Sakshi Education
Living organisms perform work in the form of executing different life processes in order to sustain life. Biological work includes all the cellular mechanisms which consume energy like muscle contraction, synthesis of macromolecules, or transport of a polar molecule or ion across a membrane. These are all endergonic mechanisms which are energy-requiring, enzyme catalysed reactions that cells must carry out in order to remain viable. The energy necessary for such reactions is derived from the exothermic hydrolysis of adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and inorganic orthophosphate (Pi). Cells derive energy from the environment to maintain a high concentration of ATP. Most cells obtain energy by the oxidation of organic fuels like glucose, although certain bacteria can oxidize inorganic compounds.
Glucose (from the Greek word glykys, meaning sweet) is the single most important organic fuel which can be oxidized in all cells by glycolysis (from the Greek word lysis, meaning a loosening or parting). Sugars other than glucose can serve as cellular fuels, but in every case at least part of the oxidation occurs via glycolysis. The sugars are converted either to glucose or to one of the intermediates in the glycolytic pathway.
The central feature of the pathway is that the oxidation of glucose can generate ATP either with or without the presence of oxygen. The six carbon compound glucose is broken down in a series of enzymatic reactions to two molecules of three carbon compound- pyruvate. In the presence of oxygen, the pyruvate produced by glycolysis can be completely oxidized to carbon dioxide and water via the citric acid cycle and oxidative phosphorylation. Sufficient energy is released to form at least 30–32 moles of ATP per mole of glucose Glycolysis is almost universal pathway which occurs in cytoplasm, this pathway was first explained by Gustav Embden and Otto Meyerhoff, so this pathway is also called as EM-pathway. This is the major pathway for the generation of ATP in tissues lacking mitochondria, e.g. RBC’s, lens, cornea. Brain is also dependent on glycolysis for the generation of ATP.
In obligate anaerobic bacteria and in eukaryotic cells that lack mitochondria, glycolysis is the sole source of ATP. Glycolysis in the absence of oxygen involves mild oxidation of glucose, yielding organic products derived from pyruvate, for example two molecules of lactate (C3H5O32) in lactic acid fermentation.
Lactic acid fermentation is the sole source of ATP in erythrocytes, which lose their mitochondria during cell differentiation. Mature red cells have relatively little metabolic demand for ATP beyond the operation of membrane ion pumps. In fact, approximately 25% of the high energy phosphate generated by glucose oxidation is diverted toward the formation of 2,3-bisphosphoglycerate, an important regulator of haemoglobin function. The lens of the eye is also clear of mitochondria, to avoid absorption and scattering of light, and relies almost exclusively on anaerobic glycolysis. Cells of the heart can use either fatty acids or glucose as fuel, brain tissue relies exclusively on the complete oxidation of glucose to carbon dioxide and water.
Pasteur Effect: The presence of oxygen causes a decrease in both glucose consumption and lactate formation in cells that can carry out aerobic oxidation of NADH, The suppression of fermentation reactions and glucose consumption under aerobic conditions is known as the Pasteur Effect. This effect aids the cell in diverting of pyruvate from lactate production towards complete oxidation to carbon dioxide and water, increases the yield of ATP per gram of glucose, decreasing the amount of glucose required to meet the energy requirements of the cell.
Why is glucose instead of some other monosaccharide such a prominent fuel
The breakdown of Glucose to pyruvate occurs in a series of ten enzymatic reactions which can be divided into two phases. 1) Preparatory phase, 2) Pay-off phase.
Preparatory Phase:
Fate of Pyruvate: Pyruvate formed by the glycolysis can be further catabolized by three possible fates,
Glucose (from the Greek word glykys, meaning sweet) is the single most important organic fuel which can be oxidized in all cells by glycolysis (from the Greek word lysis, meaning a loosening or parting). Sugars other than glucose can serve as cellular fuels, but in every case at least part of the oxidation occurs via glycolysis. The sugars are converted either to glucose or to one of the intermediates in the glycolytic pathway.
