Transcript Lecture 33 - University of Arizona

Carbohydrate Metabolism 2:
Glycogen degradation, glycogen synthesis,
reciprocal regulation of glycogen metabolism
Bioc 460 Spring 2008 - Lecture 34 (Miesfeld)
Carbohydrates in pasta are
a good way to replenish
muscle glycogen stores
Glycogen phosphorylase
enzyme is a dimer that is
regulated by both
phosphorylation and allostery
Gerty Cori won the 1947
Nobel Prize for her work on
glycogen metabolism
Key Concepts in Glycogen Metabolism
• Glycogen is a highly-branched polymer of glucose that can be quickly
degraded to yield glucose-1P which is isomerized to glucose-6P.
• Glycogen phosphorylase removes one glucose at a time from the
nonreducing ends using inorganic phosphate (Pi).
• Glycogen synthase adds glucose residues to nonreducing ends in a
reaction involving UDP-glucose; the cost of glycogen synthesis is 1
ATP/glucose residue.
• Net phosphorylation leads to glycogen degradation, whereas, net
dephosphoryation, results in glycogen synthesis.
Overview of Glycogen Metabolism
The storage form of glucose in most eukaryotic cells
(except plants) is glycogen, a large highly branched
polysaccharide consisting of glucose units joined by 1,4 and -1,6 glycosidic bonds.
The large number of branch points in
glycogen results in the generation of
multiple nonreducing ends that provide a
highly efficient mechanism to quickly
release and store glucose.
The reducing and nonreducing ends of glycogen
The nonreducing
end of glycogen
molecules refers to
the carbon that is
opposite to the
reducing end in the
ring structure. The
reducing end of a
linear glucose
molecule can be
oxidized by Cu2+ by
definition.
Nonreducing end
Reducing end
Nonreducing end
……… Reducing end
Glycogen Core Complexes
Glycogen core complexes consist of glycogenin protein and
~50,000 glucose molecules with α-1,6 branches about every 10
residues creating ~2,000 nonreducing ends. Glycogen is stored
primarily in liver and skeletal muscle cells.
Pathway Questions
Liver glycogen is used as a short term
energy source for the organism by
providing a means to store and release
glucose in response to blood glucose
levels; liver cells do not use this
glucose for their own energy needs.
Muscle glycogen provides a readily
available source of glucose during
exercise to support anaerobic and
aerobic energy conversion pathways
within muscle cells; muscle cells lack
the enzyme glucose-6-phosphatase
and therefore cannot release glucose
into the blood.
Pathway Questions
2. What are the net reactions of glycogen degradation and synthesis?
Glycogen Degradation:
Glycogenn units of glucose + Pi →
Glycogenn-1 units of glucose + glucose-6-phosphate
Glycogen Synthesis:
Glycogenn units of glucose + glucose-6-phosphate + ATP + H2O →
Glycogenn+1 units of glucose + ADP + 2Pi
Pathway Questions
3. What are the key enzymes in glycogen metabolism?
Glycogen phosphorylase – enzyme catalyzing the phosphorylysis reaction that
uses Pi to remove one glucose at a time from nonreducing ends of glycogen
resulting in the formation of glucose-1P..
Glycogen synthase - enzyme catalyzing the addition of glucose residues to
nonreducing ends of glycogen using UDP-glucose as the glucose donor.
Branching and debranching enzymes - these two enzymes are responsible for
adding (branching) and removing (debranching) glucose residues.
Pathway Questions
4. What are examples of glycogen metabolism in real life?
The performance of elite endurance athletes can benefit from a diet
regimen of carbohydrate "loading" prior to competition.
Key is to deplete glycogen before carbo loading to get 2x glycogen level.
Function of Glycogen Phosphorylase
Glycogen degradation is initiated by glycogen phosphorylase, a
homodimer that catalyzes a phosphorolysis cleavage reaction of the α1,4 glycosidic bond at the nonreducing ends of the glycogen molecule.
Inorganic phosphate (Pi) attacks the glycosidic oxygen using an acid
catalysis mechanism that releases glucose-1P as the product.
Although the standard free energy change for this phosphorylysis
reaction is positive (ΔGº' = +3.1 kJ/mol), making the reaction
unfavorable, the actual change in free energy is favorable (ΔG' = -6
kJ/mol) due to the high concentration of Pi relative to glucose-1P inside
the cell (ratio of close to 100).
Structure of Glycogen Phosphorylase
Exists as a dimer and has binding sites for glycogen and catalytic
sites that contain pyridoxal phosphate (derived from vitamin B6). The
critical Pi substrate is bound to the active site by interactions with
pyridoxal phosphate and active site amino acids.
Function of Phosphoglucomutase
The the next reaction in the glycogen degradation pathway is the
conversion of glucose-1P to glucose-6P by the enzyme
phosphoglucomutase.
Where have you seen this type of reaction before (a mutase rxn)?
Glycogen Debranching Enzyme
The glycogen debranching
enzyme (also called α-1,6glucosidase) recognizes the
partially degraded branch
structure and remodels the
substrate in a two step reaction.
Since α-1,6 branch points occur
about once every 10 glucose
residues in glycogen, complete
degradation releases ~90%
glucose-1P and 10% glucose
molecules.
