biochem lecture 3 pt 2 Flashcards
what enzyme carries out the modification/de-phosphorylation of glucose
glucose-6-phosphatase
what are phosphatases
enzymes that de-phosphorylates substrates
what is interesting about glucose-6-phosphatase
membrane associated protein that’s embedded within the membrane of ER in the cell
what is needed in order for glucose-6-phosphatase to dephosphorylate G6P
G6P has to be transported into the ER space (ER lumen) within the endoplasmic reticulum
what happens when G6P is transported to ER
phosphatase de-phosphorylates the glucose
what happens after the glucose is de-phosphorylated
glucose is transported back out of the ER lumen where it can be transported via a glucose transporter (GLUT2) into the bloodstream
what does GLUT2 do
glucose transporter, transports glucose into the bloodstream
where is glucose-6-phosphatase found
in liver, not muscles
why does it make sense that the liver has this enzyme as opposed to muscles etc.
liver needs to disseminate that glucose into the body, can’t do that without de-phosphorylating G6P
why does the muscle not need glucose-6-phosphatase
any glucose that’s released from glycogen stores in the muscle is gonna remain in the muscle cell, and used in muscle thru glycolysis
glycogen synthesis is called
glycogenesis
what is the enzyme that catalyzes the synthesis of the majority of glycogen
glycogen synthase
does glycogen synthase have the same function as glycogenin
no
what is necessary in order for glucose units to be added to the glycogen molecule
glucose has to be biochemically activated
how does the glucose get biochemically activated
process of uridylation (uridine diphosphate added to sugar –> UDP-sugar)
what is the process of uridylation
when uridine diphosphate is attached to the sugar (UDP-sugar)
describe the process of uridylation of glucose monomers
UTP has its 3rd phosphate group removed, and the remaining UDP molecule is attached to glucose
what is UDPG
uridine diphosphate glucose
what is UDP
an active donor of glucosyl units in growing polysaccharide chains
describe glycogen synthase
glycogen + UDPG –> (glycogen)n+1 + UDP
what is the point of tagging glucose w/ UDP
it’s a way of biochemically activating/targeting it for its incorporation into glycogen
what does glycogen synthase do
makes alpha-1,4 glycosidic bonds [catalyzes formation/extension of linear chains of glycogen by adding glucose units]
what does branching enzyme do
transplants a short chain, introduces a branch by forming an alpha-1,6 glycosidic bond [effectively creates the branch points within glycogen]
hexokinase
catalyzes the first step of glycolysis; helps w/ formation of glucose-6-phosphate from glucose
what are different ways to generate G6P
glycogen breakdown, glucose absorbed from diet; hexokinase forms glucose-6-phosphate from internalized glucose
where does glucose-6-phosphate go
glycolysis
what happens if you want to catalyze the synthesis of glycogen (instead of going to glycolysis)
glucose-6-phosphate needs to be isomerized into glucose-1-phosphate
what catalyzse the isomerization of G6P to G1P
phosphoglucomutase
what happens after G6P –> G1P
it creates UDP-glucose
how do you go from Glucose-1-phosphate to UDP-glucose
UDP-glucose pyrophosphorylase
what does UDP-glucose pyrophosphorylase do
uridylation of glucose to form UDP glucose (transfers phosphate from UTP, attaches UDP to glucose)
what’s up with UDP glucose
this is the form of glucose that’s going to be attached to glycogen and incorporated into glycogen’s structure
pyrophosphatase
removes or utilizse the pryophosphate generated in step 3 to generate free energy (useful for synthesis reactions)
how are these two opposing pathways regulated/controlled
hormones
insulin
hormone that lowers blood glucose levels
what does insulin inhibit
glycogen breakdown and gluconeogenesis
what does insulin promote
glycogen synthesis (to use up glucose in formation of glycogen to lower levels)
what are epinephrine and glucagon
hormones that raise blood glucose levels
what do epinephrine /glucagon stimulate/promote
glycogen breakdown and gluconeogenesis
what do epinephrine/glucagon inhibit
glycogen synthesis (to prevent glucose from being used up to make glycogen which will lower blood levels even more… we don’t want that)
what stimulates release of insulin
when you’ve ingested a carb rich meal
where does insulin come from/ is released from
pancreas
what is insulin involved in
major hormone involved in glucose uptake and storage
what does insulin do
stimulates the process that inhibits breakdown of glycogen, activates glycogen synthesis
what does insulin need to do in order to inhibit glycogen breakdown and activate glycogen synthesis
needs to be able to stimulate uptake of glucose into liver cells, muscle cells, etc.; and then stimulate process of glycogen synthesis
what is gluconeogenesis
synthesis of glucose from non-carbohydrate precursors (like lactate and pyruvate which provide carbon skeletons necessary for glucose synthesis)
what do epinephrine and glucagon do
stimulate breakdown of glycogen and stimulate gluconeogenesis
when does epinephrine/adrenaline work
fight or flight response; when you do this muscle needs energy to carry out a lot of muscle activity
where does that energy come from
need for available glucose is increased, comes in form of glycogenolysis stimulating glycogen breakdown
when does glucagon work
short-term starvation conditions
where does glucagon work
stimulates glycogen breakdown in liver
what is in liver
majro, predominant stores of glycogen
what does glucagon do
makes glucose available to brain and tissues under short-term starvation conditions
where does glucagon primarily work
in liver
what does converting G6P to glucose do
contribute to elevation of blood glucose
is there release of glucose into bloodstream in the liver?
