1. Regulation of glycogen synthesis and breakdown
Ferchmin 2017
Regulation of glycogen synthesis and breakdown
Integration of lipid and carbohydrate metabolism
Slides content
1-3 Generalities
4-6 Regulation of glycogen synthase, phosphorylase and phosphorylase b kinase
7-9 Stepwise integration of content in slides 9 and 13.
10-12 Role of insulin
14 Slides beyond 13 link glycogen metabolism with glycolysis, PPP, phosphofructokinase 2, lipid synthesis, and gene regulation.
The “strategy” used here is to present a complex biochemical process in chunks. In the next steps, the chunks are joined and additional complexity is added. So, study each step first and then add the additional complexity. My ambitious purpose is that you learn a lot by these reiterative steps and that only moderate practice will be needed for the exam.
Your comments, suggestions and venomous critiques are warmly welcomed.

2. I was informed that you are knowledgeable I cell signaling, second
messengers, receptors, and, protein kinases and phosphatases.
Based in my faith in you, I will go straight into the subject matter.
Actually, this is not too complicated. In addition, I will be available
at my and/or at my office to clarify doubts.
Unfortunately, some very intelligent fellows from outside of the UCC
insisted in decreasing the hours of lecture. As a consequence, you
did not have a lecture about sugar structure.. So, you better review
glycogen structure as soon as possible.
The granule of glycogen contains all the enzymes necessary for the synthesis and breakdown, and regulation of these processes. However the granule is not a multi-enzymatic complex like the pyruvate dehydrogenase complex (PDC). This is because the enzymes are not in fixed proportion nor are linked like in the PDC.
The next slide is to show you a sketch to indicate regulation by phosphorylation

3. The sketch illustrates a general situation where a protein kinase and a protein phosphatase regulate the concentrations of the non-phosphorylated and phosphorylated forms of a substrate
In these lectures, this sketch will be used three times as it is shown here. Moreover, it will be utilized in more complex setting several times. Therefore, it would be time-effective if you understand it now. Admittedly, there are other ways to present the same idea. However, I prefer this one.
Perhaps the following will help you to remember. Glycogen metabolism is part of the normal
homeostatic regulation of glycemia and also of the fight-or-flight response.
Hypoglycemia or expected need of more glucose requires glycogen breakdown achievable
by phosphorylation of the enzymes involved in glycogen metabolism. On the contrary,
satiety will initiate glycogen synthesis which is accomplished by dephosphorylation of
the same enzymes.
Colloquially speaking when you are chased by a hairy monster you phosphorylate your
glycogen enzymes however after a large meal and a peaceful nap you dephosphorylate
these enzymes.

4. How many enzymes are shown in this sketch?
To start with we will consider three enzymes of glycogen metabolism:
glycogen synthase, glycogen phosphorylase and glycogen phosphorylase b
kinase
Let me introduce here the
idea of the “monster”
that activates adenylate
cyclase (adenylyl cyclase)
and antagonizes
fructose-2,6-di-phosphate.
This cartoon shows the
phosphorylation and
dephosphorylation of
glycogen synthase
How many enzymes are shown in this sketch?
The inactive glycogen synthase becomes active in the presence of
glucose-6P
Glycogen synthase can be D, or dependent on glucose-6-P, or I, independent of the presence
of glucose -6-P. Please, remember that the immediate precursor of glycogen is UDP-glucose
not glucose-6-P. The latter is only an allosteric regulator of glycogen synthase.

5. b stands for inactive
Phosphorylase a, was the first enzyme ever discovered to be activated by phosphorylation. So, it was dubbed a for (active)

6. This enzyme is regulatory. Does not affect “real”
metabolites.
On top of it its is “misnamed”.
See below.
Glycogen phosphorylase b kinase also phosphorylates the synthase and should actually be called synthase phosphorylase b kinase. The phosphorylation of the alpha subunit regulates the dephosphorylation of the beta subunit. The delta subunit is calmodulin. The direct interaction of Ca2+ with calmodulin activates this enzyme. This effect is specially relevant in muscle.

7. Integration of the regulation of glycogen synthesis and breakdown
Glucagon
Epinephrine
Receptors
Adenylyl Cyclase
cAMP
ATP
AMP
Phosphodiesterase
R2C2
R2(cAMP)4 2C
ACTIVE PKA
Glycogen synthase
Phosphorylase b
Kinase
Slides 3 to 8 conceptually merged in slide 9.
Slides 10, 11 and 12 introduce the role of
insulin. All that is integrated in slide 13.
If you study sequentially the slides (3 to 13)
you will be able (hopefully) to understand the
regulation of glycogen metabolism. In slides
15 and 16 comes the role of glycolysis and
PPP in lipogenesis. The carbohydrate, lipids, and
obesity are linked in slides 17 to 21.
revise the slide #
Glycogen Phosphorylase
activation
inhibition

8. Integration of the regulation of glycogen synthesis and breakdown
Glucagon
Epinephrine
Receptors
Adenylyl Cyclase
cAMP
ATP
AMP
Phosphodiesterase
R2C2
R2(cAMP)4 2C
ACTIVE PKA
Glycogen synthase
Phosphorylase b
Kinase
Glycogen Phosphorylase
Protein Phosphatase
activation
Protein Phosphatase Inhibitor
inhibition

9. Integration of the regulation of glycogen synthesis and breakdown
Glucagon
Epinephrine
Receptors
Adenylyl Cyclase
Obviously, insulin has a role in this metabolism.
See next slides.
cAMP
ATP
AMP
Phosphodiesterase
R2C2
R2(cAMP)4
Ca2+ 2C
ACTIVE PKA
Glycogen synthase
Phosphorylase b
Kinase
Glycogen Phosphorylase
Protein Phosphatase
activation
Protein Phosphatase Inhibitor
inhibition

10. Pancreatic β-cells and hepatocytes
have the same glucokinase
and GluT2
transporter.
Why?

11. Insulin activates glycogen synthesis
You must know the
mechanism of insulin
receptor. It is by Tyr
phophorylation on the
receptor and on its
substrates

12. Insulin stimulates glucose transport in muscle and adipose cells by stimulating translocation of glucose transporter 4 (GLUT4) to the plasma membrane.