The central feature of the pathway is that the oxidation of glucose can generate ATP either with or without the presence of oxygen. The six carbon compound glucose is broken down in a series of enzymatic reactions to two molecules of three carbon compound- pyruvate. In the presence of oxygen, the pyruvate produced by glycolysis can be completely oxidized to carbon dioxide and water via the citric acid cycle and oxidative phosphorylation. Sufficient energy is released to form at least 30–32 moles of ATP per mole of glucose Glycolysis is almost universal pathway which occurs in cytoplasm, this pathway was first explained by Gustav Embden and Otto Meyerhoff, so this pathway is also called as EM-pathway. This is the major pathway for the generation of ATP in tissues lacking mitochondria, e.g. RBC’s, lens, cornea. Brain is also dependent on glycolysis for the generation of ATP.
In obligate anaerobic bacteria and in eukaryotic cells that lack mitochondria, glycolysis is the sole source of ATP. Glycolysis in the absence of oxygen involves mild oxidation of glucose, yielding organic products derived from pyruvate, for example two molecules of lactate (C3H5O32) in lactic acid fermentation.
Lactic acid fermentation is the sole source of ATP in erythrocytes, which lose their mitochondria during cell differentiation. Mature red cells have relatively little metabolic demand for ATP beyond the operation of membrane ion pumps. In fact, approximately 25% of the high energy phosphate generated by glucose oxidation is diverted toward the formation of 2,3-bisphosphoglycerate, an important regulator of haemoglobin function. The lens of the eye is also clear of mitochondria, to avoid absorption and scattering of light, and relies almost exclusively on anaerobic glycolysis. Cells of the heart can use either fatty acids or glucose as fuel, brain tissue relies exclusively on the complete oxidation of glucose to carbon dioxide and water.
Pasteur Effect: The presence of oxygen causes a decrease in both glucose consumption and lactate formation in cells that can carry out aerobic oxidation of NADH, The suppression of fermentation reactions and glucose consumption under aerobic conditions is known as the Pasteur Effect. This effect aids the cell in diverting of pyruvate from lactate production towards complete oxidation to carbon dioxide and water, increases the yield of ATP per gram of glucose, decreasing the amount of glucose required to meet the energy requirements of the cell.
Why is glucose instead of some other monosaccharide such a prominent fuel
- Glucose is one of several monosaccharides formed from formaldehyde under the prebiotic condition, and so it may have primitive source for biochemical systems.
- Glucose have low tendency, relative to other monosaccharides, to non-enzymatically glycosylate proteins. In their open chain forms, monosaccharides contain carbonyl group that reacts with the amino group of proteins and forms Schiff bases, which rearrange to form a more stable amino-ketone linkage. Such non-specifically modified proteins often do not function effectively. Glucose has a strong tendency to exist in the ring formation and consequently, relatively little tendency to modified proteins. Recall all the hydroxyl groups in the ring confirmation of ß- glucose are equatorial, contributing to the sugar's high relative stability.
The breakdown of Glucose to pyruvate occurs in a series of ten enzymatic reactions which can be divided into two phases. 1) Preparatory phase, 2) Pay-off phase.
- In the preparatory phase, glucose is converted to Glyceraldehyde-3-phosphate by the investment of ATP.
- In the pay-off phase, glyceraldehydes-3-phosphate is converted to pyruvate and the energy released of this oxidation is stored in the form of ATP and NADH.
Preparatory Phase:
- Phosphorylation of Glucose: In the first step of Glycolysis, glucose gets activated by the addition of a phosphate group at C-6 where ATP acts as the phosphate donor to glucose-6-phosphate. This irreversible reaction is catalyzed by Hexokinase. This enzyme requires Mg2+ and ATP for its activity and it is present in almost all tissues. Hexokinase IV, also called as Glucokinase is present in hepatocytes. Hexokinase and Glucokinase differ in their affinity towards the substrate. Hexokinase has low km (high affinity) and it phosphorylates different hexoses like mannose, fructose. It is inhibited by glucose-6-phophate. Glucokinase has high km (low affinity), it phosphorylates only glucose. Due to high affinity, hexokinase acts even at low concentrations of Glucose where as glucokinase acts only at high concentrations of glucose, like after a meal.
- Isomerization: The reversible isomerisation of glucose-6-phosphate (aldose) to fructose-6-phosphate (ketose) is catalyzed by phophohexose isomerase. This is reaction is critical for the glycolytic pathway because the subsequent hydrolysis between C3 and C4 requires the carbonyl at C2.
