Glycogen metabolism Flashcards
where is glucose stored as glycogen and why
liver + muscle cells
glucose osmotically active – high glucose conc –> causes water uptake or endomosis
causing cell lysis
Why is glucose storage needed
allows blood glucose conc to be constant –> constant supply of glucose to brain
what happens when glucose conc is high or low
high – glucose –> glycogen
low – glycogen –> glucose
what is advantage of glycogen and what does glycogen support
can be broken down quickly –> quick supply of glucose for energy
supports anaerobic metabolism or glycolysis
what can animals do with glucose
can convert glucose –> fat
BUT CAN’T fat –> glucose
describe structure of glycogen
polysaccharide of alpha glucose
alpha 1,4 glycosidic bond in main chain +
alpha 1,6 glycosidic bond for every 12th glucose for branching
what is glycogenesis
glucose –> glycogen in liver + muscle
glycogen metabolism
what cells does glucose enter and how
liver cells – GLUT 2
muscle cells - GLUT 4
How would glycogenesis occur if the futile cycle isn’t formed
glucose enters liver cells -GLUT 2 + muscle cells - GLUT4
glucose –> glucose-6-phosphate = catalysed by glucokinase
glucose-6-phosphate –> glycogen = catalysed by Glycogen synthase
What is GS and when is it used and how is it activated
glycogen synthase
glycogenesis - not during futile cycle
GS activated by dephosphorylation
glycogen synthesis is rate limiting step = Slowest
when is the futile cycle formed
catabolism + anabolism pathways active simultaneously in cell
formed by 1 phosphate bond broken per cycle in forming UDP cycle
how can futile cycle be prevented
GS + glycogen phosphorylase (GP) must be reciprocally regulated
by allosteric effectors + phosphorylation
how does the futile cycle work
draw diagram
glucose –> glucose-6-phosphate = hexokinase
G6P –> G1P = phosphoglucomutase
G1P + UTP–> UDP-glucose + PPi = UDP glucose phosphorylase
PPi + H20 –> 2Pi = pyrophosphatase - spontaneous + lots energy released
UDP-glucose + glycogen (n) residues –> glycogen(n+1) + UDP = Glycogen synthase
how does glycogen catabolism occur,
briefly describe the equations (3)
- glycogen(n) –> glycogen(n-1) + G1P
- G1P G6P
- G6P + H20 –> glucose + Pi
why is vitamin B6 used with pyridoxal phosphate in 1st step of glycogen catabolism
prosthetic group
how does glycogen phosphorylase become active in 1st step of glycogen catabolism
when phosphorylated
opposite to GS activation
involves braking alpha 1,4 glycosidic bond
what is the 2nd step of glycogen catabolism
G1P G6P phosphoglucomutase (Pi moved C1 --> C6) G6P goes to glycolysis to make ATP - reversible high conc of G1P --> forward reaction high conc of G6P --> back reaction
what is the 3rd step of glycogen catabolism
G6P + H20 –> glucose + Pi
glucose-6-phosphatase in ER
only in liver = release glucose for constant supply to brain
how and when does glycogen debranching happen
when 4 glucose left in branch during catabolism
3 glucose moves to main branch
breaking alpha 1,4 glycosidic bond = alpha 1,4 transglycosylase
alpha 1,6 glycosidic bond in branched glucose broken = alpha 1,6 glucosidase
how + when does glycogen branching happen
glycogen anabolism
(Amylo 1,4 –> 1,6) transglycosylase transfers segment from end of glycogen chain to hydroxyl group of C6 of glucose
forms alpha 1,6 glycosidic bond
has to be 7 glucose chain each time - specific to enzyme active site
when is insulin produced and what does it do
blood glucose high
activates glycogen synthesis to convert glucose –> glycogen
when is glucagon and adrenaline produced and what do they do
glucagon - blood glucose too low
adrenaline - stress
hormones activate glycogenolysis + inhibit glycogen synthesis
what are the different ways to regulate glycogen metabolism
glucagon
insulin
cAMP
pKA action
how is cAMP used to regulate glycogen metabolism
insulin activates phosphodiesterase which inhibits cAMP
glucagon activates cAMP
how is cAMP made inside the cells
- hormone/1st messenger binds to G protein coupled receptors
- binding activates G protein
- G protein activates adenylate cyclase
- ATP converts into cAMP = catalysed adenylate cyclase
how does insulin regulate glycogen metabolism
- insulin activates glycogen synthase phosphatase
- GSP catalysis dephosphorylation of GS
Phosphate GS (inactive) + ADP –> GS + ATP - insulin inhibits GS kinase3 + cAMP dependant PKA
causing inhibition of phosphorylation of glycogen kinase A
GS + ATP –> PGS + ADP
draw the diagram of how glucagon helps to regulation glycogen metabolism
see diagram
