Glycogen Synthesis and Mobilisation Flashcards
Where is the greater store of glycogen: liver or muscle?
Muscle - 400g
Liver - 100g
What is the main function of muscle glycogen?
to provide energy for short bursts of exercise
What is the main function of liver glycogen?
to maintain blood glucose
Outline the advantages of glycogen as a storage molecule
readily mobilisable, less osmotically active than glucose, readily mobilised to the liver to maintain blood glucose or to enter glycolytic pathways
Describe the structure of glycogen
many monomers joined together by alpha-1,4 glycosidic bonds , with alpha-1,6 branches
In what circumstances is glycogen synthesised?
insulin stimulates glycogenesis
In what circumstances is glycogen degraded?
glucagon stimulates glycogenolysis in the liver as does muscle contraction and adrenaline
How is glycogenesis facilitated in terms of GLUT transporters?
glucose enters cell through glut (GLUT2 in liver, GLUT4 elsewhere) and is converted to glucose-6-phosphate (by glucokinase/hexokinase) and if it is not used it is converted to glycogen
Outline the process of glycogen synthesis
glucose –> glucose-6-phosphate (hexokinase) –> glucose-1-phosphate –> UDP-glucose –> glucose is released from UDP-glucose and is attached to pre-existing glycogen strand (elongation by glycogen synthase) –> different enzyme facilitates branching
Outline the process of glycogen degradation
[glucose]n/glycogen –>glucose-1-phosphate + [glucose]n-1 (using glycogen phosphorylase enzyme). The glycogen remaining experiences transfers of 3 residues to expose branches which are removed by debranching enzyme producing more free glucose. The glucose-1-phosphate produced is converted to glucose-6-phosphate which can enter glycolysis immediately to supply ATP or converted to glucose in the liver (maintain blood glucose)
How is glycogen synthesis inhibited?
PKA enzyme involved: glycogen phosphorylase (glycogenolysis) is activated by phosphorylation and glycogen synthase (glycogenesis) is deactivated by phosphorylation. PKA activated by cAMP which responds to hormones; glucagon and adrenaline elevate cAMP which activates PKA (activating glycogen breakdown),
How is glycogen synthesis promoted?
Insulin up-regulates PP1 which dephosphorylates glycogen phosphorylase and glycogen synthase, and this causes increased synthesis and inhibits breakdown
What is the fate of the glucose-1-phosphate produced in glycogenolysis in the liver?
glucose-1-phosphate –> glucose-6-phosphate which is then converted to glucose through glucose-6-phosphatase
What is the fate of the glucose-1-phosphate produced in glycogenolysis in the muscles?
glucose-1-phospate –> glucose-6-phosphate, which enters directly into glycolysis
Describe the allosteric regulation of hepatic glycogenolysis
glucose inhibits glycogen phosphorylase by causing conformational change to allow PP1 to bind and dephoshorylate