1. High concentration of ATP inhibits this reaction and fructose-2,6-bisphosphate generated by PFK-2 allosterically activates the PFK-1.
- Cleavage: Fructose-1,6-bisphospahte is cleaved to generate glyceraldehyde-3-phosphate (aldose) and dihydroxyacetone phosphate (ketose). This reversible reaction is catalyzed by the enzyme aldolase.
- Interconversion:
Of the two products formed in the above reaction only glyceraldehyde-3-phosphate is used in the subsequent reactions of glycolysis. Dihydroxyacetone phosphate formed is interconverted to glyceraldehyde-3-phosphate catalyzed by the enzyme Triose phosphate isomerase. Here ketose is converted into an aldose. Ø Pay-off phase: - Oxidation:
Glyceraldehyde-3-phosphate undergoes oxidation to form 1,3-bisphosphoglycerate. This reaction is catalyzed by Glyceraldehyde-3-phosphate dehydrogenase, NAD+ acts as the hydrogen acceptor and inorganic phosphate acts as phosphate donor. This is first of the two energy conserving steps of glycolysis in which acyl phosphate is formed, which has a high standard free energy of hydrolysis. Iodoacetate and Arsenate inhibits this enzyme. - Substrate level phosphorylation:
1,3-bisphosphoglycerate transfers its high energy phosphate to ADP to form 3-phosphoglycerate and ATP. ATP is generated without the involvement of electron transport chain, so called as substrate level phophorylation. This reaction is reversible a rare example of kinase reactions. - Isomerization: The reversible shift of Phosphate from C3 of the 3-phosphoglycerate to C2 forming 2-phsophoglycerate catalyzed by phosphoglycerate mutase. This reaction involves 2,3-bisphosphoglycerate (2,3-BPG) as intermediate. In RBC’s 2,3-BPG regulates their affinity towards oxygen.
- Dehydration: This is the second energy conserving reactions of glycolysis. A molecule of water is removed from 2-phosphoglycerate to form a high energy compound phoshoenolpyruvate, catalyzed by enolase. Inspite of having the same total amount of energy, removal of water molecule from 2-phsophoglycerate redistributes the energy within the phosphoenolpyruvate increasing the free energy of hydrolysis of phosphoryl group. Flouride is the inhibitor of enolase.
- Substrate level phosphorylation: The phosphoryl group from phosphoenolpyruvate (PEP) to ADP forming pyruvate and ATP. This reaction is catalyzed by pyruvate kinase which requires K+ and either Mg2+ or Mn2+ and the reaction is irreversible and acts as the site of regulation. The large standard free energy of PEP is due to tautomerization of the enol form of pyruvate to the more stable keto form.
- Overall equation of Glycolysis: Glucose + 2NAD+ + 2ADP + 2Pi 2 Pyruvate + 2NADH + H+ + 2ATP + 2H20 As 2 molecules of ATP are invested in the two priming reactions the net gain is 2 molecules of ATP. So therefore for each molecule of glucose degraded to pyruvate, two molecules of ATP are generated.
Fate of Pyruvate: Pyruvate formed by the glycolysis can be further catabolized by three possible fates,
- Glycolysis is only the first phase of cellular respiration, the pyruvate formed is further degraded to acetyl CoA which enters into TCA cycle for the complete degradation to form CO2 and H2O and the electron transport chain drives the synthesis of ATP.
- In the vigorously contracting skeletal muscles, pyruvate is reduced to Lactate. In these muscles due to hypoxia NAD+ is not regenerated from NADH. Pyruvate accepts the electrons from NADH to form lactate and regenerate NAD+, catalyzed by Lactate dehydrogenase.
- Under anaerobic conditions pyruvate is catabolised to form ethanol and CO2, called as alcoholic or ethanol fermentation.
- Upon phosphorylation the glycolytic intermediates cannot pass through the plasma membrane as it lacks the transporters for phosphorylated intermediates.
- The high standard free energy of hydrolysis present in the phosphorylated intermediates is used in the generation of ATP.
- The phosphate present in the intermediates interacts with the active site of the enzyme lowering the activation energy.
