Tag Archives: glycogen breakdown

Glycogen: an efficient storage form of energy in aerobic conditions

What is the net energy yield for the oxidation of a glucose unit from glycogen in aerobic conditions?

Aerobic Conditions: Glycogen Structure
Fig. 1 – Glycogen Structure

In aerobic conditions, the oxidation of a free glucose to CO2 and H2O (glycolysis, Krebs cycle and oxidative phosphorylation) leads to the net production of about 30 molecules of ATP.

Glucose from the action of glycogen phosphorylase: glucose-1-phosphate release (about 90% of the removed units).

Glycogen synthesis from free glucose costs two ATP units for each molecule; a glucose-1-phosphate is released by the action of glycogen phosphorylase with recovering/saving one of the two previous ATP molecules.
Therefore in aerobic condition, the oxidation of glucose starting from glucose-6-phosphate and not from free glucose yields 31 ATP molecules and not 30 (one ATP instead of two is expended in the activation phase, 30 ATP are produced during Krebs cycle and oxidative phosphorylation: 31 ATP gained).
The net rate between cost and yield is 1/31 (an energy conservation of about 97%).
The overall reaction is:

glycogen(n glucose residues) + 31 ADP + 31 Pi → glycogen(n-1 glucose residues) + 31 ATP + 6 CO2 + 6 H2O

If we combine glycogen synthesis, glycogen breakdown and finally the oxidation of glucose to CO2 and H2O we obtain 30 molecules of ATP per stored glucose unit, that is the overall reaction is:

glucose + 29 ADP + 30 Pi → 29 ATP + 6 CO2 + 6 H2O

Glucose from the action of debranching enzyme: free glucose release (about 10% of the removed units).

The net yield in ATP between glycogen synthesis and breakdown is two ATP molecules expended because of free glucose is released.
In this case the oxidation of glucose starts from the not-prephosphorylated molecule so we obtain 30 ATP molecules.
The net rate between cost and yield is 2/30 (a energy conservation of about 93,3%).
Considering the oxidation of the glucose units from glycogen to CO2 and H2O we have an energy conservation of:

1-(((1/31)*0,9)+((2/30)*0,1))=0,9643

Conclusion

In aerobic conditions, there is the conservation of about 97% of energy into the glycogen molecule, an extremely efficient storage form of energy.

References

Arienti G. “Le basi molecolari della nutrizione”. Seconda edizione. Piccin, 2003

Cozzani I. and Dainese E. “Biochimica degli alimenti e della nutrizione”. Piccin Editore, 2006

Giampietro M. “L’alimentazione per l’esercizio fisico e lo sport”. Il Pensiero Scientifico Editore, 2005

Mahan LK, Escott-Stump S.: “Krause’s foods, nutrition, and diet therapy” 10th ed. 2000

Mariani Costantini A., Cannella C., Tomassi G. “Fondamenti di nutrizione umana”. 1th ed. Il Pensiero Scientifico Editore, 1999

Nelson D.L., M. M. Cox M.M. Lehninger. Principles of biochemistry. 4th Edition. W.H. Freeman and Company, 2004

Stipanuk M.H.. “Biochemical and physiological aspects of human nutrition” W.B. Saunders Company-An imprint of Elsevier Science, 2000

Carbohydrate ingestion 60 min before exercise

Introductory statement

Carbohydrates
Fig. 1 – Carbohydrates

An high-carbohydrate diet in the days before exercise, as well as ingestion of meals high in carbohydrate 3-4 h before exercise, better if with low glycemic index, can have positive effects on athlete’s performance.

