W7 Glycogen Metabolism Flashcards
definition of gluconeogenesis
synthesis of new glucose from non carbohydrate precursor
glucose depleted > must be synthesised from other sources
main non carbohydrate precursors used in gluconeogenesis
lactate, amino acids and glycerol
where is glycogen stored
principally stored in cytosol granules of liver and muscle
account for 10% of mass in liver and 2% of mass of muscle
use of regulation of synthesis and breakdown of glycogen in liver and muscle
liver: synthesis and breakdown of glycogen regulated to maintain blood glucose levels
muscle: synthesis and breakdown of glycogen regulated to meet energy requirements of muscle cell
3 key enzymes required for reversible degradation and synthesis of glycogen
glycogen phosphorylase and glycogen synthase modify glycogen at non reducing ends
glycogen branching and debranching enzymes modify glycogen at alpha1,6 and alpha-1,4
step one in glycogen synthesis
reversible reaction converting G1P to UDP glucose via enzyme UDP-glucose phosphorylase with the use of UTP
step 2 of glycogen synthesis
glycogen synthase can add glucose residue only if the polysaccharide chain already contains more than 4 residues
glycogen synthesis requires a primer > priming function carried out by glycogenin
how does glycogenin works
hydroxyl group of tyrosine residue in glycogenin attacks C1 of glycosyl moiety of UDP-glucose > transfer of glucose from UDP-glucose to tyrosine residue > form glucosylated tyrosine
C1 of another UDP-glucose attacked by C4 of hydroxyl group of the glucosylated tyrosine
sequence repeats to form nascent glycogen molecule of 8 glucose residues attached by 1,4 glycosidic linkages
how is activity of glycogen synthase regulated
by covalent modification and allosteric ligand alteration
multi site phosphorylation markedly changes the net charge of the enzyme at N- and C- terminal ends
how do other enzymes regulate the activity of glycogen synthase
enzyme phosphorylated ar multiple sites by glycogen synthase kinase 3 (GSK3), protein kinase A (PKA) , casein kinase (CKII) and other kinases
insulin triggers activation of glycogen synthase b (inactive) by blocking activity of GSK3 and activation phosphoprotein phosphatase
G6P favours dephosphorylation of glycogen synthase by binding to it > promote conformation that is a good substrate for PP1
glucose also promotes dephosphorylation
inhibitors/promoters of glycogen synthase
insulin blocks GSK3 > prevent glycogen synthase a to b
insulin, G6P and glucose needed to bind to PP1 > convert glycogen synthase b to a while glucagon and EPI inhibitors
difference between glycogen synthase a and b
a: dephosphorylated (active form)
b: phosphorylated (inactive form)
how does glucose activate glycogen synthase
promotes dephosphorylation
binding of glucose to glycogen phosphorylase a forces conformational change that favours dephosphorylation to glycogen phosphorylase b > allow action of PP1
steps of glycogen degradation (glycogenolysis)
release of G1P from glycogen
rearranging remaining glycogen to permit continued breakdown
conversion of G1P to G6P for further metabolism
step 1 of glycogenolysis
inorganic phosphate cleaves glycogen, catalysed by glycogen phosphorylase > produce G1P and remaining glycogen chain
step 2 of glycogenolysis
transferase activity: transfer of 3 glucose residues from branched chain > relocated to non reducing end of another glycogen chain > reattached via alpha 1,4 bond > branch shortened to single glucose residue linked by alpha 1,6 bond
alpha 1,6 glucosidase activity: cleaves alpha 1,6 glycosidic bond at branch point > release single glucose molecule
step 3 of glycogenolysis
phosphoglucomutase catalyses transfer of phosphoryl group from itself to G1P > glucose 1,6 biphosphate intermediate > different phosphoryl group transferred back to restore the enzyme to original state
G1P converted to G6P
function of glucose 6-phosphatase
cleaves phosphoryl group to form free glucose and orthophosphate
mainly present only in the liver
function of coenzyme PLP in glycogen phosphorylase
aldehyde group of PLP forms a schiff base with specific lysine side chain of enzyme > positions PLP correctly within enzyme for its catalytic role
5’ phosphate group of PLP interacts with orthophosphate (Pi) > initially act as proton donor to stabilise intermediates > then act as proton acceptor to facilitate completion of reaction
interplay between PLP and Pi enables efficient cleavage of alpha 1,4 glycosidic bonds
glycogen phosphorylase mechanism
bound HPO4- favours cleavage of glycosidic bond by donating a proton to the departing glycogen > formation of carbocation, which is also favoured by transfer of proton from protonated phosphate group of bound PLP group
combination of carbocation and orthophosphate > formation of G1P
difference between phosphorylase a and b
equilibrium for phosphorylase a favours relaxed (R) state and b favours tensed (T) state
transition from T to R state associated with structural changes in alpha helices that move a loop out of the active site of each subunit
regulatory enzyme phosphorylase kinase catalyses covalent modification
allosteric regulation of glycogen breakdown in muscle by AMP, ATP and G6P
low atp > high amp > bind to nucleotide binding site > stabilise conformation of phosphorylase b in R state
atp acts as negative allosteric effector by competing with amp to favour T state
enzyme inhibited by G6P (feedback inhibition)
why would glucose function as a negative regulator of liver phosphorylase a
when there is plenty of glucose > no need to breakdown liver glycogen
how is phosphorylase kinase activated
regulated by phosphorylation: beta subunit phosphorylated by cAMP dependent PKA
partly activated by calcium levels: delta subunit is calmodulin, a calcium sensor that stimulates many enzymes
phosphorylase kinase has highest activity only after both phosphorylation of beta subunit and activation of delta subunit by Ca binding
cascade mechanism of Epi and glucagon
binding of glucagon or Epi > activates GTP binding protein Gs > active Gs triggers rise in cAMP > activate PKA > activates phosphorylase b kinase > activates glycogen phosphorylase
