Glycogen Metabolism Flashcards
What is glycogen?
Large homopolymer of glucose, highly branched, insoluble, acts as carb energy store
What is the glycogen structure?
residues linked by a-1,4 glycosidic bonds 93% and a-1,6 g bonds 7%
How often does glycogen branch?
every 10 or so residues via a-1,6, glycosidic bonds
What is the benefits of branching?
Improves solubility and more sites available for synthesis and degradation interactions (rapid breakdown)
What are the main tissues that store glycogen?
Liver and muscle
What does liver do with glycogen?
Maintains blood glucose, release over long periods of time
What does muscle do with glycogen?
energy provision for muscle, released when required
What is the function of glycogen?
Energy reserve, readily made - mobilised
how is glycogen stored?
In granules that contain not only glycogen but enzymes and regulatory protein for synthesis
What is glucostat?
maintains constant blood glucose levels in spite of spisodic nature of food intake and fasting and flunctuations in energy demand
How does Glycogen sort fight or flight response into?
adrenaline stimulates glycogen breakdown to increase blood sugar levels and energy
what does glycogen synthesis require?
Energy
What is the activated form of glucose?
UPD-Glucose
How many moles of ATP used for per moles of UPD glucose?
2 moles
What is process of UPD-glucose to glucose 1-p by?
UDP-glucose pyrophosphorylase
What is glucosyl residue from UDP-glucose transferred to?
C-4OH group at non-reducing end of glycogen
What does glucosyl and C-4OH form?
a-1,4, glycosidic bond
What can glycongen synthase extend?
Existing chain
What primer is used for the enzyme to extend the chain?
glycogenin
What is glycogen synthesized by?
Glycogen synthase
What does the synthesis require?
branches to be made every 10 residues- branching enzymes
How is a 1-6 branch point formed?
10 glucose units added to glycogen
What does the branching enzyme do?
Breaks an a-1,4 bond and transfers block of 7 residues to interior site of glycogen molecule
How are the bonds re-attached?
a-1-6 bond
what 3 enzymes are needed to synthesize glycogen?
UDP-glucose pyrophosphorylase
Glycogen synthase
Branching enzyme (amylo-transglycosylase)
What does catabolic process of glycogen result in?
Formation of free glucose or glucose-6-phosphate
What does glycogen phosphorylase do?
Degrades glycogen by breaking a-1,4, glycosidic bonds to release glucose units one at a time from end with free 4-OH group
What does phospohorylsisdo?
Use organic phosphate
What is glucose released as?
Glucose 1-phosphate
When is breakdown of glycogen rapid?
Through cascade mechanism
What is glycogen presence aided by?
Multiple branch points
How many glucose-1-phosphate residues released and when?
7-9 for every branch point
When can glycogen phosphorylase remove glucose residues?
> 5 away from branchpoint
What happens to the remaining residues?
Added to an existing chain with an a-1,4 glycosidic bond - catalysed by transferase
What happens with the residue at the a-1,6 branching point?
Removed with glycogen-debranching enzyme
What are the 3 enzymes required to break down glycogen?
Glycogen phosphorylase, transferase enzyme, glycogen-debranching enzyme
What is G1-P converted to? and by what enzyme?
glucose-6-phosphate by phosphoglucomutase
What does liver contain that muscle does not?
glucose-6-phosphate , so free glucose formed and released for export
What does muscle not contain G-6-P?
Used locally in glycolysis
What is glycogenesis?
Synthesis
What is glycogenolysis?
Breakdown
Why can glycogenesis and glycogenolysis not occur at same time?
Prevents hydrolysis of UTP via substrate cycle
How is glycogenesis and glycogenolysis controlled?
allosteric regulation
covalent modificatio of phosphorylase and synthase enzymes
hormonal control by adrenaline, glucagon and insulin
What does allosteric regulation of glycogen phosphorylase in skeletal muscle do?
High amp activate phos b - opposed by high atp and G-6-p, exercise balance changes, amp activates phos b
What is phosphorylse a not affected by?
ATP, AMP, Glu-6
What does allosteric regulation of glycogen phosphorylase in liver do?
phos b not regulated by amp so inactive and not responsive to energy in cell, phos a deactivated by glucose
What does high glu-6-p activate?
Glycogen synthase b
When is glu-6-p low?
During muscle contraction - favours phosphorylase activation
What is glycogen synthase?
active regardless of glu-6-p levels
Which hormones reduce glycogen synthase activity?
adrenaline and glycogen
What do these hormones stimulate?
glycogen breakdown by activating glycogen phosphorylase
What does insulin stimulate?
glycogen synthase activity
What does insulin reduce?
glycogen phosphorylase activity
Where is glucagon secreted and act on?
a-cells in pancreas
acts on liver to stimulate glycogen breakdown to glucose
Where is glucose release?
in bloodstream
What hormones causes glycogen breakdown?
adrenaline
What causes glycogen breakdown in the muscles?
Adrenaline
What receptor does adrenaline bind to?
B-adrenergic receptor on PM of target cell
What pathway does adrenaline work on?
G-protein - bind 2 cAMP molecules to PKA to make it active
What does phos b convert to?
phos a - degrades glycogen
What amplifies the response of glycogen release?
Cascade
When turns off activation cascade?
decline in cAMP - dephosphorylation restores enzymes
What does insulin act on?
liver to stimulate glycogen synthesis from glucose
What is insulin used for?
dephosphorylation of enzymes, glycogen synthase on, glycogenesis
What does glucagon act on?
Phosphorylation of enzymes , glycogen synthase off, glycogenolysis
