Glycogen metabolism

Glycogen metabolism

1- The main stores of glycogen in the body are found in skeletal
muscle
Those serve as fuel reserve for the synthesis OF ATP during
muscle Contraction
2-in the liver where glycogen is used to maintain blood glucose
particularly during the early stages of fasting
3-Glycogen is highly branched polymer of ?-D glucose
_the primary glycosidic linkage is ?-(1- 4) linkage
_ after about 8-10 glucosyl residues .there is branch containing
an ?(1-6) linkage
4- UDP-glucose is the building block of glycogen
UDP-glucose is synthesized from glucose-1-phosphate and UTP
by the enzyme UDP-glucose pyrophosphorylase
Glucose from UDP-glucose is transformed to non-reducing
ends of glycogen by glycogen synthase which makes ?(1-4)
linkages .
5- branches are formed by the enzyme glucosyl ?(1-4) ?(1-6)
transferase ,which transfers a chain of five to eight residues from
non-reducing end of the glycogen chain
( breaking an ?(1-4) linkage to another residue in the chain
6-glycogen phosphorylase cleaves the ?(1-4) bonds between glycosyl
residues of non-reducing of the glycogen chains , producing glucose -
1-phosphate
Glycogen phosphorylase requires pyrodoxal phosphate as co-enzyme
.this sequential degradation continues until four glucose unite remain
on each chain before branch point .
The resulting structure called limit dextrin .
7-the enzyme oligo ?(1-4) ?(1-4 glucan trasferase removes the
outer three of the four glucosyl residues attached at branch and
transfers them to the non reducing end of another chain where they
can be converted to glycosyl -1- phosphate by glycogen
phosphorylase .
8- the remaining single glucose residue attached to an ?(1-6)linkage
is removed hydrolytically by the enzyme amylo-?(1-6)glycosidase this
lead to the release of free glucose .
9- glucose -1-phosphate is converted to glucose -6-phosphate by the
enzyme phosphoglycomutase .
10-in the muscle glucose -6-phosphate enters glycolysis in the liver
the phosphate is removed by the enzyme glucose-6-phosphase
releasing free glucosethat can be used to maintain blood glucose
level s at the beginning of the fast .
11-a deficiency of glucose -6-phosphase causes glycogen storage
disease type Ia (von gierkes disease )
This disease result in an in ability of liver to provide free glucose to
the body during fast. It affects both glycogen degradation and the
last step of gluconeogenesis and causes severe
Fasting hypoglycemia.
12- glycogen synthase and glycogen phosphorylase are allosterically
regulated .