Glukoneogeneseit is a pathway leading to the synthesis of glucose from pyruvate and other non-carbohydrate precursors, even in non-photosynthetic organisms.
It is found in all microorganisms, fungi, plants and animals and the reactions are essentially the same, leading to the synthesis of one molecule of glucose from two molecules of pyruvate. That's essentially how it isGlycolysisvice versa, which instead goes from glucose to pyruvate and shares seven enzymes with it.
Glukogenolysediffers greatly from gluconeogenesis: it does not lead toonce againProduction of glucose from non-carbohydrate precursors as shown by their general response:
Glycogen or (glucose)Norte→ n Glucose molecules
The following discussion focuses on the gluconeogenesis that occurs in higher animals and particularly in the mammalian liver.
- Why is gluconeogenesis important?
- Where does gluconeogenesis take place?
- Irreversible stages of gluconeogenesis
- From pyruvate to phosphoenolpyruvate
- Phosphoenolpyruvate precursors: pyruvate or alanine
- Phosphoenolpyruvate precursor: lactate
- From fructose-1,6-bisphosphate to fructose-6-phosphate
- From glucose-6-phosphate to glucose
- Gluconeogenesis: energetically expensive
- From pyruvate to phosphoenolpyruvate
- Coordinated regulation of gluconeogenesis and glycolysis.
- Regulation der Gluconeogenese
- PFK-1, FBPase-1 and fructose-2,6-bisphosphate
- PEP carboxiquinasa
- Regulation der Gluconeogenese
- Precursors of gluconeogenesis
- glucogenic amino acids
- Ketogenic Amino Acids
Why is gluconeogenesis important?
Gluconeogenesis is an essential metabolic pathway for at least two reasons.
- Ensures the maintenance of a reasonableblood sugar levelsif the liverGlycogenis almost sold out and nocarbohydratesYou will be taken.
- It is important to keep blood glucose within the normal range of 3.3 to 5.5 mmol/L (60 and 99 mg/dL) because many cells and tissues are highly or completely dependent on glucose for their ATP -to meet needs; Examples are red blood cells, neurons, skeletal muscles working in oxygen-poor conditions, renal medulla, testes, eye lens and cornea, and embryonic tissues. For example, the brain's glucose requirement is about 120 g/day, which corresponds to:
more than 50% of the whole body stores the monosaccharide, about 210 g, of which 190 g is stored in the liver andMuscle glycogen, and 20 g are found freely in body fluids;
about 75% of the daily requirement for glucose, about 160 g.
During fasting, between meals, or overnight, blood glucose levels remain in the normal range due to hepatic glycogenolysis and the release offatty acidsof adipose tissue and ketone bodies by the liver.fatty acidsand ketone bodies are preferentially utilized by skeletal muscle, thereby sparing glucose for glucose-dependent cells and tissues, primarily red blood cells and neurons. After about 18 hours of fasting or during intense and prolonged physical activityGlycogenReserves are exhausted and may no longer be sufficient. At this point, gluconeogenesis becomes important when carbohydrates are not ingested.
And the importance of gluconeogenesis is underscored by the fact that unconsciousness occurs at blood glucose levels below 2 mmol/L.
- Excretion of pyruvate would result in loss of ability to produce ATP through aerobic respiration, i.e. more than 10 molecules of ATP for every molecule of oxidized pyruvate.
Where does gluconeogenesis take place?
In higher animals, gluconeogenesis occurs in the liver, renal cortex, and epithelial cells of the small intestine, i. H. the enterocytes.
Quantitatively, the liver is the main site of gluconeogenesis, accounting for about 90% of synthesized glucose, followed by the renal cortex with about 10%. The liver's fundamental role is based on its size; In fact, based on wet weight, the renal cortex produces more glucose than the liver.
In the renal cortex, gluconeogenesis occurs in the cells of the proximal tubule, the part of the nephron that immediately follows the glomerulus. Much of the glucose produced in the kidney is utilized by the renal medulla, while the kidney's role in maintaining blood sugar levels becomes more important during prolonged fasting and liver failure. However, it must be emphasized that the kidneys, unlike the liver, do not have any significant glycogen stores and only contribute to maintaining blood sugar homeostasis through gluconeogenesis and not through glycogenolysis.
Part of the gluconeogenetic pathway also occurs in skeletal muscle, cardiac muscle and the brain, albeit to a very small extent. In adults, the muscle weighs about 18 times that of the liver; so yoursonce againGlucose synthesis can be of quantitative importance. However, the release of glucose into the circulation does not occur because, unlike the liver, renal cortex, and enterocytes, these tissues lack glucose-6-phosphatase (EC 18.104.22.168), an enzyme that catalyzes the final step in glucose circulation. (See below). .