13. or inhibition of inhibition is as good as activation
Integration of the regulation of glycogen synthesis and breakdown
Glucagon
Epinephrine
Receptors
Heart, muscle and other
Insulin-R
PI3-K
PKB/Akt
GSK-3
Adenylyl Cyclase
Remember that
(-1) x (-1)=+1
or inhibition of inhibition is as good as activation
cAMP
ATP
AMP
Phosphodiesterase
R2C2
R2(cAMP)4
Ca2+ 2C
ACTIVE PKA
Glycogen synthase
Phosphorylase b
Kinase
Glycogen Phosphorylase
Protein Phosphatase
activation
Protein Phosphatase Inhibitor
inhibition
Akt inhibits glycogen synthase kinase 3 (GSK-3) which then stops inhibiting the glycogen synthase

14. We will leave glycogen metabolism and return to glycolysis,
fructose-2,6-bisphosphate, xylulose-5-P, and expand the regulatory
big picture

15. Take home message.
When you are chased by
a monster your liver
won’t synthesize
glycogen.

16. 2) Nonoxidative steps of pentose phosphate shunt
Misplaced slide?
No, it is integration
of glucose metabolism
with lipid synthesis
2) Nonoxidative steps of pentose phosphate shunt
Xylulose-5P the pentose that makes you FAT
Xylulose-5P the pentose that makes you FAT

17. Phosphofructokinase-2 Fructose-2,6-bis-phosphatase
Regulation of fructose-2,6-bis phosphate synthesis and break down
HEXOSES
Fructose-6-P
Fructose-2,6-bis-phosphate
ATP
ADP
Lipogenesis
by activation of
hepatic glycolysis
PKA
ATP
Phosphofructokinase-2
PP2A Pi
cAMP
Bifunctional enzyme.
Phosphorylated is phosphatase
Dephosphorylated is kinase
PKA is just that, protein kinase A
Fructose-2,6-bis-phosphatase
Fructose-2,6-P
Fructose-6-P
H2O Pi
xylulose-5-P comes from PPP when there is plenty of glucose
PP2A is protein phosphatase 2A

18. This “repeated” slide was placed for you to practice the metabolism of fructose-2,6-P

19. Role of 2,6-Fructose bisphosphate and phosphofructokinases-1 and -2
Phosphofructokinase-1 is the well-known glycolytic enzyme; phosphofructokinase-2 is exclusively a regulatory enzyme. PFK-2 has kinase activity when nonphosphorylated and catalyzes the phosphorylation of fructose-6-P in carbon 2.
When phosphorylated, phosphofructokinase-2 has phosphatase activity and catalyzes the dephosphorylation of carbon 2 of 2,6-FDP.
The role of 2,6-FDP is to "convey" to the liver cells that there is plenty of hexoses and that glycolysis has to be activated to support fatty acid synthesis. At the same time, gluconeogenesis has to be inhibited. In liver glycolysis is inhibited by cAMP, which indirectly inhibits phosphofructokinase-1 (by lowering 2,6-FDP and activating 1,6-FDP phosphatase) and liver pyruvate kinase (as seen above). In muscle, cAMP activates glycolysis by activation of glycogenolysis. This difference in regulation reflects the different roles of glycolysis in both organs.
You should recall from the regulation of glycolysis that 2,6-FDP activates the hepatic PFK-1 by removing the inhibitory effect of ATP. Also 2,6-FDP enhances the action of PFK-1 by inhibition the 1,6- FDP phosphatase.
In cardiac muscle phosphofructokinase-2 is a different isozyme than in liver and the phosphorylated muscle phosphofructokinase-2 is active allowing cyclic-AMP to activate glycolysis. It is unlikely that the you will find this in the NBE.

20. Abbreviated glycolysis, gluconeogenesis, and pentose shunt pathways and roles of Xu5P and Fru-2,6-P2 in activation of PP2A and PFK, respectively.
Abbreviated glycolysis, gluconeogenesis, and pentose shunt pathways and roles of Xu5P and Fru-2,6-P2 in activation of PP2A and PFK, respectively. The scheme illustrates the formation of Xu5P from fructose 6-phosphate (F6P) and glyceraldehyde 3-phosphate (GAP) by transketolase, which activates PP2A. Xu5P PP2A then activates 6-phosphofructo-2-kinase/Fru-2,6-P2 phosphatase (PF2K/Pase) to increase Fru-2,6-P2, resulting in activation of PFK. The same PP2A also activates ChREBP in the activation of lipogenic gene transcription, including ATP citrate lyase (ACL), acetyl-CoA carboxylase (ACC), and fatty acid synthase (FAS). Glu, glucose; HMP, hexose monophosphate; TCA, tricarboxylic acid cycle.
Kabashima T et al. PNAS 2003;100:
©2003 by The National Academy of Sciences

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