Lecture 2: Glycogen metabolism (Chapter 15)

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Lecture 2 Glycogen metabolism (Chapter 15)
First.
Fig. 15.1
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Review Animals use glycogen for ENERGY
STORAGE. Glycogen is a highly-branched polymer
of glucose units
Basic structure is similar to that of
amylopectin, but with only about 8 to 12 glucose
units between branch points (n 4 to 6).
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GLYCOGEN BREAKDOWN INSIDE CELLS Glycogen's
glucose units are mobilized by their sequential
removal from the glucan chain's nonreducing ends
that is, the ends that lack a C1-OH group.
This is the reducing end of glucose
Fig. 15-2a
The ends of some sugars have a free anomeric
carbon, which can act as a mild reducing
agent. (In glycogen, however, the reducing end
is actually bound by a protein named GLYCOGENIN.)
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The branched structure of glycogen permits the
rapid release of glucose simultaneously from
every non-reducing end of every branch
(These red arrows point to the non-reducing ends.)
Only ONE reducing end per molecule
Fig. 15-2b (modified)
(Note that the number of glucose units between
branch points in this figure is not accurate.
Dont let this confuse you!)
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Reminder The reducing end is bound by GLYCOGENIN
GLYCOGENIN
Fig. 15-2b (modified)
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Why use glycogen to store energy rather than just
using fat? (Since fat is more abundant than
glycogen in the body and also stores
energy) 1. Muscles "mobilize" (i.e., convert to
energy) glycogen faster than fat. 2. Fatty
acid residues cannot be metabolized
anaerobically (that is, without oxygen). (If
you want to burn fat while you are exercising,
you must be able to breathe fairly
easily.) 3. Animals cannot convert fat to
glucose, so fat metabolism cannot maintain
blood glucose levels. (Glucose is brain food"
it is the major energy form that crosses the
blood-brain barrier.)
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Glycogenolysis (or glycogen breakdown) requires 3
major enzymes 1) GLYCOGEN PHOSPHORYLASE
(Fig. 15-4 more later)
2) GLYCOGEN DEBRANCHING ENZYME (Fig. 15-6)
3) PHOSPHOGLUCOMUTASE (Fig. 15-7)
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Glycogenolysis requires 3 major enzymes 1)
GLYCOGEN PHOSPHORYLASE (or simply
PHOSPHORYLASE) See Fig. 15-4 (next slide) for
GPs reaction mechanism. Note that GP catalyzes
bond cleavage by PHOSPHOROLYSIS, as opposed to
HYDROLYSIS. The overall reaction is
Glycogen(n residues) Pi
lt---gt Glycogen (n-1) G-1-P
inorganic phosphate
Glucose-1-phosphate
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GLYCOGEN PHOSPHORYLASE MECHANISM
Fig. 15-4 Phosphorylase has a random
sequentialenzyme mechanism that involves PLP
(pyridoxyl-5-phosphate), a vitamin B6
derivative
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NOTE Phosphorylase only releases units that
are 5 or more from the branch point, leaving a
LIMIT BRANCH.
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Glycogenolysis requires 3 major enzymes
2) GLYCOGEN DEBRANCHING ENZYME (Fig. 15-6)
GDE has two enzymatic activities A) A
debranching transglycosylase
activity B) An hydrolysis activity
A
B
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Glycogenolysis requires 3 major enzymes
3) PHOSPHOGLUCOMUTASE reaction G-1-P
lt---gt G-1,6-P lt---gt G-6-P

Glucose-1,6-bisphosphate
Glucose-6-phosphate
Fig. 15-7 Phosphoglucomutase Mechanism
(Note that this reaction is fully reversible.)
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Fig. 15-1 G-6-P is a major intermediate in
glucose metabolism
Glucose-6-phosphatase hydrolyzes G-6-P to Glucose
Pi in LIVER
Important in nucleotide synthesis
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Fig. 15-1 G-6-P is a major intermediate in
glucose metabolism
Brief overview next...
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Glycogen SYNTHESIS requires 3 major enzymes, and
occurs by a SEPARATE PATHWAY from
glycogenolysis 1) UDP-GLUCOSE
PYROPHOSPHORYLASE (Fig. 15-9) G-1-P UTP
lt---gt UDP-glucose (UDPG) PPi
Uridine triphosphate Uridine diphosphate
glucose inorganic
pyrophosphate 2) GLYCOGEN
SYNTHASE (Fig. 15-10) UDPG Glycogen(n units)
lt---gt UDP Glycogen(n1 units) This reaction
must be primed by GLYCOGENIN 3) GLYCOGEN
BRANCHING ENZYME (Fig. 15-11) or AMYLO
(1,4--gt 1,6) TRANSGLYCOSYLASE.

