How the Body Metabolizes Glycogen: With Doctor Bones "The Funny Man of Health" aka Don R. Mueller, Ph.D.

1. The Science Behind Health
With Doctor Bones (Don R. Mueller, Ph.D.)
The Funny Man of Health
Educator
Entertainer
J
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Scientist

2. This is a brief tour through the Metabolism of Glycogen
(the storage form for glucose in humans):

3. Metabolism is generally divided into two basic reaction types:
(1) Anabolic reactions – those chemical reactions involving the
synthesis or construction of new compounds. Constructing
proteins from amino acids, for example, is an anabolic process.
(2) Catabolic reactions – chemical reactions that are concerned
with the breakdown of compounds. The oxidation of glucose
to form CO2, H2O and energy is a catabolic reaction. The
balanced chemical equation for this process is as follows:
Metabolism is a term that is used to describe all of the
chemical reactions, which take place in living organisms.
C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy

4. Anabolic reactions need energy to occur. Adenosine triphosphate
(ATP) comes in handy for these reactions because the release of
phosphate (P) from ATP is a useful source of energy. The hydrolysis
of ATP (addition of H2O) is shown below.
Removing a phosphate (P) group from ATP, in a process that adds
H2O, releases approximately 7 kilocalories of energy/mole of ATP.

5. Adenosine triphosphate (ATP) consists of Adenine (a purine),
Ribose (a sugar) and three phosphate (PO4) groups.
The ATP molecule provides a more convenient form of energy for
cells to meet their many energy requirements.

6. In a process known as cellular respiration, energy from the
breakdown of glucose is captured in the high-energy phosphate
bonds of ATP. Most energy-consuming activities of cells are
powered by ATP. By hydrolyzing ATP to release its energy, various
endothermic reactions can be “energized.”
Just a few of the cellular processes utilizing ATP for energy include:
• constructing proteins from amino acids
• making polysaccharides from simple sugars
• creating fats from fatty acids and glycerol
• synthesizing nucleosides for DNA and RNA
• nerve impulses
• muscle contraction

7. Catabolic reactions release energy. They are known as exothermic
reactions. A catabolic process can be used, for example, to convert
the lower-energy ADP to the higher-energy ATP. This is a part of the
ATP cycle, which is depicted as follows:

8. Glycogen – the intermediate “energy storage” molecule.
We humans can store excess glucose in a form called glycogen,
which is stored predominantly in the liver and the skeletal muscles.
The following frame illustrates a glycogen molecule. The numbers
given in red specify the positions of the carbon atoms in the
glucose molecule, which contains 6 carbon atoms.
The figure also illustrates the two different ways in which the
glucose rings are linked together in glycogen. These “glycosidic”
linkages are designated, 1-4 and 1-6, meaning that carbon atom 1
in one glucose ring is connected either to carbon atom 4 or carbon
atom 6 in the other glucose ring, respectively.
Metabolizing Glycogen

9. Glycogen
It should be noted that the 1-4
linkages are found in the straight
chains and the 1-6 linkages, form
the branches in glycogen, which is
a highly-branched molecule.

10. Breaking Apart the Glycogen Polymer
Splitting glycogen into separate
glucose molecules is a catabolic
process known as glycogenolysis or
the "splitting of glycogen."
In glycogenolysis, the glycogen
polymer is first converted to
glucose-1-phosphate using the
enzyme glycogen phosphorylase,
which employs inorganic phosphate
in the form of HPO4
2- to remove
the glucose-1-phosphate molecules
from the non-reducing ends of
glycogen molecules.

11. Glucose-1-phosphate is then converted into glucose-6-phosphate
by the enzyme phosphoglucomutase.
Glucose-6-phosphate may enter into glycolysis or it can lose its
phosphate group in a process called dephosphorylation and be
released into the blood as glucose.

12. The enzyme glucose-6-phosphatase catalyzes the removal of
phosphate. This takes place mainly in the liver, as most other
tissues of the body (with the exception of the small intestine and
the kidneys) lack this enzyme.
For example, glucose released from muscle glycogen, which is in
the form of glucose-6-phosphate, can only be used in glycolysis,
and not to maintain blood glucose levels, as muscles lack glucose-
6-phosphatase.
The liver is vital in maintaining
adequate blood glucose levels by
converting excess glucose to glycogen
(called glycogenesis) or by converting
glycogen to glucose (glycogenolysis)
when glucose levels become too low.

13. Two different enzymes are needed to cleave the respective 1-4
and 1-6 glycosidic linkages in glycogen:
Glycogen phosphorylase cleaves the alpha (1-4) linkages.
Amylo-1,6-glucosidase,
cleaves the
alpha 1-6 linkage.

14. The figure below shows how both alpha 1-4 and beta 1-4 linkages
are constructed. The alpha 1-4 linkage in glycogen, for example,
designates that two glucose rings are linked together starting
from carbon atom number 1 on one glucose ring, to carbon atom
number 4 on the opposing glucose ring. In this figure, you can also
see how an alpha 1-4 bond connects the two glucose rings from
below the respective planes of the glucose molecules, whereas
the beta 1-4 bond connects the two glucose rings from across
their respective planes.

15. Phosphorolytic cleavage is achieved with the addition of
phosphoric acid (H3PO4) across the respective bond. Glycogen
phosphorylase does this in splitting glycogen into glucose. The
hydrolytic cleavage of a chemical bond is the addition of water
(H2O) across the bond. This is called hydrolysis.
The following two frames present the
respective metabolic summaries for:
1. Carbohydrate Metabolism
2. Metabolism of Carbohydrates, Proteins
and Fats