Therefore, the production of glucose-6-phosphate, including glycogenolysis, does not contribute to the maintenance of blood sugar levels and only helps to replenish glycogen stores, which are small in the brain and mainly limited to astrocytes. For these tissues, particularly skeletal muscle due to its large mass, the contribution to blood glucose homeostasis arises only from the small amount of glucose released in the reaction catalyzed by enzymatic debranching (EC 22.214.171.124) of glycogenolysis .
In terms of cellular localization, most of the reactions take place in the cytosol, some in the mitochondria, and the final step) within the endoplasmic reticulum cisternae.
Irreversible stages of gluconeogenesis
As stated above, gluconeogenesis is essentially reverse glycolysis. And of the ten reactions that make up gluconeogenesis, seven are shared with glycolysis; These reactions have a ΔG close to zero and are therefore easily reversible. However, under intracellular conditions, the total ΔG for glycolysis is about -63 kJ/mol (-15 kcal/mol) and for gluconeogenesis about -16 kJ/mol (-3.83 kcal/mol). are irreversible.
The irreversibility of the glycolysis pathway is due to three highly exergonic reactions that cannot be used in gluconeogenesis, listed below.
- The phosphorylation of glucose to glucose-6-phosphate catalyzed by hexokinase (EC 126.96.36.199) or glucokinase (EC 188.8.131.52).
ΔG = -33,4 kJ/mol (-8 kcal/mol)
ΔG°' = -16,7 kJ/mol (-4 kcal/mol)
- Phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate catalyzed by phosphofructokinase-1 or PFK-1 (EC 184.108.40.206)
ΔG = -22,2 kJ/mol (-5,3 kcal/mol)
ΔG°' = -14,2 kJ/mol (-3,4 kcal/mol)
- The conversion of phosphoenolpyruvate or PEP to pyruvate catalyzed by pyruvate kinase (EC 220.127.116.11)
ΔG = -16,7 kJ/mol (-4,0 kcal/mol)
ΔG°' = -31,4 kJ/mol (-7,5 kcal/mol)
In gluconeogenesis, these three steps are bypassed by enzymes that catalyze irreversible steps towards glucose synthesis: this ensures the irreversibility of the metabolic pathway.
These reactions are discussed below.
From pyruvate to phosphoenolpyruvate
The first step in gluconeogenesis that bypasses an irreversible step in glycolysis, i. H. the reaction catalyzed by pyruvate kinase is the conversion of pyruvate to phosphoenolpyruvate.
Phosphoenolpyruvate is synthesized by two reactions catalyzed by enzymes in this order:
- Pyruvatcarboxylase (EC 18.104.22.168);
- Phosphoenolpyruvat-Carboxykinase oder PEP-Carboxykinase (EC 22.214.171.124).
Pyruvat → Oxalacetat → Phosphoenolpyruvat
Pyruvate carboxylase catalyzes the carboxylation of pyruvate to oxaloacetate using ATP. The enzyme requires the presence of magnesium or manganese ions.
Pyruvate + HCO3–+ ATP → Oxalacetat + ADP + PEU
Discovered by Merton Utter in 1960, the enzyme is a mitochondrial protein composed of four identical subunits, each with catalytic activity. The subunits contain a biotin prosthetic group covalently attached through an amide bond to the ε-amino group of a lysine residue, which acts as a carrier for activated CO.2during the reaction. An allosteric binding site for acetyl-CoA is also present on each subunit.
It should be noted that the pyruvate carboxylase catalyzed reaction leading to the production of oxaloacetate also provides intermediates for the citric acid cycle or Krebs cycle.
Phosphoenolpyruvate carboxykinase is present in approximately equal amounts in the mitochondria and cytosol of hepatocytes. Isoenzymes are encoded by separate nuclear genes.
The enzyme catalyzes the decarboxylation and phosphorylation of oxaloacetate to phosphoenolpyruvate in a reaction in which GTP acts as a high-energy phosphate donor. PEP carboxykinase requires the presence of magnesium and manganese ions. The reaction is reversible under normal cell conditions.
Oxalacetat + GTP ⇄ PEP + CO2+ BIP
This reaction produces CO2the same molecule that is added to pyruvate in the pyruvate carboxylase catalyzed reaction is removed. The carboxylation-decarboxylation sequence is used to activate pyruvate since decarboxylation of oxaloacetate facilitates the formation of phosphoenolpyruvate, making it thermodynamically viable.
More generally, the carboxylation-decarboxylation sequence strongly promotes endergonic reactions and also occurs in the citric acid cycle wherevia pentose phosphate, also called the hexose monophosphate pathway, and in the synthesis of fatty acids.
PEP carboxykinase levels are very low before birth, while their activity increases many-fold a few hours after birth. Because of this, gluconeogenesis is activated after birth.
