Gluconeogenesis Control of the Blood Glucose

Peter A. Mayes, PhD, DSc, & David A. Bender, PhD

BIOMEDICAL IMPORTANCE

Gluconeogenesis is the term used to include all pathways responsible for converting noncarbohydrate precursors to glucose or glycogen. The major substrates are the glucogenic amino acids and lactate, glycerol, and propionate. Liver and kidney are the major gluco-neogenic tissues. Gluconeogenesis meets the needs of the body for glucose when carbohydrate is not available in sufficient amounts from the diet or from glycogen reserves. A supply of glucose is necessary especially for the nervous system and erythrocytes. Failure of gluconeogenesis is usually fatal. Hypoglycemia causes brain dysfunction, which can lead to coma and death. Glucose is also important in maintaining the level of intermediates of the citric acid cycle even when fatty acids are the main source of acetyl-CoA in the tissues. In addition, gluconeogenesis clears lactate produced by muscle and erythrocytes and glycerol produced by adipose tissue. Propionate, the principal glucogenic fatty acid produced in the digestion of carbohydrates by ruminants, is a major substrate for gluconeogenesis in these species.

GLUCONEOGENESIS INVOLVES GLYCOLYSIS, THE CITRIC ACID CYCLE, & SOME SPECIAL REACTIONS (Figure 19-1)

Thermodynamic Barriers Prevent a Simple Reversal of Glycolysis

Three nonequilibrium reactions catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase prevent simple reversal of glycolysis for glucose synthesis (Chapter 17). They are circumvented as follows:

A. Pyruvate & Phosphoenolpyruvate

Mitochondrial pyruvate carboxylase catalyzes the car-boxylation of pyruvate to oxaloacetate, an ATP-requir-ing reaction in which the vitamin biotin is the co-enzyme. Biotin binds CO2 from bicarbonate as carboxybiotin prior to the addition of the CO2 to pyru vate (Figure 45-17). A second enzyme, phosphoenolpyruvate carboxykinase, catalyzes the decarboxylation and phosphorylation of oxaloacetate to phosphoenolpyruvate using GTP (or ITP) as the phosphate donor. Thus, reversal of the reaction catalyzed by pyruvate kinase in glycolysis involves two endergonic reactions.

In pigeon, chicken, and rabbit liver, phospho-enolpyruvate carboxykinase is a mitochondrial enzyme, and phosphoenolpyruvate is transported into the cy-tosol for gluconeogenesis. In the rat and the mouse, the enzyme is cytosolic. Oxaloacetate does not cross the mitochondrial inner membrane; it is converted to malate, which is transported into the cytosol, and converted back to oxaloacetate by cytosolic malate dehydrogenase. In humans, the guinea pig, and the cow, the enzyme is equally distributed between mitochondria and cytosol.

The main source of GTP for phosphoenolpyruvate carboxykinase inside the mitochondrion is the reaction of succinyl-CoA synthetase (Chapter 16). This provides a link and limit between citric acid cycle activity and the extent of withdrawal of oxaloacetate for gluconeo-genesis.

B. Fructose 1,6-Bisphosphate & Fructose 6-Phosphate

The conversion of fructose 1,6-bisphosphate to fructose 6-phosphate, to achieve a reversal of glycolysis, is catalyzed by fructose-1,6-bisphosphatase. Its presence determines whether or not a tissue is capable of synthesizing glycogen not only from pyruvate but also from triosephosphates. It is present in liver, kidney, and skeletal muscle but is probably absent from heart and smooth muscle.

C. Glucose 6-Phosphate & Glucose

The conversion of glucose 6-phosphate to glucose is catalyzed by glucose-6-phosphatase. It is present in liver and kidney but absent from muscle and adipose tissue, which, therefore, cannot export glucose into the bloodstream.

