5 Yield of Energy from Glucose

Dr. Ramesh Kothari

  1. Objectives
  • Glucose as the key metabolite in plants, animals and many microorganisms
  • Cellular respiration
  • Yield of energy from glucose
  • Biochemistry behind ATP yield from glucose

  

 

2. Concept Map

Step coenzyme yield ATP yield Source of ATP
Glycolysis preparatory phase −2 Phosphorylation of glucose and fructose 6-phosphate uses two ATP from the cytoplasm.
Glycolysis pay-off phase 4 Substrate-level phosphorylation
2 NAD 3 or 5 Oxidative phosphorylation : Each NADH produces net 1.5 ATP (instead of usual 2.5) due to NADH transport over the mitochondrial membrane
Oxidative decarboxylation of pyruvate 2 NADH 5 Oxidative phosphorylation
Krebs cycle 2 Substrate-level phosphorylation
6 NADH 15 Oxidative phosphorylation
2 FADH2 3 Oxidative phosphorylation
Total yield 30 or 32 ATP From the complete oxidation of one glucose molecule to carbon dioxide and oxidation of all the reduced coenzymes.

Table 1: Overall yield of ATP from glucose

 

 

3. Description

 

3.1 Glucose

  • Glucose is a monosaccharide (aldohexose) which occupies a central position in the metabolism of plants, animals and many microorganisms.
  • It is a good fuel due to its relative rich potential energy

Figure 1: D and L forms of glucose 

 

3.2 Cellular Respiration:

  • The main source of energy in a cell is the breakdown of carbohydrates, particularly, glucose.
  • The formula given below shows the complete breakdown of glucose to carbon dioxide and water in aerobic conditions:
  • The breakdown of glucose releases a bulk amount of free energy,  G°′= -686 kcal/mol.
  • Glucose is oxidized inside the cells in a sequence of steps leading to the synthesis of Adenosine Tri Phosphate.
  • Table -1 represents the reactions taking place during complete oxidation of one glucose molecule into carbondioxide.
  • If we talk theoretically, one glucose molecule yields 38 ATP molecules during cellular respiration, but, practically some ATPs are utilized for moving pyruvate, phosphate and Adenosine Di Phosphat into the mitochondria.
  • All are actively transported with the help of carriers that make use of the stored energy in the proton electrochemical gradient.
  • Figure 2 describes complete catabolism of glucose taking place in a eukaryortic cell.

Figure 2: Typical Eukaryotic cell

 

3.3 Glycolysis:

The sequential steps in glycolysis are explained below:

  1. Enzyme hexokinase catalyzes the conversion of glucose to glucose 6-phosphate and ATP is converted to ADP.
  2. Phosphoglucoisomerase converts glucose 6-phosphate is to fructose 6-phosphate.

Figure 3: Glycolysis

  1. Enzyme phosphofructokinase (PFK) dephosphorylates ATP to ADP to form fructose 1,6 bisphosphate from fructose 6-phosphate.

4. Two three C molecules, glyceraldehyde 3-phosphate and dihydroxyacetone phosphate are formed by splitting of a six C molecule, ie, fructose 1,6-bisphosphate by the enzyme aldolase.

5. Triose phosphate isomerase rapidly converts DHAP to glyceraldehyde 3-phosphate that can be utilized for the rest of glycolysis. This step is an equilibrium reaction as for each six C molecule (fructose 1,6-bisphosphate), two 3 C molecules (glyceraldehydes 3-phosphate) go on with the pathway.

6. Inorganic phosphate and NAD+ are utilized by glyceraldehyde 3-phosphate dehydrogenase to catalyze conversion of glyceraldehyde 3-phosphate to 1, 3-bisphosphoglycerate and NADH. The oxidation of the aldehyde group of the glyceraldehyde 3-phosphate generates this new high-energy phosphate bond.

7. ATP is synthesized from the newly created high-energy phosphate bond of 1, 3-bisphosphoglycerate. Phosphoglycerate kinase catalyzes the transfer of the phosphoryl group from the 1, 3-bisphosphoglycerate to ADP, generating Adenosine Tri Phosphate and 3-phosphoglycerate.

8. Phosphoglycerate mutase converts 3-phosphoglycerate to 2-phosphoglycerate. The phosphate group moves to a different carbon atom within the same molecule.

9. Enolase catalyzes the dehydration of 2-phosphoglycerate to form phosphoenolpyruvate (PEP). This reaction converts the low-energy phosphate ester bond of 2-phosphoglycerate into the high-energy phosphate bond of PEP.

10. The last reaction involves the enzyme pyruvate kinase which catalyzes the transfer of the phosphoryl group from PEP to ADP to form Adenosine Tri Phosphate and pyruvate. This reaction is physiologically irreversible.