The sum of the reactions catalyzed by pyruvate carboxylase and phosphoenolpyruvate carboxykinase is:
Pyruvat + ATP + GTP + HCO3–→ PEP + ADP + PIB + PEU+CO2
ΔG°' of the reaction is equal to 0.9 kJ/mol (0.2 kcal/mol), while the standard free energy change associated with the formation of pyruvate from phosphoenolpyruvate by the kinase inversion of the pyruvate reaction is +31.4 kJ /mol (7.5 kcal/mol). Wart).
Although ΔG°' of the two steps leading to the formation of PEP from pyruvate is slightly positive, the actual free energy change (ΔG) calculated from the intracellular concentrations of the intermediates is strongly negative, -25 kJ/mol ( - 6 kcal/mol). This is due to the rapid consumption of phosphoenolpyruvate in other reactions, keeping its concentration at very low levels. Therefore, the synthesis of PEP from pyruvate is irreversible under cellular conditions.
It is important to note that the metabolic pathway for the formation of phosphoenolpyruvate from pyruvate depends on the precursor: pyruvate or alanine or lactate.
Phosphoenolpyruvate precursors: pyruvate or alanine
The shunt responses described below are predominant when the glucogenic precursor is alanine or pyruvate.
Pyruvate carboxylase is a mitochondrial enzyme, therefore pyruvate must be transported from the cytosol into the mitochondrial matrix. This is mediated by transporters located in the inner mitochondrial membrane called MPC1 and MPC2. ThisProteinWhen associated, they form a hetero-oligomer that facilitates the transport of pyruvate.
Pyruvate can also be produced from alanine in the mitochondrial matrix by transamination in the reaction catalyzed by alanine aminotransferase (EC 126.96.36.199), while the amino group is then converted to urea via the urea cycle.
Since the enzymes involved in the final stages of gluconeogenesis, with the exception of glucose-6-phosphatase, are cytosolic, oxaloacetate produced in the mitochondrial matrix is transported to the cytosol. However, there are no oxaloacetate transporters in the inner mitochondrial membrane. The passage to the cytosol occurs as a result of its reduction to malate, which, on the contrary, can pass through the inner mitochondrial membrane. The reaction is catalyzed by mitochondrial malate dehydrogenase (EC 188.8.131.52), an enzyme also involved in the citric acid cycle where the reaction occurs in reverse. In the reaction, NADH is oxidized to NAD.+.
Oxalacetat + NADH + H+⇄ Malate + NAD+
Although the ΔG°' of the reaction is highly positive, under physiological conditions the ΔG is close to zero and the reaction is easily reversible.
Malate crosses the inner mitochondrial membrane via a component of the malate-aspartate shuttle, the malate-α-ketoglutamate transporter. Once in the cytosol, malate is reoxidized to oxaloacetate in the reaction catalyzed by cytosolic malate dehydrogenase. In this NAD reaction+reduces NADH.
Malate + NAD+→ Oxalacetat + NADH + H+
Note: The malate-aspartate shuttle is the most active shuttle for transporting reducing NADH equivalents from the cytosol to the mitochondria. It is found in the mitochondria of the liver, kidneys and heart.
The reaction allows for the transport of mitochondrial reducing equivalents in the form of NADH into the cytosol. This transfer is necessary for gluconeogenesis because in the cytosol NADH, oxidized in the reaction catalyzed by glyceraldehyde-3-phosphate dehydrogenase (EC 184.108.40.206), occurs in very low concentrations with a [NADH]/[NAD+] ratio equals 8×10-4, about 100,000 times smaller than that observed in mitochondria.
Finally, oxaloacetate is converted to phosphoenolpyruvate in the reaction catalyzed by PEP carboxykinase.
Phosphoenolpyruvate precursor: lactate
Lactate is one of the most important gluconeogenic precursors. It is made for example by:
- red blood cells, which depend entirely on anaerobic glycolysis for ATP production;
- Skeletal muscle during intense physical activity, d. H. under oxygen starved conditions when the rate of glycolysis exceeds the rate of the citric acid cycle and oxidative phosphorylation.
If lactate is the gluconeogenic precursor, PEP synthesis occurs via a different pathway than previously seen. In the cytosol of NAD hepatocytes+the concentration is high and lactate is oxidized to pyruvate in the reaction catalyzed by the hepatic isoenzyme lactate dehydrogenase (EC 220.127.116.11). In the NAD reaction+reduces NADH.
Lactate + NAD+→ Pyruvate + NADH + H+
Production of cytosolic NADH obviates the need for export of reducing equivalents from mitochondria.