Gluconeogenesis

Figure 19-1. Major pathways and regulation of gluconeogenesis and glycolysis in the liver. Entry points of glucogenic amino acids after transamination are indicated by arrows extended from circles. (See also Figure 16-4.) The key gluconeogenic enzymes are enclosed in double-bordered boxes. The ATP required for gluconeogenesis is supplied by the oxidation of long-chain fatty acids. Propionate is of quantitative importance only in ruminants. Arrows with wavy shafts signify allosteric effects; dash-shafted arrows, covalent modification by reversible phosphorylation. High concentrations of alanine act as a "gluconeogenic signal" by inhibiting glycolysis at the pyruvate kinase step.

Figure 19-1. Major pathways and regulation of gluconeogenesis and glycolysis in the liver. Entry points of glucogenic amino acids after transamination are indicated by arrows extended from circles. (See also Figure 16-4.) The key gluconeogenic enzymes are enclosed in double-bordered boxes. The ATP required for gluconeogenesis is supplied by the oxidation of long-chain fatty acids. Propionate is of quantitative importance only in ruminants. Arrows with wavy shafts signify allosteric effects; dash-shafted arrows, covalent modification by reversible phosphorylation. High concentrations of alanine act as a "gluconeogenic signal" by inhibiting glycolysis at the pyruvate kinase step.

D. Glucose 1-Phosphate & Glycogen

The breakdown of glycogen to glucose 1-phosphate is catalyzed by phosphorylase. Glycogen synthesis involves a different pathway via uridine diphosphate glucose and glycogen synthase (Figure 18-1).

The relationships between gluconeogenesis and the glycolytic pathway are shown in Figure 19-1. After transamination or deamination, glucogenic amino acids yield either pyruvate or intermediates of the citric acid cycle. Therefore, the reactions described above can account for the conversion of both glucogenic amino acids and lactate to glucose or glycogen. Propionate is a major source of glucose in ruminants and enters gluco-neogenesis via the citric acid cycle. Propionate is esteri-fied with CoA, then propionyl-CoA, is carboxylated to d-methylmalonyl-CoA, catalyzed by propionyl-CoA carboxylase, a biotin-dependent enzyme (Figure 19-2). Methylmalonyl-CoA racemase catalyzes the conversion of d-methylmalonyl-CoA to l-methylmalonyl-CoA, which then undergoes isomerization to succinyl-CoA catalyzed by methylmalonyl-CoA isomerase. This enzyme requires vitamin B12 as a coenzyme, and deficiency of this vitamin results in the excretion of methylmalonate (methylmalonic aciduria).

C15 and C17 fatty acids are found particularly in the lipids of ruminants. Dietary odd-carbon fatty acids upon oxidation yield propionate (Chapter 22), which is a substrate for gluconeogenesis in human liver.

Glycerol is released from adipose tissue as a result of lipolysis, and only tissues such as liver and kidney that possess glycerol kinase, which catalyzes the conversion of glycerol to glycerol 3-phosphate, can utilize it. Glycerol 3-phosphate may be oxidized to dihydroxyacetone phosphate by NAD+ catalyzed by glycerol-3-phos-phate dehydrogenase.

SINCE GLYCOLYSIS & GLUCONEOGENESIS SHARE THE SAME PATHWAY BUT IN OPPOSITE DIRECTIONS, THEY MUST BE REGULATED RECIPROCALLY

Changes in the availability of substrates are responsible for most changes in metabolism either directly or indirectly acting via changes in hormone secretion. Three mechanisms are responsible for regulating the activity of enzymes in carbohydrate metabolism: (1) changes in the rate of enzyme synthesis, (2) covalent modification by reversible phosphorylation, and (3) allosteric effects.

