W7 Glycogen Metabolism

Question 1

definition of gluconeogenesis

Answer 1

synthesis of new glucose from non carbohydrate precursor

glucose depleted > must be synthesised from other sources

Question 2

main non carbohydrate precursors used in gluconeogenesis

Answer 2

lactate, amino acids and glycerol

Question 3

where is glycogen stored

Answer 3

principally stored in cytosol granules of liver and muscle

account for 10% of mass in liver and 2% of mass of muscle

Question 4

use of regulation of synthesis and breakdown of glycogen in liver and muscle

Answer 4

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

Question 5

3 key enzymes required for reversible degradation and synthesis of glycogen

Answer 5

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

Question 6

step one in glycogen synthesis

Answer 6

reversible reaction converting G1P to UDP glucose via enzyme UDP-glucose phosphorylase with the use of UTP

Question 7

step 2 of glycogen synthesis

Answer 7

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

Question 8

how does glycogenin works

Answer 8

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

Question 9

how is activity of glycogen synthase regulated

Answer 9

by covalent modification and allosteric ligand alteration

multi site phosphorylation markedly changes the net charge of the enzyme at N- and C- terminal ends

Question 10

how do other enzymes regulate the activity of glycogen synthase

Answer 10

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

Question 11

inhibitors/promoters of glycogen synthase

Answer 11

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

Question 12

difference between glycogen synthase a and b

Answer 12

a: dephosphorylated (active form)

b: phosphorylated (inactive form)

Question 13

how does glucose activate glycogen synthase

Answer 13

promotes dephosphorylation

binding of glucose to glycogen phosphorylase a forces conformational change that favours dephosphorylation to glycogen phosphorylase b > allow action of PP1

Question 14

steps of glycogen degradation (glycogenolysis)

Answer 14

release of G1P from glycogen

rearranging remaining glycogen to permit continued breakdown

conversion of G1P to G6P for further metabolism

Question 15

step 1 of glycogenolysis

Answer 15

inorganic phosphate cleaves glycogen, catalysed by glycogen phosphorylase > produce G1P and remaining glycogen chain

Question 16

step 2 of glycogenolysis

Answer 16

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

Question 17

step 3 of glycogenolysis

Answer 17

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

Question 18

function of glucose 6-phosphatase

Answer 18

cleaves phosphoryl group to form free glucose and orthophosphate

mainly present only in the liver

Question 19

function of coenzyme PLP in glycogen phosphorylase

Answer 19

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

Question 20

glycogen phosphorylase mechanism

Answer 20

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

Question 21

difference between phosphorylase a and b

Answer 21

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

Question 22

allosteric regulation of glycogen breakdown in muscle by AMP, ATP and G6P

Answer 22

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)

Question 23

why would glucose function as a negative regulator of liver phosphorylase a

Answer 23

when there is plenty of glucose > no need to breakdown liver glycogen

Question 24

how is phosphorylase kinase activated

Answer 24

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

Question 25

cascade mechanism of Epi and glucagon

Answer 25

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