Pyruvate enters the mitochondrial matrix to be converted to oxaloacetate in the reaction catalyzed by pyruvate carboxylase. In mitochondria, oxaloacetate is converted to phosphoenolpyruvate in the reaction catalyzed by mitochondrial pyruvate carboxylase. Phosphoenolpyruvate leaves the mitochondria via an anion transporter located in the inner mitochondrial membrane and, once in the cytosol, continues on the pathway of gluconeogenesis.
Note: The synthesis of glucose from lactate can be considered part of theChoriclusthat occurs in the liver.
From fructose-1,6-bisphosphate to fructose-6-phosphate
The second step in gluconeogenesis, bypassing an irreversible step in the glycolysis pathway, ie the reaction catalyzed by PFK-1, is the dephosphorylation of fructose-1,6-bisphosphate to fructose-6-phosphate.
This reaction, catalyzed by fructose-1,6-bisphosphatase or FBPasi-1 (EC 18.104.22.168), a Mg2+Dependent enzyme located in cytosol leads to hydrolysis of C-1-phosphate from fructose-1,6-bisphosphate, without ATP production.
Fructose-1,6-bisphosphat + H2O → Fructose-6-Phosphate + PEU
The ΔG°' of the reaction is -16.3 kJ/mol (-3.9 kcal/mol), so it is an irreversible reaction.
From glucose-6-phosphate to glucose
The third step in gluconeogenesis, which bypasses an irreversible step in the glycolysis pathway, ie the reaction catalyzed by hexokinase or glucokinase, is the dephosphorylation of glucose-6-phosphate to glucose.
This reaction is catalyzed by the catalytic subunit of glucose-6-phosphatase, a protein complex found in the endoplasmic reticulum membrane of hepatocytes, enterocytes and renal proximal tubular cells. The glucose-6-phosphatase complex consists of a glucose-6-phosphate catalytic subunit and a glucose-6-phosphate transporter called glucose-6-phosphate translocase or T1.
The catalytic subunit of glucose-6-phosphatase has its active site on the luminal side of the organelle. That is, the enzyme catalyzes the release of glucose not into the cytosol but into the lumen of the endoplasmic reticulum.
Glucose-6-phosphate is localized both from gluconeogenesis arising in the reaction catalyzed by glucose-6-phosphate isomerase or phosphoglucose isomerase (EC 22.214.171.124) and from glycogenolysis arising in that catalyzed by phosphoglucomutase Reaction (EC 126.96.36.199) arises. . . . in the cytosol and must enter the lumen of the endoplasmic reticulum to be dephosphorylated. Its transport is mediated by glucose-6-phosphate translocase.
The catalytic subunit of glucose-6-phosphatase, a Mg2+As an enzyme-dependent enzyme, it catalyzes the final step in both gluconeogenesis and glycogenolysis. And like the reaction catalyzed by fructose-1,6-bisphosphatase, this reaction results in the hydrolysis of a phosphate ester.
Glucose-6-Phosphat + H2O → Glucose + PEU
It should also be noted that due to the orientation of the active site, the cell separates this enzymatic activity from the cytosol, preventing the glycolysis taking place in the cytosol from being terminated by the enzymatic action on glucose-6-phosphate.
The ΔG°' of the reaction is -13.8 kJ/mol (-3.3 kcal/mol), so it is an irreversible reaction. Conversely, if the reaction were instead catalyzed by hexokinase/glucokinase, transfer of a phosphate group from glucose-6-phosphate to ADP would be required. Such a reaction would have a ΔG of +33.4 kJ/mol (+8 kcal/mol) and would therefore be strongly endergonic. Similar considerations can be made for the FBPase-1 catalyzed reaction.
glucose and PEUThe group appears to be transported to the cytosol by different transporters termed T2 and T3, the latter being an anion transporter.
Finally, glucose leaves the hepatocytes via the GLUT2 membrane transporter, enters the bloodstream, and is transported to the tissues that need it. In contrast, under physiological conditions, as mentioned above, the glucose produced by the kidney is mainly used by the renal medulla.
Gluconeogenesis: energetically expensive
As with glycolysis, much of the energy consumed is consumed in the irreversible phases of the process.
Six high-energy phosphate bonds are consumed: two by GTP and four by ATP. In addition, two molecules of NADH are required for the reduction of two molecules of 1,3-bisphosphoglycerate in the reaction catalyzed by glyceraldehyde-3-phosphate dehydrogenase. The oxidation of NADH causes the lack of production of 5 ATP molecules, which are synthesized when the electrons of the reduced coenzyme are used in oxidative phosphorylation.
Furthermore, these energetic considerations show that gluconeogenesis is not simply reverse glycolysis, in which case it would require the consumption of two molecules of ATP, as shown by the general glycolysis equation.