no
what does liver have
glucose-6-phosphate which allows for release of glucose from liver (by converting G6P to glucose)
what does glycogenolyis generate
glucose-6-phosphate, gonna be used in glycolysis
what is there a lowering of in liver during glycogenolysis
lowering of glycolytic activity (glycolysis); b/c you don’t wanna use up glucose that’s released from glycogen; you wanna make it available to other body parts
what does liver favor
lowering of glycolysis
what does muscle favor
increase in glycolytic activity, b/c muscle will be metabolically active
what stimulates release of insulin from pancreas
ingesting carb-rich meal, hella glucose present in bloodstream (when you break down food, lots of carbs being released, many carbs are gonna be in form of glucose)
how do hormones work
binding of hormone to its receptor triggers activation of specific signal transduction pathway
what is the overall role of insulin
glucose uptake and storage; favors glycogen synthesis and inhibits glycogen breakdown
what does insulin do in the muscle (and liver???)
increases glycolysis
why does it make sense that insulin stimulates glycolysis
b/c if you’re gonna promote glycogen synthesis, you will need ATP to drive that synthesis/anabolic process
why are you stimulating glycolysis as opposed to deriving energy from breakdown of glycogen
we have hella glucose around, some can be utilized for glycolysis (to make ATP to drive synthesis), but majority is used for glycogen synthesis
so what two things is glucose being used
glycolysis AND glycogen synthesis
what does insulin promote and inhibit
promotes glycogen synthesis AND inhibits glycogen breakdown
how does insulin activate glycogen synthesis
insulin binds to its receptor, stimulates a kinase called PKB (upregulates it)
what does PKB do
kinase, inhibits GSK3 (another kinase)
what does GSK3 do when active
inhibits glycogen synthase
what does PKB inhibiting GSK3 do
activates glycogen synthase
what is glycogen synthase
enzyme that promotes/carries out glycogen synthesis
what does inhibiting the inhibitor of glycogen synthase (GSK3) do
elevates glycogen synthase activity, and you get glycogen synthesis
how does insulin inhibit glycogen breakdown?
insulin stimulates insulin sensitive protein kinase
what does insulin sensitive protein kinase do
activates PP1 (protein phosphatase 1)
what does PP1 do
de-phosphorylates a bunch of stuff
what does blocking activity of GSK-3 while simultaneously activating PP1 do
de-phosphorylates any residues on glycogen synthase (anything that was phosphorylated earlier by GSK3, those phosphate groups are removed) –> elevates activity of synthase
what does combined effects of inhibiting gGSK-3 while promoting de-phosphorylation of glycogen synthase (PP1) do
increases/elevates activity of synthase (remove any phosphate groups attached to AA residues on glycogen synthase polypeptide)
what else does PP1 dephosphorylate
glycogen phosphorylase and phosphorylase kinase, decreasing activity of both
does phosphorylating and de-phosphorylating always have the same outcome
no, depends on the specific protein
what does phosphorylating/de-phosphorylating do to glycogen synthase
phosphorylating it inhibits it, de-phosphorylating it activates it
what does phosphorylating/de-phosphorylating do to glycogen phosphorylase and phosphorylase kinase
phosphorylating it activates it, de-phosphorylating it inhibits it
what is net effect of PP1 inhibiting the kinase that would normally activate the phosphorylase AND de-phosphorylating the glycogen phosphorylase
reducing activity of both enzymes, inhibiting glycogen breakdown
what do you need to do when you have low blood glucose
stimulate breakdown of glycogen while also inhibiting glycogen synthesis
what does releasing epinephrine/glucagon do
activates adenylyl cyclase which produces cAMP
what is cAMP
allosteric effector
what does cAMp activate
PKA (protein kinase A)
what does PKA do
phosphorylates glycogen synthase and glycogen phosphorylase kinase
what does phosphorylating glycogen synthase do
inhibits it
what does PKA activation and activation of GSK3 (cuz its not being inhibited) do
inhibits glycogen synthase (phosphorylating glycogen synthase inhibits it)
what else does PKA do in addition to phosphorylating glycogen phosphorylase kinase
phosphorylates the glycogen phosphorylase kinase
what does activating phosphorylase kinase do
phosphorylates and activates phosphorylase
what does activating phosphorylase do
favors activation of glycogen breakdown
describe what happens in low blood glucose conditions
glucagon activates enzymes necessary to promote glycogen breakdown, and inhibits enzyme glycogen synthase
what does hormonal control involve
activation of signal transduction pathways
what do epinephrine and glucagon directly do
activate adenylyl cyclase
how do they activate adenylyl cyclase
through G protein system
what does adenylyl or adenylate cyclase do
takes ATP, lops off pyrophosphate, cyclizes remaining AMP
what is cAMP a major allosteric regulator of
PKA
what is another name of PKA
cyclic AMP dependent protein kinase A
what does PKA do
phosphorylates and activates phosphorylase kinase
what does activated phosphorylase kinase do
phosphorylates and activates glycogen phosphorylase
what does activation of glycogen phosphorylase result in
breakdown of glycogen
is insulin a G protein linked receptor
no
what is cAMP
allosteric regulator
where does cAMP bind to
regulatory site on PKA to bring about changes and increased activity of PKA
what does adenylyl cyclase do
generates cAMP from ATP, PKA, serial phosphorylation or kinase cascade
what does pKA do
PKA phosphorylates and activates the phosphorylase kinase, that being active now will phosphorylate and activate the glycogen phosphorylase, this is where you stimulate the breakdown of glycogen
dose glucose have different fates in muscle vs liver?