For many years it has been suggested that ingestion of carbohydrates 30-60 min before exercise may adversely affect performance because it could cause hypoglycemia (blood glucose < 3.5 mmol/l or < 63 mg/l), a major contributor to fatigue. In fact, a typical athlete’s mantra is: “Avoid carbohydrate in the hour before exercise”!
What is the reason of that?
Glucose ingestion may cause hyperglycaemia followed by hyperinsulinaemina that may result in:

  • a rapid decline in glycemia 15-30 minutes after the onset of exercise, called rebound or reactive hypoglycaemia, most likely the result of:

I. an increase in muscle glucose uptake (due to the mobilization of GLUT-4 transporters by the action of insulin but also from physical activity itself);
II. the reduction in liver glucose output;

  • in addition, higher availability of carbohydrates to the muscle stimulates glycolysis and this, in combination to insulin-induced inhibition of lipolysis in both adipose tissue and muscle, results in a reduction in fat oxidation (apparently long-chain fatty acids, not medium-chain fatty acids). This may lead to premature glycogen depletion and early onset of fatigue (glycogen would be almost the only available fuel for working muscle).
    This effect is temporary, approximately lasting only for the first 20 min of exercise so, it is likely that this little glycogen breakdown has no significant effect on exercise performance.

Therefore, at least in theory, carbohydrate ingestion 60 minutes before exercise could affect performance but only two studies (Foster et al. 1979, e Kovisto et al. 1981) have reported a reduced endurance capacity while the majority of studies have reported no change or an improvement in performance.
To clarify these results, a systematic series of studies was done in trained subjects. The conclusion of these studies was that:

  • There is no effect of pre-exercise carbohydrate feeding on performance, even though in some cases hypoglycaemia did develop”.
  • There was no relationship between low blood glucose concentrations and performance”. (Jeukendrup and Killer S.C. 2010)

Conclusion

Ingestion of meals rich in carbohydrates 3-4 h before exercise is important for the increase of liver and muscle glycogen stores, or for their resynthesis in previously depleted muscle and liver.
Carbohydrate ingestion 30-60 min before exercise may be important in topping-up liver glycogen stores which serve to maintain blood glucose concentrations during exercise.
Based on the currently available scientific evidences, there is no reason to avoid carbohydrates 60 min before the onset of exercise, because they don’t seem to have any detrimental effect on performance.

References

Glycogen: an efficient storage form of energy in anaerobic conditions

What is the net energy yield for the oxidation of a glucose unit from glycogen in anaerobic conditions?

In anaerobic conditions, the oxidation of a free glucose to lactate leads to the net production of two molecules of ATP.

Anaerobic Conditions: Glycolysis to Lactate
Fig. 1 – Glycolysis to Lactate

Glucose from the action of glycogen phosphorylase: glucose-1-phosphate release (about 90% of the removed units).

Glycogen synthesis from free glucose costs two ATP units for each molecule; a glucose-1-phosphate is released by the action of glycogen phosphorylase, with recovering/saving of one of the two previous ATP molecules.
Therefore the oxidation of glucose to lactate starting from glucose-6-phosphate and not from free glucose yields three ATP molecules and not two (one ATP is expended in the activation stage instead of two, 4 ATP are produced in the third stage: three ATP gained).
The net rate between cost and yield is 1/3 (an energy conservation of about 66,7%).
The overall reaction is:

glycogen(n glucose residues) + 3 ADP + 3 Pi → glycogen(n-1 glucose residues) + 2 lactate + 3 ATP

If we combine glycogen synthesis, glycogen breakdown and finally glycolysis to lactate we obtain only one ATP molecule per stored glucose unit, that is the overall sum is:

glucose + ADP + Pi → 2 lactate + ATP

Glucose from the action of debranching enzyme: free glucose release (about 10% of the removed units).

The net yield in ATP between glycogen synthesis and breakdown is two ATP molecules expended because of free glucose is released.
In this case the oxidation of glucose starts from the not-prephosphorylated molecule and it yields two ATP molecules.
Therefore the net yield in ATP is zero.
Considering the oxidation of the glucose units from glycogen to lactate we have an energy conservation of:

1-(((1/3)*0,9)+((2/2)*0,1))=0,60

Conclusion

In anaerobic conditions, there is the conservation of about 60% of energy into the glycogen molecule, a good storage form of energy.