Induction & Repression of Key Enzyme Synthesis Requires Several Hours

The changes in enzyme activity in the liver that occur under various metabolic conditions are listed in Table 19-1. The enzymes involved catalyze nonequilibrium (physiologically irreversible) reactions. The effects are generally reinforced because the activity of the enzymes catalyzing the changes in the opposite direction varies reciprocally (Figure 19-1). The enzymes involved in the utilization of glucose (ie, those of glycolysis and li-pogenesis) all become more active when there is a superfluity of glucose, and under these conditions the enzymes responsible for gluconeogenesis all have low activity. The secretion of insulin, in response to increased blood glucose, enhances the synthesis of the key

ch2 I

COO-Propionate

ACYL-CoA SYNTHETASE

PROPIONYL-CoA CARBOXYLASE

tin\

D-Methyl-malonyl-CoA

METHYLMALONYL-CoA RACEMASE

Intermediates of citric acid cycle

COO I

ch2 I

METHYLMALONYL-CoA ISOMERASE

B12 coenzyme

L-Methyl-malonyl-CoA

Figure 19-2. Metabolism of propionate.

Table 19-1. Regulatory and adaptive enzymes of the rat (mainly liver).

Activity In

Inducer

Repressor

Activator

Inhibitor

Carbohydrate Feeding

Starvation and Diabetes

Enzymes of glycogenes

Glycogen synthase system

is, glycoly

Insulin

Insulin Glucose 6-phosphate1

Glucagon (cAMP) phos-phorylase, glycogen

Hexokinase

Glucose 6-phosphate1

Glucokinase

î

;

Insulin

Glucagon (cAMP)

Phosphofructokinase-1

î

;

Insulin

Glucagon (cAMP)

AMP, fructose 6-phosphate, Pi, fructose 2,6-bisphos-phate1

Citrate (fatty acids, ketone bodies),1 ATP,1 glucagon (cAMP)

Pyruvate kinase

î

;

Insulin, fructose

Glucagon (cAMP)

Fructose 1,6-bisphosphate1, insulin

ATP, alanine, glucagon (cAMP), epinephrine

Pyruvate dehydro-genase

î

;

CoA, NAD+, insulin,2 ADP, pyruvate

Acetyl-CoA, NADH, ATP (fatty acids, ketone bodies)

Enzymes of gluconeoge

Pyruvate carboxylase

nesis

î

Glucocorticoids, glucagon, epinephrine (cAMP)

Insulin

Acetyl-CoA1

ADP1

Phosphoenolpyruvate carboxykinase

;

î

Glucocorticoids, glucagon, epi-nephrine (cAMP)

Insulin

Glucagon?

Fructose-1,6-bisphosphatase

;

î

Glucocorticoids, glucagon, epi-nephrine (cAMP)

Insulin

Glucagon (cAMP)

Fructose 1,6- bisphosphate, AMP, fructose 2,6-bisphos-phate1

Glucose-6-phosphatase

;

î

Glucocorticoids, glucagon, epi-nephrine (cAMP)

Insulin

Enzymes of the pentose

Glucose-6-phosphate dehydrogenase

phospha

Insulin

6-Phosphogluconate dehydrogenase

î

;

Insulin

"Malic enzyme"

î

;

Insulin

ATP-citrate lyase

î

;

Insulin

Acetyl-CoA carboxylase

î

;

Insulin?

Citrate,1 insulin

Long-chain acyl-CoA, cAMP, glucagon

Fatty acid synthase

î

;

Insulin?

2In adipose tissue but not in liver.

1Allosteric.

2In adipose tissue but not in liver.

enzymes in glycolysis. Likewise, it antagonizes the effect of the glucocorticoids and glucagon-stimulated cAMP, which induce synthesis of the key enzymes responsible for gluconeogenesis.

Both dehydrogenases of the pentose phosphate pathway can be classified as adaptive enzymes, since they increase in activity in the well-fed animal and when insulin is given to a diabetic animal. Activity is low in diabetes or starvation. "Malic enzyme" and ATP-citrate lyase behave similarly, indicating that these two enzymes are involved in lipogenesis rather than gluconeogenesis (Chapter 21).