Glukose + 2 ADP + 2 PEU+ 2 NAD+→ 2 Pyruvato + 2 ATP + 2 NADH + 2 H++ 2 hours2Ö
Here is the general equation for gluconeogenesis:
2 Pyruvate + 4 ATP + 2 GTP + 2 NADH++ 2 hours++ 4 hours2O → Glukose + 4 ADP + 2 BIP + 6 PEU+ 2 NAD+
At least in the liver, the ATP needed for gluconeogenesis comes primarily from the oxidation of fatty acids or the carbon skeletons of amino acids, depending on the "fuel" available.
Coordinated regulation of gluconeogenesis and glycolysis.
If glycolysis and gluconeogenesis were active simultaneously at high rates in the same cell, the only products would be ATP consumption and heat production, particularly in the irreversible steps of both pathways, and nothing else.
For example, considering PFK-1 and FBPasi-1:
ATP + Fructose-6-phosphate → ADP + Fructose-1,6-bisphosphate
Fructose-1,6-bisphosphat + H2O → Fructose-6-Phosphate + PEU
The sum of the two reactions is:
ATP + H2O → ADP + PEU+ heat
Two reactions occurring simultaneously in opposite directions result in a waste cycle or substrate cycle. These seemingly pointless cycles allow for the regulation of opposing metabolic pathways. In fact, different enzymes are involved in a substrate cycle, at least two whose activity can be regulated separately. Such regulation would not be possible if a single enzyme acts in both directions. The activity of the enzymes involved is modulated by:
- allosteric mechanisms;
- covalent modifications such as phosphorylation and dephosphorylation;
- Changes in the concentration of the enzymes involved due to changes in their synthesis/degradation ratio.
Allosteric mechanisms are very fast and immediately reversible, occurring within milliseconds. The others, triggered by signals from outside the cell, such as hormones such as insulin, glucagon, or epinephrine, occur on a time scale of seconds or minutes, and hours with changes in enzyme concentration.
This allows coordinated regulation of the two pathways and ensures that when pyruvate enters gluconeogenesis, the flow of glucose through the glycolytic pathway is slowed and vice versa.
Regulation der Gluconeogenese
The regulation of gluconeogenesis and glycolysis involves the unique enzymes of each pathway, not the common ones.
While the main checkpoints of glycolysis are the reactions catalyzed by PFK-1 and pyruvate kinase, the main checkpoints of gluconeogenesis are the reactions catalyzed by fructose-1,6-bisphosphatase and pyruvate carboxylase.
The other two enzymes unique to gluconeogenesis, glucose-6-phosphatase and PEP-carboxykinase, are regulated at the transcriptional level.
In mitochondria, pyruvate can be converted into:
- Acetyl-CoA, in reactions catalyzed byPyruvate Dehydrogenase Complex, reaction linking glycolysis to the Krebs cycle;
- Oxaloacetate, in the reaction catalyzed by pyruvate carboxylase to continue the gluconeogenesis pathway.
The metabolic fate of pyruvate depends on the availability of acetyl-CoA, ie the availability of fatty acids in the mitochondria.
When fatty acids are available, their β-oxidation leads to the production of acetyl-CoA, which enters the Krebs cycle and leads to the production of GTP and NADH. When the cell's energy needs are met, oxidative phosphorylation decreases, [NADH]/[NAD+] increases, NADH inhibits the citric acid cycle, and acetyl-CoA accumulates in the mitochondrial matrix. Acetyl-CoA is a positive allosteric effector of pyruvate carboxylase and a negative allosteric effector of pyruvate kinase. In addition, it inhibits the pyruvate dehydrogenase complex both through feedback inhibition and through phosphorylation through activation of a specific kinase.
This means that when the cell is subjected to a high energy load, the formation of acetyl-CoA from pyruvate decreases, while the conversion of pyruvate to glucose is stimulated. Therefore, acetyl-CoA is a molecule that indicates that no further oxidation of glucose is required for energy production and that glucogenic precursors can be used for glucose synthesis and storage.
On the other hand, when acetyl-CoA levels fall, the activity of pyruvate kinase and the pyruvate dehydrogenase complex increases, and with it the flux of metabolites through the citric acid cycle. This supplies energy to the cell.
In summary, when the cell is under high energy load, pyruvate carboxylase is active since the first checkpoint of gluconeogenesis determines the fate of pyruvate in the mitochondria.
The second important control point in gluconeogenesis is the reaction catalyzed by fructose-1,6-bisphosphatase. The enzyme is allosterically inhibited by AMP. When AMP levels are high and consequently ATP levels are low, gluconeogenesis decreases. This means that, as seen above, FBPase-1 is active when the cell's energy load is high enough to support itonce againGlukosesynthese.
