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Biochem Muscle Weakness Glycogen


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descriptive name for glycogen
structure of glycogen (bonds)
branched chain homopolysaccharide made from alpha-D-gluc

alpha 1,4 glycosidic linkages
alpha 1,6 about every 8-10 glucose units
where is glycogen stored
cytoplasm in large hydrated granules, each containing ~100,000 gluc units
primarily in liver and muscles
is more glycogen stored in muscle or liver
total glycogen stored in muscle (because there is so much more muscle) is three times that of liver, even though liver has higher concentrations of glycogen
why not store glucose in fat? or as free glucose?
cannot convrt fat back to glucose, only glucose to fat

free glucose is osmotically active so need glycogen
how does the function of glycogen differ in liver and muscle?
liver maintains blod glu
muscles supply fuel reserve
where is glycogen utilized
75% brain, the rest is erythrocytes skeletal and heart muscles
how long does the glycogen supply last in the liver
less than 24 hours
what is gluconeogenesis and why would you need glycogen if you have this pathway?
- the synthesis of glu from small molecules like aa (happens in liver)
is much SLOWER, need fast release hence glycogen
muscles and glycogen, why doesnt glucose diffuse out of muscles when it is made from muscle glycogen?
muscles lack Glucose-6-phosphatase and therefore glucose found in muscles is in the G-6-P form that doesnt diffuse out of cells.
Four broad steps of glycogen synthesis
synthesis if UDP glucose
adding of primer
explain biochemical process of glycogen synthesis
glucose is taken up into the cell and phosphorylated by hexokinase (glucokinase in liver)

G-6-P is then mutated to G-1-P via phosphoglucomutase

G-1-P is converted to UDP-gluc using UTP
via uridylyltransferase or UDPGLc pyrophosphorylase
explain biochemical process of glycogen synthesis continued
use specific aa sites
UDP-glucose is attached to glycogenin at OH of tyr 194 via autoglucosylation

glycogenin has a glucosyltransferase activity (note: udp is kicked out in the reaction)
explain biochemical process of glycogen synthesis continued
new glycosidic bond formed between incoming(activated) C1 OH and the accepting C4
via glycogen synthase
adds three at a time
alpha 1,4 glycosidic linkage
what is special about glycogen synthase?
it is rate limiting and regulated
is glycogen synthase's Km for small glycogen molecules large or small
it's large(ie low affinity)
glycogen branching
glucosyl 4:6 transferase
how does glucosyl 4:6 transferase work
transfers 6-8 gluc residues from end of chain to another residue forming an alpha 1,6 bond

hence its 4:6 b/c it breaks an alpha 1,4 bond and makes an alpha 1,6 bond
is glucose added to the reducing or nonreducing end of a chain?
non reducing
why branch?
increases glycogen solubility
increases nonreducing ends to which glucosyl residues can be added
describe glycogenolysis
glycogen phosphorylase uses Pi to cleave alpha 1,4, requires PLP (B6 derivative)as coenz

debranching enzyme: 4-alpha-D-glucotransferase, removes outer 3 of limit dextrin and attaches to a nonreducing end, amylo-alpha1,6,glucosidase cleaves last glucose
decribe the debranching enzyme and the two catalytic sites (bifunctional)
debranching enzyme: 4-alpha-D-glucotransferase, removes outer 3 of limit dextrin and attaches to a nonreducing end, amylo-alpha1,6,glucosidase cleaves last glucose usig water
whats the ratio of glucoses released from phosphorylase to debranching enz
how else can you degrade glycogen?
lysosomal glycogen degradation 1-3%
glycogen phosphorylase is active or inactive when phosphorylated
active when phosphorylated

a form active
b form inactive
what activates glycogen phosphorylase
what inactivates it
activated via phosphorylase kinase using ATP

inactivated using phosphoprotein phosphatase
REGULATION of phosphorylase kinase
what regulates the regulator
phosphorylated is active

phos'd by protein kinase A

dephos'd by phosphoprotein phosphatase
glycogen phosphorylase chemical characteristics
T and R states,
a is active b/c P shifts equilibrium to R state
how is PKA . . released from regulatory sites
H activates Gprotein receptor causes formation of cAMP
cAMP binds regulatory sites on PKA and releases catalytic portion
what does PKA inhibit and how
phosphoprotein phostphatase

it P's protein phosphatase inhibitor activating it so that protein phosphatase cant change things back into their inactive forms
PKA activates phosphorylase kinase which activates glycogen phosphatase releasing glucose
PKA inhibits phosphoprotein phosphatase by activating it's inhibitor(protein phosphatase inhibitor) so that

glycogen phosphorylase isnt turned off
summary of cAMP activation
inhibits synyhesis (glycogenesis)
increases catabolism (glycogenolysis)
activates glycogen phosphorylase kinase and therefore glycogen phosphorylase

inactivates glycogen synthase (because glycogen phosphorylase kinase is activated)

activates protein phosphatase inhibitor
is glycogen synthase active or inactive when p'd
synthesis vs catabolism and phosphorylation
synthesis is inactive when p'd

catab is active when p'd
Insulin and glycogen synthesis
inhibition of protein kinases,
activation of phosphoprotein phosphatase

glycogen synthase is de P'd (active)
phosphorylase kinase and phosphorylase are de P'd (inactive)
what are allosteric effectors?
bind enz regulatory site to promote R or T state
what is the effector of phophorylase kinase b (catab)
Ca, will activate this even though it's dephosed

binds delta subunit, conf change activates gamma
what is the effector of phosphorylase b
AMP, activates it, in muscles
effector of phosphorylase a
glucose, inhibits it , only hepatic
effector of synthase b
G-6-P, activates, in both liver and muscle
how does Ca activate glycogenolysis in liver?
Ca increased in liver due to epi binding alpha-adrenergic receptors

this activates phospholipase C generating IP3 and DAG
IP3 causes Ca release in cytosol which activates glycogenolysis
AMP to ATP ratio, if increased reflects what?
low energy
AMP is an allosteric effector of?
phosphorylase b, activates
high amp to atp ratio means what
activation of catabolism

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