the metabolism of glycogen

The Science Behind Health

With Doctor Bones (Don R. Mueller, Ph.D.)

The Funny Man of Health

Educator

Entertainer

J

U

GG

L

E

R

Scientist

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

Metabolism is generally divided into two basic reaction types:

(1) Anabolic reactions – those chemical reactions involving thesynthesis or construction of new compounds. Constructingproteins from amino acids, for example, is an anabolic process.

(2) Catabolic reactions – chemical reactions that are concernedwith the breakdown of compounds. The oxidation of glucoseto form CO2, H2O and energy is a catabolic reaction. Thebalanced chemical equation for this process is as follows:

Metabolism is a term that is used to describe all of thechemical reactions, which take place in living organisms.

C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy

Anabolic reactions need energy to occur. Adenosine triphosphate(ATP) comes in handy for these reactions because the release ofphosphate (P) from ATP is a useful source of energy. The hydrolysisof ATP (addition of H2O) is shown below.

Removing a phosphate (P) group from ATP, in a process that addsH2O, releases approximately 7 kilocalories of energy/mole of ATP.

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.

In a process known as cellular respiration, energy from thebreakdown of glucose is captured in the high-energy phosphatebonds of ATP. Most energy-consuming activities of cells arepowered by ATP. By hydrolyzing ATP to release its energy, variousendothermic 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

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

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 theglucose molecule, which contains 6 carbon atoms.

The figure also illustrates the two different ways in which theglucose rings are linked together in glycogen. These “glycosidic”linkages are designated, 1-4 and 1-6, meaning that carbon atom 1in one glucose ring is connected either to carbon atom 4 or carbonatom 6 in the other glucose ring, respectively.

Metabolizing Glycogen

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.

Breaking Apart the Glycogen Polymer

Splitting glycogen into separateglucose molecules is a catabolicprocess known as glycogenolysis orthe "splitting of glycogen."

In glycogenolysis, the glycogenpolymer is first converted toglucose-1-phosphate using theenzyme glycogen phosphorylase,which employs inorganic phosphatein the form of HPO4

2- to removethe glucose-1-phosphate moleculesfrom the non-reducing ends ofglycogen molecules.

Glucose-1-phosphate is then converted into glucose-6-phosphateby the enzyme phosphoglucomutase.

Glucose-6-phosphate may enter into glycolysis or it can lose itsphosphate group in a process called dephosphorylation and bereleased into the blood as glucose.

The enzyme glucose-6-phosphatase catalyzes the removal ofphosphate. This takes place mainly in the liver, as most othertissues of the body (with the exception of the small intestine andthe kidneys) lack this enzyme.

For example, glucose released from muscle glycogen, which is inthe 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 maintainingadequate blood glucose levels byconverting excess glucose to glycogen(called glycogenesis) or by convertingglycogen to glucose (glycogenolysis)when glucose levels become too low.

Two different enzymes are needed to cleave the respective 1-4and 1-6 glycosidic linkages in glycogen:

Glycogen phosphorylase cleaves the alpha (1-4) linkages.

Amylo-1,6-glucosidase, cleaves the

alpha 1-6 linkage.

The figure below shows how both alpha 1-4 and beta 1-4 linkagesare constructed. The alpha 1-4 linkage in glycogen, for example,designates that two glucose rings are linked together startingfrom carbon atom number 1 on one glucose ring, to carbon atomnumber 4 on the opposing glucose ring. In this figure, you can alsosee how an alpha 1-4 bond connects the two glucose rings frombelow the respective planes of the glucose molecules, whereasthe beta 1-4 bond connects the two glucose rings from acrosstheir respective planes.

Phosphorolytic cleavage is achieved with the addition ofphosphoric acid (H3PO4) across the respective bond. Glycogenphosphorylase does this in splitting glycogen into glucose. Thehydrolytic 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