References

Arienti G. “Le basi molecolari della nutrizione”. Seconda edizione. Piccin, 2003

Cozzani I. and Dainese E. “Biochimica degli alimenti e della nutrizione”. Piccin Editore, 2006

Giampietro M. “L’alimentazione per l’esercizio fisico e lo sport”. Il Pensiero Scientifico Editore, 2005

Mahan LK, Escott-Stump S.: “Krause’s foods, nutrition, and diet therapy” 10th ed. 2000

Mariani Costantini A., Cannella C., Tomassi G. “Fondamenti di nutrizione umana”. 1th ed. Il Pensiero Scientifico Editore, 1999

Nelson D.L., M. M. Cox M.M. Lehninger. Principles of biochemistry. 4th Edition. W.H. Freeman and Company, 2004

Stipanuk M.H.. “Biochemical and physiological aspects of human nutrition” W.B. Saunders Company-An imprint of Elsevier Science, 2000

Blood glucose levels and liver

Blood glucose levels and hepatic glycogen

One of the main functions of the liver is to participate in the maintaining of blood glucose levels within well defined range (in the healthy state before meals 60-100 mg/dL or 3.33-5.56 mmol/L). To do it the liver releases glucose into the bloodstream in:

  • fasting state;
  • between meals;
  • during physical activity.

Blood glucose levels and hepatic glucose-6-phosphatase

In the liver, glycogen is the storage form of glucose which is released from the molecule not as such, but in the phosphorylated form i.e. with charge, the glucose-1-phosphate (this process is called glycogenolysis). The phosphorylated molecule can’t freely diffuse from the cell, but in the liver it is present the enzyme glucose-6-phosphatase that hydrolyzes glucose-6-phosphate, produced from glucose-1-phosphate in the reaction catalyzed by phosphoglucomutase, to glucose (an irreversible dephosphorylation).

glycogen(n glucose residues) + Pi → glucose-1-phosphate + glycogen(n-1 glucose residues)

glucose-1-phosphate ↔ glucose-6-phosphate

glucose-6-phosphate + H2O → glucose + Pi

Then, glucose can diffuse from the hepatocyte, via a transporter into the plasma membrane called GLUT2, into the bloodstream to be delivered to extra-hepatic cells, in primis neurons and red blood cells for which it is the main, and for red blood cells the only energy source (neurons, with the exception of those in some brain areas that can use only glucose as energy source, can derive energy from another source, the ketone bodies, which becomes predominant during periods of prolonged fasting).

Note: the liver obtains most of the energy required from the oxidation of fatty acids, not from glucose.

Glucose-6-phosphatase is present also in the kidney and gut but not in the muscle and brain; therefore in these tissues glucose-6-phosphate can’t be released from the cell.
Glucose-6-phosphatase plays an important role also in gluconeogenesis.

Glucose-6-phosphatase is present into the membrane of endoplasmic reticulum and the hydrolysis of glucose-6-phosphate occurs into its lumen (therefore this reaction is separated from the process of glycolysis). The presence of a specific transporter, the glucose-6-phosphate translocase, is required to transport the phosphorylated molecule from citosol into the lumen of endoplasmic reticulum. Although a glucose transporter is present into the membrane of endoplasmic reticulum, most of the released glucose is not transported back into the cytosol of the cell but is secreted into the bloodstream. Finally, an ion transporter transports back into the cytosol the inorganic phosphate released into the endoplasmic reticulum.

References

Arienti G. “Le basi molecolari della nutrizione”. Seconda edizione. Piccin, 2003

Cozzani I. and Dainese E. “Biochimica degli alimenti e della nutrizione”. Piccin Editore, 2006

Giampietro M. “L’alimentazione per l’esercizio fisico e lo sport”. Il Pensiero Scientifico Editore, 2005

Mahan LK, Escott-Stump S.: “Krause’s foods, nutrition, and diet therapy” 10th ed. 2000

Mariani Costantini A., Cannella C., Tomassi G. “Fondamenti di nutrizione umana”. 1th ed. Il Pensiero Scientifico Editore, 1999

Nelson D.L., M. M. Cox M.M. Lehninger. Principles of biochemistry. 4th Edition. W.H. Freeman and Company, 2004

Stipanuk M.H.. “Biochemical and physiological aspects of human nutrition” W.B. Saunders Company-An imprint of Elsevier Science, 2000