Glycogenesis and Glycogenolysis
A) What is glycogen?
- It is the storage form of excess glucose in blood formed from a great number of glucose α-unit and synthesized and stored mainly in skeletal muscle and liver.
Site of storage:
- Mainly the skeletal muscle and liver
- Glycogen stored in the liver: its function is to maintain normal blood glucose level during fasting and between meals.
- Glycogen stored in the muscle: its function isn’t maintenance blood glucose level but is only immediate source of energy for muscle contraction (i.e. depleted during contraction and then reformed during rest).
- Glycogen is homopolysaccharide formed from branched D-glucose units linked by 1,4 and 1 ,6 glycosidic bonds but 1-4-linkage is the primary glycosidic bond.
- Each branch is made of 6 -12 glucose units.
- At the branching point, the chain is attached by 1-6 linkage.
- Extensively branched and compact than starch.
Why branched? Because degrading enzymes work on the non-reducing ends of branches thus the More branches is the more nonreduing ends and the faster degradation is.
Why glucose is stored as polymer glycogen polymer? Because the concentration of glycogen in the liver cells is 0.01μM which is equivalent to 0.4 μM glucose, thus if 0.4 glucose is stored, it will lead to high osmolarity of the liver cell causing cell rupture:
B) Processes of glycogen metabolism:
We know that glycogen is the storage form of glucose and this means that our body synthesize glycogen when glucose concentration increase and break it down back to glucose when the cells need glucose in case of low blood glucose level …. This means that glycogen metabolism consists of 2 processes; glycogen synthesis which is called glycogenesis and glycogen degradation which is called glycogenolysis
- Glycogenesis (= glycogen synthesis):
- It is the process of glycogen synthesis which occurs when blood glucose level is high after a meal rich in carbohydrate or during rest.
- Glycogen synthase enzyme for adding glucosyl units in an α-[1,4]-linkage of the branch
- This enzyme is inactivated by phosphorylation.
- Branching enzyme
- Glycogenolysis (= glycogen degradation):
- It is the process of glycogen degradation which occurs when blood glucose level is low during fasting, between meals or during high exercises.
- Glycogen phosphorylase enzyme for removing glucosyl units that form an α-[1,4]-linkage of the branch
- This enzyme is activated by phosphorylation.
- Debranching enzyme
Don’t panic from this diagram:-
Glycogenesis pathway:- (demonstrated by the blue lines)
Suppose that glucose will be stored in liver in the form of glycogen. First, Glucose will be converted to Glucose-6-phospate (G6P) … in this reaction, a phosphate group is added to glucose and hence hexokinase enzyem or glucoinase enzyme is needed (as we said before in glycolysis) …. Second, G6P will be converted into glucose-1-phosphate (G1P) by make a mutation in G6P by the enzyme phosphoglucomutase which just transfer the phosphate group from carbon number one of the glucose molecule to the carbon number six …. Third, G1P react with uridyl triphosphate (UTP) where the G1P will replace 2 phosphate groups of the UTP (as shown in the following figure) to produce UDP-Glucose… Fourth, Here will glycogen synthase appear and start to add the glucose of the UDP-glucose to the glucose strip [make it from (glucose)2 to (glucose)n+1 ] .. Fifth, the branching enzyme start to connect the glucose strips to make glycogen as seen in the previous figure…. But note that, glycogen synthase is in an inactive form called glycogen synthase D which should be activated by kinase enzyme to glycogen synthase I.
Glycogenolysis pathway:- (demonstrated by the green lines)
As we said before that there are enzymes that first break the branches called debranching enzymes, so, first these debranching enzymes should break the branches…. Second, glycogen phosphorylase start to break the glucose strips that were branches into individual glucose units and convert it into G1P … Third, G1P will converted to G6P by phosphoglucomutase which will transfer the phosphate group from carbon number 1 to carbon number 6 ….. Fourth, G6P continue the way according to the body needs where if there is low blood sugar then the liver will convert the G6P to glucose to enter the blood to be carried to all tissues or it can be enter glycolysis if it in muscles…But note that, glycogen phosphorylase is in an inactive form called glycogen phosphorylase b which should be activated by kinase enzyme to glycogen phosphorylase a.
See the regulation of Glycogen metabolism in page 2