Glycogen, TCA Cycle and Mitochondria (Lecture 7)

Question 1

what are the covalent modifications of a protein?

Answer 1

reversible:
when a covalent bond can be undone (i.e. kinase/phosphatase)

irreversible;
when a covalent bound is permanently destroyed (i.e. protease)

Question 2

what is the purpose of monocyclic enzyme cascades:

Answer 2

covalent modifications of the target enzyme

Question 3

purpose of bicyclic enzyme cascades:

Answer 3

covalent modification of one of the modifying enzymes ion addition to the target enzyme

the regulator is regulated by something upstream

this modulates signal transduction cascades

Question 4

name an example of a bicyclic enzyme cascade:

Answer 4

PKA phosphorylates phosphorylase to make it more active with the use fo ATP

phosphorylase (the modulator) is then seams to be active. this activated the enzyme glycogen phosphorylase A in order to active this enzyme via a second round of phosphorylation

phosphoprotein phosphatase 1 will do the inverse of PKA by inhibiting both glycogen phosphorylate and phosphorylase kinase

Question 5

regulation of glycogen metabolism involved 2 regulatory pathways:

Answer 5

allosteric control of glycogen phosphorylase and glycogen synthase

covalent modification by cascade phosphorylation

Question 6

glycogen phosphorylase

Answer 6

when active, glycogen phosphorylase will breakdown glycogen

Question 7

what are the conformations of glycogen phosphorylase and how are they controlled ?

Answer 7

there are two conformations: T(tense) and R (relaxed)

these 2 conformations are controlled by allosteric effectors

under physiological conditions, phosphorylase b is un the T state conformation (most inactive, dephosphorylated)

under physiological conditions, phosphorylase a is un the R state conformation (most active, phosphorylated)

Question 8

explain the regulation of glycogen phosphorylase?

Answer 8

the the presence of ATP (high energy) and/or high G6P (high glucose), the R state of phosphorylase b is converted into the T form (most inactive). this is done via allosteric control

the T form of phosphorylase b is then under covalent control via phosphorylase kinase (phosphorylates) with 2ATP to form a less inactive T form and then it is converted into the most active form of phosphorylase a

conversely, the most active phosphorylase a will be converted into the from in the presence fo glucose (liver).

phosphoprotein phosphatase will then dephosphorylate the R state with 2H20 into the most inactive T form

in the presence of AMP (indicator of low ATP) the T state is converted into the R form of phosphorylase b

Question 9

the proportion go active glycogen phosphorylase is determined by:

Answer 9

the rates of covalent modifications

Question 10

what is the role of phosphorylase kinase ?

Answer 10

phosphorylase kinase A will activate glycogen phosphorylase A via phosphorylation for glycogen breakdown

it also inactivated glycogen synthase via phosphorylation

Question 11

phosphorylase kinase B vs phosphorylase kinase A

Answer 11

B:
inactive, active is high [calcium]

A:
active even at low [calcium]

Question 12

what are the 4 subunits found on phosphorylase kinase

Answer 12

alpha/beta= regulatory subunits

they are phosphorylated by PKA and dephosphorylated by PP1

gamma =catalytic subunit

phosphorylates both glycogen phsophoryalse and glycogen synthase

delta: calmodulin (CaM) confers to the calcium sensitivity

Question 13

how is phosphorylase kinase modulated: (2 inputs)

Answer 13

hormonal (epinephrine) via cAMP

neural via release of calcium for muscle contraction and glycogen degradation to form glucose

Question 14

what does protein kinase A do?

Answer 14

PKA will activate phosphorylase kinase A, and PP1-inhibitor and inhibit glycogen synthase via phosphorylation

Question 15

what is PKA dependent on and why?

Answer 15

cAMP

PKA is a heterodimer

cAMP will bind to 2 regulatory subunits causing the 2 catalytic subunits to dissociate and act as kinases on target proteins

the [cAMP] will determine the fraction of PKA in the active form

(not that activation of PKA does not depend on covalent modulators, only ones of an allosteric nature)

Question 16

what are PKA’s target proteins

Answer 16

PKA will phosphorylate:

PP1 inhibitor (active)

glycogen synthase (inactive)

phosphorylase kinase (active)

Question 17

what does glycogen synthase do?

Answer 17

builds up glycogen from UDP-glucose

Question 18

when glycogen synthase is phosphorylated, its:

Answer 18

less active

Question 19

when glycogen synthase is dephosphorylated, its:

Answer 19

more active

Question 20

how is glycogen synthase covalently regulated?

Answer 20

phosphorylated (inactivated) when:

• phosphorylase kinase is active
• cAMP stimulated kinase is active
• glycogen synthase kinase (GSK3-B) is active
• PP1c is inhibited (presence of epinephrine)

dephosphorylated (active) when:

• PP1c active
• phosphorylase kinase b inhibited
• low cAMP
• GSK3-B inhibited (presence of insulin)

Question 21

how is glycogen synthase regulated allosterically?

Answer 21

glycogen synthase b ONLY

G6P facilitates dephosphorylation of glycogen synthase B via PP1c for synthesis of glycogen

increase glucose, ATP that are able top phosphorylate glucose –> G6P (fed state).

to render it active

Question 22

how does PP1c inhibit glycogen breakdown? (4)

Answer 22

dephosphorylation of glycogen phosphorylase

dephosphorylation of phosphorylase kinase A

dephosphorylation of glycogen synthase to activate glycogen synthesis

dephosphorylation of its own inhibitory peptide: PP1c -inhibitor

Question 23

when is PP1c inhibited and how it this controlled ?

Answer 23

when PP1c-inhibitor is phosphorylated

PP1c inhibitor is phosphorylated via PKA and PP1c

Question 24

describe the process off PP1c inhibitor

Answer 24

PP1c inhibitor is phsiphgoryalted via PKA, thus rendering the inhibitor active

the PP1c inhibitor + Pi will bind to PP1c to inactivate PP1c

when cAMP is low, there’s a decrease in PKA activity, thus less phosphorylation of PP1c inhibitor (now less active)

this increases the probability that PP1c will regulate itself to become active

active PP1c will dephosphorylate its inhibitor, thus rendering it inactive

active PP1c will inactivate glycogen phosphorylase A and phosphorylase kinase A

active PP1c will activate glycogen synthase

Question 25

how does glycogen bind to PP1c?

Answer 25

via an intermediate regulatory protein Gm subunit

Question 26

explain the hormonal regulation of PP1c activity in a normal to fed state

Answer 26

in a fed state, glucose levels increase and this allows for an increase in insulin secretion

the non phosphorylated glycogen-Gm-PP1c complex will be phosphorylated at position 1 via insulin stimulated protein kinase (with ATP)

this results in a more active complex and an increase in glycogen synthesis

Question 27

explain the hormonal regulation of PP1c activity in a normal to stress state

Answer 27

in conditions of stress, theres an increase in epinephrine release and an increase in cAMP

this will trigger the phosphorylation via PKA at 2 sites

this will release the PP1c catalytic subunit (dissociates due to a loss in affinity)

this results in a increase in glycogen breakdown

Question 28

explain the hormonal regulation of PP1c activity in a fed state state of stress

Answer 28

With PKA and ATP, the single phosphorylation will turn into a double phosphorylation, thus causing glycogen break down

the inverse can happen with no ATO

Question 29

Gm subunit

Answer 29

tethers the catalytic PP1 subunit to glycogen to facilitate the dephosphorylation of other proteins bound to glycogen

Question 30

insulin

Answer 30

glycogen synthesis

PP1c more active

Question 31

epinephrine

Answer 31

glycogen break down

PP1c less active