Is there a difference in the amount
of energy that can be recovered
from glucose-1P and glucose?
Regulation of Glycogen Phosphorylase Activity
Activity is regulated by both covalent modification (phosphorylation)
and by allosteric control (energy charge).
Glycogen phosphorylase is found in cells in two conformations:
• active conformation, R form
• inactive conformation, T form
Phosphorylation of serine 14 (Ser 14) shifts the equilibrium in favor of
the active R state.
This phosphorylated form of glycogen phosphorylase is called
phosphorylase a (active), and the unphosphorylated form is called
phosphorylase b. It is the same polypeptide, just a different name.
Regulation of Glycogen Phosphorylase Activity
The enzyme responsible for phosphorylating glycogen phosphorylase b
to activate it, is phosphorylase kinase which is a downstream target of
glucagon and epinephrine signaling, as well as, insulin signaling.
Tissue-specific isozymes of glycogen phosphorylase
The activity of glycogen phosphorylase
can also be controlled by allosteric
regulators, which binds to the T state
and shifts the equilibrium to the R state.
Liver and muscle isozymes of
glycogen phosphorylase are
allosterically-regulated in different
ways, which reflects the unique
functions glycogen in these two tissues.
Tissue-specific isozymes of glycogen phosphorylase
Liver glycogen phosphorylase a, but
not muscle glycogen phosphorylase a
is subject to allosteric inhibition by
glucose binding which shifts the
equilibrium from the R to T state.
When liver glycogen phosphorylase a
(phosphorylated form) is shifted to the
T state, it is a better substrate for
dephosphorylation by PP-1 than is the
R state.
Why does it make sense that muscle
glycogen phosphorylase b, but not
liver glycogen phosphorylase b,
would be allosterically activated by
AMP in the absence of hormone
signaling?
Hint: what does the liver do with the
glucose-6P that is produced?
Glycogen synthase catalyzes glycogen synthesis
The addition of glucose units to the nonreducing ends of glycogen by
the enzyme glycogen synthase requires the synthesis of an
activated form of glucose called uridine diphosphate glucose (UDPglucose).
The rapid hydrolysis of PPi by the abundant cellular enzyme
pyrophosphatase results in a highly favorable coupled reaction.
Why does rapid conversion of PPi --> 2 Pi result in a more favorable reaction?
Glycogen Synthase Reaction
Glycogen synthase transfers the glucose unit of UDP-glucose to the C-4
carbon of the terminal glucose at the nonreducing end of a glycogen chain.
The UDP moiety is released and UTP is regenerated in a reaction involving
ATP and the enzyme nucleoside diphosphate kinase.
Glycogen Branching Enzyme
Once the chain reaches a length of 11 glucose residues, the
glycogen branching enzyme transfers seven glucose units from the
end of the chain to an internal position using a α-1,6 branchpoint.
Growing Glycogen Tree - Starting with Glycogenin
Protein
Regulation of Glycogen Synthase Activity
The activity of glycogen synthase is also primarily controlled by
reversible phosphorylation.
Dephosphorylation activates glycogen synthase, whereas, glycogen
phosphorylase is activated by phosphorylation.
In this case, the active glycogen synthase a (active) form is
dephosphorylated and favors the R state, whereas, the inactive
glycogen synthase b form is phosphorylated and favors the T state.
The “a” form is always the active form; glycogen phosphorylase “a” is
phosphorylated, whereas, glycogen synthase “a” is dephosphorylated.
Regulation of Glycogen Synthase Activity
Hormone activation of glycogen synthase activity is mediated by insulin, which
promotes the activation of glycogen synthase by stimulating PP-1 activity.
Epinephrine and glucagon signaling leads to inactivation of glycogen synthase.
Reciprocal regulation of glycogen metabolism
Since glycogen phosphorylase and glycogen synthase have
opposing effects on glycogen metabolism, it is critical that their activities
be reciprocally regulated to avoid futile cycling and to efficiently control
glucose-6P concentrations within the cell.
What is the metabolic
logic of glucose inhibition
of glycogen
phosphorylase activity
and activation of
glycogen synthase?
Hormone signaling in
liver cells
Net phosphorylation
drives glycogen
degradation, and net
dephosphorylation
drives glycogen
synthesis.
Glucagon signaling
cAMP triggers two
types of
phosphorylation
circuits in muscle
cells; one that
stimulates glycogen
degradation and a
second that inhibits
glycogen synthesis.
Insulin signaling
Insulin signaling
results in
dephosphorylation
of glycogen
metabolizing
enzymes and
elevated rates of
glycogen synthesis.
Human glycogen storage diseases
A number of human
diseases have been
identified that affect
glycogen metabolism.
Disease symptoms in
many cases include:
liver dysfunction due to
excess glycogen fastinginduced hypoglycemia
(low blood glucose
levels) in the most
severe diseases, death
at an early age.
Human glycogen storage diseases
von Gierke's disease is due to a deficiency in the enzyme glucose-6phosphatase which causes a build-up of glycogen in the liver because glucose6P accumulates and activates glycogen synthase. McArdle's disease harbor
defects in muscle glycogen phosphorylase. These individuals suffer from
exercise-induced muscle pain due to their inability to degrade muscle glycogen.