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Glycogen metabolism - Biochemistry
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BIOCHEMISTRY – LECTURE SIX PART TWOglycogen metabolism
Glycogen is a readily accessible storage form of glucose. It can be used to
provide glucose rapidly, unlike fat, which is a longer-term storage form of
energy. The speed of utilization of glycogen is due to the structure of glycogen
and the efficiency that the structure allows in the breakdown to provide glucose.
The speed at which glycogen can be utilized is vital for its functions – its role in
glucose homeostasis (providing glucose quickly to correct low blood glucose) and
provision of glucose to muscle during vigorous activity. Glycogen is a long chain
of glucose molecules held together by a-1,4 glycosidic bonds (between carbon 1
of one glucose and carbon 4 of another). In addition to this there are branches.
Cellulose, despite also being made from glucose molecules too, has very
different properties to glycogen. It is a long chain of glucose molecules held
together by B-1,4 glycosidic bonds. Because of having a-1,4 glycosidic bonds,
the shape of the glycogen molecule is curved (same with starch). Cellulose has a
straight shape and is a structural molecule and despite being made from
glucose, we can not utilise is as a source of energy. It is our main source of fibre,
passing through our gut unchanged.
Unlike proteins, where there is only one type of bond between amino acid
subunits (peptide bond), in carbohydrate polymers there are different bonds
which affect the molecules properties. Starch and glycogen with a-1,4 glycosidic
bonds can be used as a source of glucose, cellulose with B-1,4 glycosidic bonds
cannot. Chitin is also a polymer of glucose like molecules (contains an acetylated
amino group) linked by B-1,4 glycosidic bonds.
The ability of glycogen to be utilised rapidly is due to the branched nature of the
molecule. Glycogen contains branches off the main a-1,4 chain. These branch
points are from carbon 6 of one glucose molecule and carbon 1 of another. The
more branches, the more rapidly the glucose stored in the molecule can be
utilised. The branching in starch varies from none at all in amylose starch to one
branch point every 30 glucose molecules in amylopectin. Glycogen has a branch
point every 10-glucose molecules. This relatively high degree of branching
relates to the speed animals require access to glucose compared to plants. The
branches can have branches too.
Glycogen is found in high concentration in the cytoplasm of liver cells and
skeletal muscle cells.
Glycogen in the liver is a store for the whole body. It is broken down to
provide glucose when the concentration of glucose in the blood drops.
Glycogen in skeletal muscle is a source of glucose just for it. It is utilized
during strenuous muscle activity.
The different role of glycogen in muscle vs. liver results in slight difference in the
metabolism of the molecule. The reactions required for glycogen synthesis and
breakdown are catalysed by a series of enzymes – a metabolic pathway. Enzyme
catalysis of the steps in absolutely required to allow the reactions to occur at a
meaningful rate. As with all metabolic pathways, it allows for the control of the
pathway, by control of enzymes activity. It also allows for control between the
antagonistic pathways (synthesis and breakdown) to be linked or co-ordinated.
, The formation of glycogen is called glycogenesis, the breakdown of glycogen is
called glycogenolysis. In the liver glycogenesis occurs during times when glucose
is in excess and glycogenolysis occurs when glucose is required. Glycogenesis is
stimulated by insulin (produced when blood glucose is high) and glycogenolysis
is stimulated by glucagon (produced when blood glucose concentration is low)
and adrenaline (as part of the fight or flight response).
Glycogenesis:
1. The first step is the phosphorylation of glucose to form glucose 6-
phosphate. This is catalysed by either hexokinase (in muscle) or
glucokinase (in liver). There are two differences between the two enzymes
that reflect the different roles of glycogen in the different tissues.
Glucokinase has a much lower affinity for glucose (higher K m) than
hexokinase. This allows the tissues to make greater use of glucose before
the liver. This prevents the liver competing for the available glucose.
Unlike hexokinase, glucokinase is not inhibited by the product, glucose 6-
phosphate. This lack of feedback inhibition allows the liver to continue
synthesising glycogen when blood glucose concentration is high.
2. The next step is the isomerisation of glucose 6-phosphate to glucose 1-
phosphate. This type of isomerisation (where a phosphate group position
is moved) are catalysed by mutase enzymes. In this case it is
phosphoglucomutase.
3. The next step is the addition of the glucose 1-phosphate molecule to a
molecule (uridine diphosphate, UDP) which acts as a carrier. The reaction
is between glucose 1-phosphate and uridine triphosphate (UTP), a high
energy molecule. The hydrolysis of the terminal phosphate bond drives
the reaction. The reaction is catalysed by the enzyme, UDP-glucose
pyrophosphorylase.
4. In the last step, glucose is added to the glycogen molecule (from the
activated UDP-glucose molecule) to the non-reducing carbon 4 end of the
glycogen molecule, with the release of UDP. This step is catalysed by
glycogen synthase, glycogen synthase is the rate limiting (slowest) step in
the pathway. It is by controlling the activity of this enzyme that cells
control the activity of the pathway and glycogenesis overall.
It is glycogen synthase that catalyses the final step in glycogenesis, the addition
of another glucose molecule to the growing glycogen chain. It catalyses the
slowest step in the pathway. It is also the activity of this enzyme that is used to
control the whole pathway, turning it on or off. But glycogen synthase is limited
in its capabilities. Its unable to initiate polymerisation, it requires a primer of at
least 4 glucose molecules to start work on. The formation of the primer (usually 8
glucose molecules) is catalysed by glycogenin, an enzyme which is bound to
glycogen synthase. Glycogen synthase requires glycogenin bound to be active.
When the two dissociate (and glycogenin remains bound to the glycogen
molecule), synthesis stops. Its unable to catalyse the formation of a-1,6 bonds. A
different ‘branching enzyme’ [amylo-(1,4-1,6)-transglucosylase] is required for
this.
Glycogenolysis:
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