Covalent Modification by Reversible Phosphorylation Is Rapid

Glucagon, and to a lesser extent epinephrine, hormones that are responsive to decreases in blood glucose, inhibit glycolysis and stimulate gluconeogenesis in the liver by increasing the concentration of cAMP. This in turn activates cAMP-dependent protein kinase, leading to the phosphorylation and inactivation of pyruvate kinase. They also affect the concentration of fructose 2,6-bisphosphate and therefore glycolysis and gluco-neogenesis, as explained below.

Allosteric Modification Is Instantaneous

In gluconeogenesis, pyruvate carboxylase, which catalyzes the synthesis of oxaloacetate from pyruvate, requires acetyl-CoA as an allosteric activator. The presence of acetyl-CoA results in a change in the tertiary structure of the protein, lowering the Km value for bicarbonate. This means that as acetyl-CoA is formed from pyruvate, it automatically ensures the provision of oxaloacetate and, therefore, its further oxidation in the citric acid cycle. The activation of pyruvate carboxylase and the reciprocal inhibition of pyruvate dehydrogen-ase by acetyl-CoA derived from the oxidation of fatty acids explains the action of fatty acid oxidation in sparing the oxidation of pyruvate and in stimulating gluco-neogenesis. The reciprocal relationship between these two enzymes in both liver and kidney alters the metabolic fate of pyruvate as the tissue changes from carbohydrate oxidation, via glycolysis, to gluconeogenesis during transition from a fed to a starved state (Figure 19-1). A major role of fatty acid oxidation in promoting gluconeogenesis is to supply the requirement for ATP. Phosphofructokinase (phosphofructokinase-1) occupies a key position in regulating glycolysis and is also subject to feedback control. It is inhibited by citrate and by ATP and is activated by 5'-AMP. 5'-AMP acts as an indicator of the energy status of the cell. The presence of adenylyl kinase in liver and many other tissues allows rapid equilibration of the reaction:

Thus, when ATP is used in energy-requiring processes resulting in formation of ADP, [AMP] increases. As [ATP] may be 50 times [AMP] at equilibrium, a small fractional decrease in [ATP] will cause a severalfold increase in [AMP]. Thus, a large change in [AMP] acts as a metabolic amplifier of a small change in [ATP]. This mechanism allows the activity of phosphofructokinase-1 to be highly sensitive to even small changes in energy status of the cell and to control the quantity of carbohydrate undergoing glycolysis prior to its entry into the citric acid cycle. The increase in [AMP] can also explain why glycolysis is increased during hypoxia when [ATP] decreases. Simultaneously, AMP activates phosphory-lase, increasing glycogenolysis. The inhibition of phos-phofructokinase-1 by citrate and ATP is another explanation of the sparing action of fatty acid oxidation on glucose oxidation and also of the Pasteur effect, whereby aerobic oxidation (via the citric acid cycle) inhibits the anaerobic degradation of glucose. A consequence of the inhibition of phosphofructokinase-1 is an accumulation of glucose 6-phosphate that, in turn, inhibits further uptake of glucose in extrahepatic tissues by allosteric inhibition of hexokinase.

Fructose 2,6-Bisphosphate Plays a Unique Role in the Regulation of Glycolysis & Gluconeogenesis in Liver

The most potent positive allosteric effector of phospho-fructokinase-1 and inhibitor of fructose-1,6-bisphos-phatase in liver is fructose 2, 6-bisphosphate. It relieves inhibition of phosphofructokinase-1 by ATP and increases affinity for fructose 6-phosphate. It inhibits fructose-1,6-bisphosphatase by increasing the Km for fructose 1,6-bisphosphate. Its concentration is under both substrate (allosteric) and hormonal control (cova-lent modification) (Figure 19-3).