yes
what happens to glucose in muscle
glucose-1-phosphate monomers will be converted to glucose 6 phosphate –> glycolysis
what does glycolysis help muscle do
muscle contraction, metabolic activity, increased muscle activity
what happens to glucose in liver
glucose-1-phosphate –> glucose-6-phosphate, de-phosphorylated and released into blood
covalent modification
phosphorylation/de-phosphorylation
allosteric regulation
cAMP
what kind of regulation is allosteric regulation
post-translational
how does reciprocal control work
covalent modification AND allosteric regulation
is phosphorylation reversible
yeah
examples of allosteric effectors of glycogen phosphorylase
too much ATP or AMP etc.
what does abundant ATP , high [ ] of G6P do
inhibits phosphorylase
why does abundant ATP have effect of inhibiting
lot of glucose available to cell, won’t need more glycogen breakdown
what does high AMP concentration indicate
hella ATP utilized, so less ATP; needs glycogen breakdown, so activates phosphorylase
how does cell make decisions
based on relative concentration of allosteric effectors
example of covalent modification of glycogen phosphorylase
serine residues on phosphorylase can be phosphorylated and de-phosphorylated
what does de-phosphorylation favor
lowered activity
what does phosphorylation favor
increased activity
what regulates phosphorylase activity
both covalent and noncovalent control
what is the active form of phosphorylase
a dimer
T state
tense state; inactive form of enzyme
R state
relaxed state, active form
what do both covalent (phosphorylation) and non-covalent (allosteric) effects do
contribute to overall inactivity/activity of glycogen phosphorylase or synthase
how is glycogen synthase covalently modified
by phosphorylating a serine residue by protein kinase A
describe phosphorylated form of glycogen synthase
inactive
describe phosphorylated form of phosphorylase
active
is there reciprocal control of phosphorylase and glycogen synthase by covalent modification?
yes, because phosphorylation activates one and inactivates another [2 diametrically opposing effects of phosphorylation]
what is another component of this system
glycogen targeting protein or GM
what is GM
scaffolding or recruiter for various other players important for glycogen metabolism (phosphorylase, synthase etc.)
how many sites are on GM
2 sites; GM site 1 and GM site 2
do both sides have the same effect
no, depending on which site is phosphorylated it has different outcomes
what does insulin do w/r to GM
stimulates phosphorylation of GM site 1, activates PP1 which de-phosphorylates glycogen phosphorylase kinase, glycogen phosphorylase, and glycogen synthase
what does epinephrine do w/r to GM
stimulates phosphorylation of GM site 2, causes PP1 to dissociate from glycogen particle, prevents PP1 access to glycogen phosphorylase and glycogen synthase (this activates PKA)
whaat activates PKA
epinephrine
what does PKA do
phosphorylates an inhibitor protein of PP1
net effect of epinephrine
breakdown of glycogen
what does insulin pathway do
inhibits glycogen synthase kinase 3
what does GSK3 do
phosphorylates and inactivates glycogen synthase
what does insulin do
stimulates activity of PKB
what does PKB do
phosphorylates GSK3, inhibiting it
what happens if you inhibit GSK3
prevents phosphorylation of glycogen synthase and simultaneously activates PP1
net effect of inhibiting GSK3
de-phosphorylation of glycogen synthase, means you’re gonna activate it
effect of insulin
promoting glycogen synthase activation overall
effect of insulin mediated inhibition of GSK3 and activation of PP1
favors activation of glycogen synthase
what do insulin, G6P and glucose all do
favor activation of PP1
what do glucagon/epinephrine do
favor inhibition of PP1 (through PKA pathway)
what does insulin do to GSK3
inhibits it
what does insulin do
stimulates glucose uptake, hexokinase activity, glycogen synthesis
is there G6P in muscle
no
what happens to glucose in muscle
G1P monomers or glucose absorbed into muscle from bloodstream is gonna be converted to G6P where it’s trapped and stays in muscle
what happens if muscle needs a lot of ATP during strenuous activity
G6P is diverted to glycolysis, used for ATP production
what happens when you have conditions that favor glycogen synthesis and storage in the muscle
G6P isomerized to G1P, converted to UDP glucose, used by glycogen synthase to produce more glucose
what is the whole idea
glucose that enters muscle cell or released from glycogen in muscle cell stays in muscle
is there release of glucose from muscle
no