 

  • There are two distinct methods by which cells synthesize ATP:

   (1) Oxidative phophorylation

(2) Substrate level phosphorylation

 

  • Oxidative phosphorylation involves the electron transport chain where the generation of ATP is linked to the oxidation of Nicotinamide Adenine Dinucleotide Hydrogenase (NADH) and FADH2 to NAD+ and FAD respectively. This involves the generation of a proton gradient across the inner mitochondrial membrane.
  • The examples of substrate level phosphorylation are the two reactions in glycolysis catalyzed by phosphoglycerate kinase and pyruvate kinase. Here, ATP is synthesized by the direct transfer of a phosphate from a sugar–phosphate intermediate to ADP.
  • Synthesis of GTP by succinate dehydrogenase in the citric acid cycle is third example of substrate level phosphorylation. The GTP can be utilized to phosphorylate ADP to form ATP.
  • Glycolysis takes place in the cytosol in eukaryotic cells
  • Pyruvate is then transported into mitochondria. Here, its complete oxidation to Carbon dioxide and water yields most of the ATP derived from glucose catabolism.
  • Figure 4 depicts the sequential step in the metabolism of pyruvate, ie, its oxidative decarboxylation in the presence of coenzyme A (CoA), which serves as a transporter of acyl groups in various metabolic reactions.
  • One carbon of pyruvate is released as carbon dioxide, and the remaining two carbons are added to CoA to form acetyl CoA involving reduction of NAD+ to NADH.

Figure 4: Oxidative decarboxylation of pyruvate.

 

3.4 Citric Acid Cycle:

  • The cycle carries out the oxidation of acetyl groups from acetyl CoA to CO 2 with the production of four pairs of electrons, stored initially in the reduced electron carriers NADH and FADH2 (Figure 4).
  • The citric acid cycle is also known as Kreb’s cycle and consists of eight stages:

1. Citrate synthase catalyzes the condensation of 2 C compound acetyl CoA and 4 C compound oxaloacetate to form 6 C compound citrate. This step is irreversible.

2. Aconitase converts citrate to isocitrate (6C) by a two step isomerization where cis aconitate is produced as intermediate.

3. Isocitrate dehydrogenase oxidizes isocitrate to α-ketoglutarate (5C) and carbon dioxide. IDH (Isocitrate dehydrogenase) is an enzyme found in mitochondria which requires NAD+, which is reduced to NADH.

4. α-Ketoglutarate is oxidized to succinyl CoA (4C) and carbon dioxide by the α-ketoglut arate dehydrogenase complex consisting of three enzymes and uses NAD+ as a cofactor.

5. Succinyl CoA synthetase converts succinyl CoA to succinate (4C). Succinyl- CoA bond breaks to release the energy to synthesize either GTP (mainly in animals) or ATP (exclusively in plants).

6. Succinate dehydrogenase oxidizes succinate to fumarate (4C). FAD is bound firmly to the enzyme and is reduced to FADH2.

7. A hydration reaction is catalysed by fumarase to convert fumarate to malate (4C). This reaction requires addition of a water molecule.

8. Malate dehydrogenase oxidizes malate to oxaloacetate (4C) where NAD+ is again required by the enzyme as a cofactor to accept the free pair of electron and produce NADH.

 

Figure 5: Citric Acid Cycle

 

  • One molecule of GTP, three of NADH, and one of FADH2 molecules are produced after each turn of the Kreb’s cycle.
  • Six molecules of CO2 are formed at the end of citric acid cycle which indicates completion of oxidation of glucose.

 

  1. Electron Transport Chain:

Figure 6: Electron Transport Chain

 

  • The electrons of NADH and FADH2 combine with Oxygen during oxidative phosphorylation. The energy released from this process drives the synthesis of Adenosine Tri Phosphate from Adenosine Di Phosphate.
  • The electrons are transferred from NADH to Oxygen and this process releases a large amount of free energy.

 

When each pair of electrons are transferred, 52.5 kcal/mol of free energy is released ( G°′= -52.5 kcal/mol).

  • For harvesting this energy in usable form, a gradual process of passage of electrons takes place through a sequence of carriers, which constitute the electron transport chain (Figure 5).
  • In eukaryotic cells, the components of the electron transport chain are located in the inner mitochondrial membrane.
  • In aerobic bacteria, the components of the electron transport chain are located in the plasma membrane.
  • The transfer of electrons from NADH to Oxygen yields sufficient energy to drive the synthesis of 3 molecules of ATP.
  • Electrons from FADH2 enter the electron transport chain at a lower energy level, so their transfer to Oxygen yields less usable free energy, only two ATP molecules.

 

  1. Summary
  • Glycolysis and citric acid cycle yields a total of 4 molecules of ATP directly from each glucose molecule.
  • In addition, ten molecules of NADH (two from glycolysis, two from the conversion of pyruvate to acetyl CoA, and six from the citric acid cycle) and two molecules of FADH2 are formed.
  • The remaining energy derived from the breakdown of glucose comes from the reoxidation of NADH and FADH 2, with their electrons being transferred through the electron transport chain to (eventually) reduce O2 to H2O.
  • Theoritically a total of 38 ATP are formed from complete oxidation of glucose , but practically 36 ATP are produced due to loss of some ATP in transport.
  • In this lecture we learnt about:
  • Catabolism of glucose
  • Glycolysis
  • Kreb’s Cycle
  • Electron Transport Chain
  • Yield of ATP from glucose