Fructose 2,6-bisphosphate is formed by phosphory-lation of fructose 6-phosphate by phosphofructoki-nase-2. The same enzyme protein is also responsible for its breakdown, since it has fructose-2, 6-bisphos-phatase activity. This bifunctional enzyme is under the allosteric control of fructose 6-phosphate, which stimulates the kinase and inhibits the phosphatase. Hence, when glucose is abundant, the concentration of fructose 2,6-bisphosphate increases, stimulating glycolysis by activating phosphofructokinase-1 and inhibiting

Glycogen Glucose t I

Glycogen Glucose

Glycogen Entering Glycolysis

Pyruvate

Figure 19-3. Control of glycolysis and gluconeoge-nesis in the liver by fructose 2,6-bisphosphate and the bifunctional enzyme PFK-2/F-2,6-Pase (6-phospho-fructo-2-kinase/fructose-2,6-bisphosphatase). (PFK-1, phosphofructokinase-1 [6-phosphofructo-1-kinase]; F-1,6-Pase, fructose-1,6-bisphosphatase. Arrows with wavy shafts indicate allosteric effects.)

Pyruvate

Figure 19-3. Control of glycolysis and gluconeoge-nesis in the liver by fructose 2,6-bisphosphate and the bifunctional enzyme PFK-2/F-2,6-Pase (6-phospho-fructo-2-kinase/fructose-2,6-bisphosphatase). (PFK-1, phosphofructokinase-1 [6-phosphofructo-1-kinase]; F-1,6-Pase, fructose-1,6-bisphosphatase. Arrows with wavy shafts indicate allosteric effects.)

fructose-1,6-bisphosphatase. When glucose is short, glucagon stimulates the production of cAMP, activating cAMP-dependent protein kinase, which in turn inactivates phosphofructokinase-2 and activates fructose 2,6-bisphosphatase by phosphorylation. Therefore, glu-coneogenesis is stimulated by a decrease in the concentration of fructose 2,6-bisphosphate, which deactivates phosphofructokinase-1 and deinhibits fructose-1,6-bis-phosphatase. This mechanism also ensures that glu-cagon stimulation of glycogenolysis in liver results in glucose release rather than glycolysis.

Substrate (Futile) Cycles Allow Fine Tuning

It will be apparent that the control points in glycolysis and glycogen metabolism involve a cycle of phosphory-lation and dephosphorylation catalyzed by: glucokinase and glucose-6-phosphatase; phosphofructokinase-1 and fructose-1,6-bisphosphatase; pyruvate kinase, pyruvate carboxylase, and phosphoenolypyruvate carboxykinase; and glycogen synthase and phosphorylase. If these were allowed to cycle unchecked, they would amount to futile cycles whose net result was hydrolysis of ATP. This does not occur extensively due to the various control mechanisms, which ensure that one reaction is inhibited as the other is stimulated. However, there is a physiologic advantage in allowing some cycling. The rate of net glycolysis may increase several thousand-fold in response to stimulation, and this is more readily achieved by both increasing the activity of phosphofructokinase and decreasing that of fructose bisphosphatase if both are active, than by switching one enzyme "on" and the other "off' completely. This "fine tuning" of metabolic control occurs at the expense of some loss of ATP.

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Responses

  • Vanessa
    How does starving affect gluconegeneis and glycogenesis?
    4 years ago
  • marie
    How does fructose enter the glycolytic pathway in liver and in muscle and adipose tissue?
    4 years ago
  • Lucas Pfeiffer
    Which amino acids control blood sugar?
    4 years ago
  • Fre-Swera
    What role does gluconeogenesis have with selected amino acids lactate and glycerol?
    4 years ago
  • BUCCA
    What makes a given amino acid capable of synthesizing glucose glucogenic?
    4 years ago
  • Dennis
    How to prevent gluconeogenesis affect diebetics?
    4 years ago
  • Balbo
    Why does gluconeogenesis disturbc fatty acid oxidation?
    4 years ago
  • Kifle
    Why does the failure of the alanine load to induce gluconeogenesis?
    3 years ago
  • enza
    Which thermodynamic barriers prevent reversal of glycolysis?
    29 days ago

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