In contrast, PFK-1, the corresponding glycolytic enzyme, is allosterically activated by AMP and ADP and allosterically inhibited by ATP and citrate, the latter as a result of the condensation of acetyl-CoA and oxaloacetate. For this reason:
- when AMP levels are high, gluconeogenesis decreases and glycolysis accelerates;
- When ATP levels are high, or when acetyl-CoA or citrate are present in sufficient concentrations, gluconeogenesis is promoted while glycolysis is slowed.
Elevated levels of citrate indicate that citric acid cycle activity may be decreasing; In this way, pyruvate can be used in the synthesis of glucose.
PFK-1, FBPase-1 and fructose-2,6-bisphosphate
The liver plays a key role in maintaining blood sugar homeostasis: this requires regulatory mechanisms that coordinate the consumption and production of glucose. Two hormones are mainly involved: glucagon and insulin. They act intracellularly through fructose-2,6-bisphosphate or F2,6BP, an allosteric effector of PFK-1 and FBPase-1. This molecule is structurally related to fructose-1,6-bisphosphate but is not an intermediate in glycolysis or gluconeogenesis.
It was discovered in 1980 by Emile Van Schaftingen and Henri-Gery Hers as a potent activator of PFK-1. The following year, the same researchers showed that it is also a potent FBPase-1 inhibitor.
When fructose-2,6-bisphosphate binds to the allosteric site of PFK-1, it decreases the enzyme's affinity for ATP and citrate, allosteric inhibitors, while increasing the enzyme's affinity for fructose-6-phosphate, its substrate PFK- 1 is practically inactive in the absence of fructose-2,6-bisphosphate and in the presence of physiological concentrations of ATP, fructose-6-phosphate, and the allosteric effectors AMP, ATP, and citrate. On the other hand, the presence of fructose-2,6-bisphosphate activates PFK-1 and thus stimulates glycolysis in hepatocytes. At the same time, fructose-2,6-bisphosphate delays gluconeogenesis by inhibiting fructose-1,6-bisphosphate even in the absence of AMP. However, the effects of fructose-2,6-bisphosphate and AMP on FBPase-1 activity are synergistic.
The concentration of fructose-2,6-bisphosphate is regulated by the relative rates of synthesis and breakdown. It is synthesized from fructose-6-phosphate in the reaction catalyzed by phosphofructokinase-2 or PFK-2 (EC 188.8.131.52) and to fructose-6-phosphate in the reaction catalyzed by fructose-2,6-bisphosphatase or FBPasi-2 hydrolyzed (EC 184.108.40.206). These two enzyme activities reside in a single bifunctional enzyme or tandem enzyme. In the liver, the balance of these two enzyme activities is regulated by insulin and glucagon, as described below.
It's released into the system when blood sugar levels drop, signaling the liver to increase and decrease the use of glucose for its own needsonce againSynthesis of glucose and its release from glycogen stores.
After binding to specific membrane receptors, glucagon stimulates hepatic adenylate cyclase (EC 220.127.116.11) to synthesize 3',5'-cyclic AMP or cAMP, which activates cAMP-dependent protein kinase or protein kinase A or PKA (EC18.104.22.168). ). The kinase catalyzes the phosphorylation of a specific serine residue (Ser32) of PFK-2/FBPase-2. As a result of phosphorylation, phosphatase activity increases while kinase activity decreases. This reduction due to the increase in KMetrofructose-6-phosphate causes a decrease in fructose-2,6-bisphosphate levels, which in turn inhibits glycolysis and stimulates gluconeogenesis. Therefore, in response to glucagon, the liver produces more glucose, allowing the organ to counteract the drop in blood sugar levels.
Note: Like adrenaline, glucagon stimulates gluconeogenesis by also increasing the availability of substrates such as glycerol and amino acids.
After binding to specific membrane receptors, insulin activates a protein phosphatase, phosphoprotein phosphatase 2A or PP2A, which catalyzes the removal of the phosphate group from PFK-2/FBPase-2, thereby increasing PFK-2 activity and decreasing PFK-2 activity. 2nd -2 activity. (At the same time, insulin also stimulates a cAMP phosphodiesterase, which hydrolyzes cAMP to AMP.) This increases the level of fructose-2,6-bisphosphate, which in turn inhibits gluconeogenesis and stimulates glycolysis.
In addition, fructose-6-phosphate allosterically inhibits FBPase-2 and activates PFK-2. It should be noted that the activities of PFK-2 and FBPase-2 are inhibited by their reaction products. However, the main effectors are the level of fructose-6-phosphate and the phosphorylation state of the enzyme.
In contrast to pyruvate carboxylase and fructose-1,6-bisphosphatase, the catalytic subunit of glucose-6-phosphatase is not subject to allosteric or covalent regulation. Its activity is modulated at the transcriptional level. Low blood sugar and glucagon, that is, factors that lead to increased glucose production, and glucocorticoids stimulate their synthesis, which, on the contrary, is inhibited by insulin.