Regulation of Glycolysis:
Metabolic regulation generally occurs at the first committed step after which the substrate has only one way to go. As the glycolytic intermediates are the precursors for several other metabolic pathways, glycolysis is regulated at more than one step. Free energy diagram of glycolysis suggests there are three steps where glycolysis is regulated. Free energy change depends on two factors, a) Standard free energy difference between reactants and products and b) Concentrations of reactants and products. Glycolysis is regulated at the three irreversible reactions catalyzed by Hexokinase, Phosphofructokinase-1 (PFK-1) and Pyruvate kinase.
Hexokinase:
Glucose enters into the cycle by the phosphorylation which is catalyzed by hexokinase. Hexokinase is inhibited by the product it has formed, so the phosphorylation of glucose is inhibited if there is increased concentration of glucose-6-phosphate (feedback inhibition). Alternatively, AMP or ADP acts as activator.
Phosphofructokinase-1:
PFK-1 catalyzes the reaction that commits the glucose for glycolysis. In addition to the substrate binding sites PFK-1 is allosterically regulated. ATP inhibits PFK-1 by binding to its allosteric site and by lowering the affinity of enzyme towards fructose-6-phosphate. Citrate, a key intermediate in the aerobic oxidation of pyruvate, fatty acids and amino acids, serves as an allosteric regulator of PFK-1; high concentration of citrate inhibits the PFK-1 further reducing the flow of glucose through glycolysis. The most important regulator of PFK-1 is fructose-2,6-bisphosphate. Fructose-2,6-bisphosphate allosterically activates PFK I by decreasing the K m for fructose-6-phosphate. Fructose-2,6-bisphosphate is synthesized from fructose-6-phosphate by the enzyme Phosphofructokinase-2 (PFK-2) and it is hydrolyzed by the enzyme fructose-2,6-bisphosphatase. These two reactions are catalyzed by a single bifunctional enzyme. Pyruvate kinase: ATP, Acetyl CoA and Citrate inhibits pyruvate kinase whereas it is activated by fructose-1,6-bisphophate, ADP and Phosphoenol pyruvate. Pyruvate kinase is activated in its dephosphrylated form and it is inactivated when it is phosphorylated by cAMP dependent protein kinase
Metabolic regulation generally occurs at the first committed step after which the substrate has only one way to go. As the glycolytic intermediates are the precursors for several other metabolic pathways, glycolysis is regulated at more than one step. Free energy diagram of glycolysis suggests there are three steps where glycolysis is regulated. Free energy change depends on two factors, a) Standard free energy difference between reactants and products and b) Concentrations of reactants and products. Glycolysis is regulated at the three irreversible reactions catalyzed by Hexokinase, Phosphofructokinase-1 (PFK-1) and Pyruvate kinase.
Hexokinase:
Glucose enters into the cycle by the phosphorylation which is catalyzed by hexokinase. Hexokinase is inhibited by the product it has formed, so the phosphorylation of glucose is inhibited if there is increased concentration of glucose-6-phosphate (feedback inhibition). Alternatively, AMP or ADP acts as activator.
Phosphofructokinase-1:
PFK-1 catalyzes the reaction that commits the glucose for glycolysis. In addition to the substrate binding sites PFK-1 is allosterically regulated. ATP inhibits PFK-1 by binding to its allosteric site and by lowering the affinity of enzyme towards fructose-6-phosphate. Citrate, a key intermediate in the aerobic oxidation of pyruvate, fatty acids and amino acids, serves as an allosteric regulator of PFK-1; high concentration of citrate inhibits the PFK-1 further reducing the flow of glucose through glycolysis. The most important regulator of PFK-1 is fructose-2,6-bisphosphate. Fructose-2,6-bisphosphate allosterically activates PFK I by decreasing the K m for fructose-6-phosphate. Fructose-2,6-bisphosphate is synthesized from fructose-6-phosphate by the enzyme Phosphofructokinase-2 (PFK-2) and it is hydrolyzed by the enzyme fructose-2,6-bisphosphatase. These two reactions are catalyzed by a single bifunctional enzyme. Pyruvate kinase: ATP, Acetyl CoA and Citrate inhibits pyruvate kinase whereas it is activated by fructose-1,6-bisphophate, ADP and Phosphoenol pyruvate. Pyruvate kinase is activated in its dephosphrylated form and it is inactivated when it is phosphorylated by cAMP dependent protein kinase
Published date : 31 May 2014 02:34PM
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