
The final reaction of gluconeogenesis, the formation of glucose, occurs in the lumen
of the endoplasmic reticulum, where glucose-6-phosphate is hydrolyzed by glucose-6-phosphatase to
produce glucose.
Glucose is shuttled into the cytoplasm by glucose transporters located in the endoplasmic
reticulum's membrane.
Regulation While most steps in gluconeogenesis are the
reverse of those found in glycolysis, three regulated and strongly exergonic reactions
are replaced with more kinetically favorable reactions.
Hexokinase/glucokinase, phosphofructokinase, and pyruvate kinase enzymes of glycolysis
are replaced with glucose-6-phosphatase, fructose-1,6-bisphosphatase, and PEP carboxykinase.
This system of reciprocal control allow glycolysis and gluconeogenesis to inhibit each other
and prevent the formation of a futile cycle.
The majority of the enzymes responsible for gluconeogenesis are found in the cytoplasm;
the exceptions are mitochondrial pyruvate carboxylase and, in animals, phosphoenolpyruvate
carboxykinase.
The latter exists as an isozyme located in both the mitochondrion and the cytosol.
The rate of gluconeogenesis is ultimately controlled by the action of a key enzyme,
fructose-1,6-bisphosphatase, which is also regulated through signal transduction by cAMP
and its phosphorylation.
Most factors that regulate the activity of the gluconeogenesis pathway do so by inhibiting
the activity or expression of key enzymes.
However, both acetyl CoA and citrate activate gluconeogenesis enzymes.
Due to the reciprocal control of the cycle, acetyl-CoA and citrate also have inhibitory
roles in the activity of pyruvate kinase.
Global control of gluconeogenesis is mediated by glucagon; it triggers phosphorylation of
enzymes and regulatory proteins by Protein Kinase A resulting in inhibition of glycolysis
and stimulation of gluconeogenesis.
Recent studies have shown that the absence of hepatic glucose production has no major
effect on the control of fasting plasma glucose concentration.
Compensatory induction of gluconeogenesis occurs in the kidneys and intestine, driven
by glucagon, glucocorticoids, and acidosis.
References
External links Overview at indstate.edu
Interactive diagram at uakron.edu The chemical logic behind gluconeogenesis