Also the k.Metrofor glucose-6-phosphate is significantly larger than the range of physiological concentrations for glucose-6-phosphate itself. Therefore, it is said that the activity of the enzyme depends almost linearly on the concentration of the substrate, ie the enzyme is controlled by the substrate level.
The enzyme is mainly regulated at the level of synthesis and degradation. For example, high levels of glucagon or fasting increase protein production, stabilize your mRNA, and increase your transcription rate. Elevated levels of glucose or insulin in the blood have adverse effects.
Xylulose-5-phosphate, a product of the pentose phosphate pathway, is a recently discovered regulatory molecule. Stimulates glycolysis and inhibits gluconeogenesis by controlling fructose-2,6-bisphosphate levels in the liver.
If the blood sugar level rises, e.g. After a carbohydrate-rich meal, activation of the glycolysis and hexose monophosphate pathways in the liver occurs. The xylulose-5-phosphate produced activates protein phosphatase 2A which, as indicated above, dephosphorylates PFK-2/FBPase-2, inhibits FBPase-2 and stimulates PFK-2. This leads to an increase in the concentration of fructose-2,6-bisphosphate and then to the inhibition of gluconeogenesis and stimulation of glycolysis, resulting in increased production of acetyl-CoA, the main substrate forLipidSynthesis. At the same time, there is increased flux through the hexose monophosphate shunt, resulting in the production of NADPH, a source of electrons forLipidSynthesis. Finally, PP2A also dephosphorylates carbohydrate-sensitive element-binding protein, or ChREBP, a transcription factor that activates expression of liver genes for lipid synthesis. Therefore, in response to an increase in blood sugar levels, lipid synthesis is stimulated.
Therefore, it is clear that xylulose-5-phosphate is an important regulator of carbohydrate and fat metabolism.
Precursors of gluconeogenesis
Besides the already mentioned pyruvate, the main gluconeogenic precursors are lactate, glycerol, most amino acids and in general any compound that can be converted intoPyruvateo Oxalacetat.
Glycerin is released throughLipolysisin adipose tissue. With the exception of propionyl-CoA, it is the only usable part of the lipid moleculeonce againGlucose synthesis in animals.
Glycerol enters gluconeogenesis or glycolysis, depending on the cellular energy load, as dihydroxyacetone phosphate, or DHAP, which is synthesized in two steps.
In the first step, glycerol is phosphorylated to glycerol-3-phosphate in the reaction catalyzed by glycerol kinase (EC 22.214.171.124) using an ATP.
Glycerin + ATP → Glycerin-3-phosphat + ADP + PEU
The enzyme is absent from adipocytes but is present in the liver; This means that glycerol must reach the liver to be further metabolized.
Glycerol-3-phosphate is then oxidized to dihydroxyacetone phosphate in a reaction catalyzed by glycerol-3-phosphate dehydrogenase (EC 126.96.36.199). In this NAD reaction+reduces NADH.
Glycerin-3-phosphat + NAD+⇄ Diidroxiacetona fosfato + NADH + H+
During prolonged fasting, glycerol is the main gluconeogenic precursor, accounting for about 20% of glucose production.
glucogenic amino acids
Pyruvate and oxaloacetate are the gateways for glucogenic amino acids, i.e. those whose carbon skeleton or parts of it can be usedonce againGlukosesynthese.
Amino acids result from the catabolism ofProtein, both from food and from endogenous proteins such as those from skeletal muscle in the fasted state or during intense and prolonged exercise.
The catabolic processes of each of the twenty amino acids that make up proteins converge to form seven major products: acetyl-CoA, acetoacetyl-CoA, α-ketoglutarate, succinyl-CoA, fumarate, oxaloacetate, and pyruvate.
Except acetyl-CoA, acetoacetyl-CoA, the other five molecules can be used for gluconeogenesis. This means that gluconeogenic amino acids can also be defined as those whose carbon skeleton or parts thereof can be converted into one or more of the above-mentioned molecules.
Entry points for gluconeogenic amino acids are shown below.
- Pyruvate: Alanine, Cysteine, Glycine, Serine, Threonine and Tryptophan.
- Oxaloacetate: aspartate and asparagine.
- α-Ketoglutarate: Glutamate, Arginine, Glutamine, Histidine und Proline.
- Succinyl-CoA: Isoleucin, Methionin, Threonin und Valin.
- Fumarate: phenylalanine and tyrosine.
Citric acid cycle intermediates, α-ketoglutarate, succinyl-CoA and fumarate enter the gluconeogenic pathway after conversion to oxaloacetate.
The use of the carbon skeletons of amino acids requires the removal of the amino group. Alanine and glutamate, key molecules in the transport of amino groups from extrahepatic tissues to the liver, are the major glucogenic amino acids in mammals. Alanine is the main gluconeogenic substrate in the liver; This amino acid is transported through the muscle liver and other peripheral tissuesGlucose-Alanine Cycle.
Ketogenic Amino Acids
Acetyl-CoA and acetoacetyl-CoA cannot be used for gluconeogenesis and are precursors to fatty acids and ketone bodies. The stoichiometry of the citric acid cycle makes it clear why they can't get used to itonce againGlukosesynthese.
Acetyl-CoA condenses in the citrate synthase-catalyzed reaction with oxaloacetate to form citrate, a 6-carbon molecule instead of 4 like oxaloacetate. However, although the two carbon atoms of acetyl-CoA become part of the oxaloacetate molecule, two carbon atoms are oxidized and eliminated as CO.2, in reactions catalyzed by isocitrate dehydrogenase (EC 188.8.131.52) and the α-ketoglutarate dehydrogenase complex. Therefore, acetyl-CoA creates no net carbon gain for the citric acid cycle.
Furthermore, the reaction leading to the formation of acetyl-CoA from pyruvate and catalyzed by the pyruvate dehydrogenase complex, which is the bridge between glycolysis and the Krebs cycle, is irreversible and there is no other pathway, acetyl-CoA to convert in pyruvate.
Pyruvate + NAD++ CoASH → Acetyl-CoA + NADH + H++C02
For this reason, amino acids whose breakdown produces acetyl-CoA and/or acetoacetyl-CoA are called ketogenic.
Only leucine and lysine are uniquely ketogenic.
Note: Plants, yeast and many bacteria can use acetyl-CoAonce againGlucose synthesis as they possess the glyoxylate cycle. This cycle shares four reactions with the citric acid cycle, two unique enzymes, isocitrate lyase (EC184.108.40.206) and malate synthase (EC 220.127.116.11), but lacks the decarboxylation reactions. Therefore, organisms possessing this pathway can utilize fatty acids for gluconeogenesis.
Five amino acids, isoleucine, phenylalanine, tyrosine, threonine, and tryptophan, are glucogenic and ketogenic because part of their carbon structure can be used for gluconeogenesis while the other gives rise to ketone bodies.
Propionate, a three-carbon fatty acid, is a gluconeogenic precursor because, like propionyl-CoA, the active molecule, it can be converted to succinyl-CoA.
The various sources of propionate are discussed below.
- It can arise from the β-oxidation of odd-chain fatty acids such as margaric acid, asaturated fatty acidwith 17 carbon atoms. These fatty acids are rare compared to monochain fatty acids, but they are present in the fatty acid in significant amountsLipidossome marine organisms, ruminants and plants. In the final step of the β-oxidation sequence, the substrate is a five-carbon fatty acid. This means that once oxidized and split into two fragments, it produces acetyl-CoA and propionyl-CoA.
- Another source is the oxidation of branched chain fatty acids, where alkyl branches have an odd number of carbon atoms. An example is phytanic acid, which is produced in ruminants by the oxidation of phytol, a breakdown product of chlorophyll.
- In ruminants, propionate is also made from glucose. Glucose is released as cellulose is broken down by bacterial cellulase (EC 18.104.22.168) in the rumen, one of the four chambers that make up the stomach of these animals. These microorganisms then ferment to convert glucose into propionate, which after absorption can be used for gluconeogenesis, fatty acid synthesis, or for energy.
In ruminants, where gluconeogenesis tends to be a continuous process, propionate is the main gluconeogenic precursor.
- Propionate can also arise from the breakdown of valine, leucine, and isoleucine (see above).
The oxidation of propionyl-CoA to succinyl-CoA involves three reactions that occur in the liver and other tissues.
In the first reaction, propionyl-CoA is carboxylated to D-methylmalonyl-CoA in the reaction catalyzed by propionyl-CoA carboxylase (EC 22.214.171.124), an enzyme that requires biotin. This reaction consumes one ATP.
Propionyl-CoA + HCO3–+ ATP → D-Methylmalonyl-CoA+ ADP + PEU
In the subsequent reaction, catalyzed by methylmalonyl-CoA epimerase (EC 126.96.36.199), D-methylmalonyl-CoA epimerizes to its L-stereoisomer.
D-Methylmalonyl-CoA ⇄ L-Methylmalonyl-CoA
Finally, L-methylmalonyl-CoA undergoes intramolecular rearrangement to succinyl-CoA in the reaction catalyzed by methylmalonyl-CoA mutase (EC 188.8.131.52). This enzyme requires 5-deoxyadenosylcobalamin or coenzyme B12, a derivative of cobalamin or vitamin B12, as a coenzyme.
L-Methylmalonyl-CoA ⇄ Succinyl-CoA
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