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GENERAL RULES FROM ABOVE BIOSYNTHETIC AND
DEGRADATIVE PATHWAYS OF METABOLISM ARE (ALMOST)
ALWAYS COMPLETELY DIFFERENT. THAT IS, THEY USED
DIFFERENT ENZYMES. POLYMERIZATION OF MONOMERIC
UNITS INTO MACROMOLECULES USUALLY REQUIRES
A PRIMER TO INITIATE THE REACTION. THAT
IS, THE FIRST TWO UNITS CANNOT BE LINKED BY THE
ENZYME THAT DOES THE POLYMERIZATION.
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1. GLYCOGEN PHOSPHORYLASE (or simply
PHOSPHORYLASE)

• Removes GLUCOSE UNITS from the NONREDUCING
• ends of GLYCOGEN.
• Is a FAST enzyme the outermost branches of
  glycogen
• are degraded in seconds in muscle tissue.
• Is a dimer of identical 842-residue subunits
  (Fig. 15-3).

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1. GLYCOGEN PHOSPHORYLASE (continued)

• Catalyzes the CONTROLLING STEP in glycogen
• breakdown.
• The standard-state free-energy change (?G') for
  
• phosphorylase reactions is 3.1 kJ/mol, but
  the
• intracellular Pi / G1P ratio is about 100,
  so ?G
• in vivo is actually about - 6 kJ/mol.

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1. GLYCOGEN PHOSPHORYLASE (continued)

• It is a highly and complexly regulated enzyme,
  both by
• ALLOSTERIC INTERACTIONS (Fig. 15-13) ATP,
• G6P glucose inhibit it AMP activates it
• and
• COVALENT MODIFICATION by phosphorylation
• and dephosphorylation (Fig. 15-5).
• Yields 2 major forms of phosphorylase
• Phosphorylase A Has a phosphoryl group
  esterified to
• Ser-14 in each subunit (more active)
• Phosphorylase B Is not phosphorylated (less
  active)
• Look at Kinemages Exercise 14 on the CD
  with VVP textbook!

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1. GLYCOGEN PHOSPHORYLASE (continued)

• Only releases units that are 5 or more from the
  branch.
• WHY?

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1. GLYCOGEN PHOSPHORYLASE (continued)

• Only releases units that are 5 or more from the
  branch.
• WHY?

• Robert Fletterick (www.ucsf.edu/pibs/faculty/flett
  erick.html)
• solved the 3D structure of Phosphorylase A Its
  crevice can admit 4 or 5sugar residues, but it is
  too narrow to admit a branch.

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Fig. 15-1 G-6-P is a major intermediate in
glucose metabolism
NEXT...
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Glycogen SYNTHESIS requires 3 major enzymes, and
occurs by a SEPARATE PATHWAY from
glycogenolysis 1) UDP-GLUCOSE
PYROPHOSPHORYLASE (Fig. 15-9) G-1-P UTP
lt---gt UDP-glucose (UDPG) PPi
Uridine triphosphate Uridine diphosphate
glucose inorganic
pyrophosphate 2) GLYCOGEN
SYNTHASE (Fig. 15-10) UDPG Glycogen(n units)
lt---gt UDP Glycogen(n1 units) This reaction
must be primed by GLYCOGENIN 3) GLYCOGEN
BRANCHING ENZYME (Fig. 15-11) or AMYLO
(1,4--gt 1,6) TRANSGLYCOSYLASE.

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1) UDP-GLUCOSE PYROPHOSPHORYLASE (Fig. 15-9)
G-1-P UTP lt---gt UDP-glucose (UDPG)
PPi Uridine triphosphate
Uridine diphosphate glucose inorganic
pyrophosphate
The DG of this reaction is nearly ZERO, but the
PPi formed is hydrolyzed to 2 Pi (orthophosphate)
in a highly EXERGONIC reaction the the
omnipresent enzyme, INORGANIC PYROPHOSPHATASE.
Therefore, the overall reaction is also highly
exergonic DG (kJ/mol) GIP UTP
lt--gt UDPG PPi 0 H2O PPi --gt
2 Pi - 33.5 GIP
UTP lt--gt UDPG 2 Pi - 33.5

OVERALL
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UDPG is a HIGH ENERGY compound that can donate
GLYCOSYL units to the growing glycogen chain. No
further energy is required for glycogen synthesis.
IMPORTANT GENERAL NOTE The cleavage of a
nucleoside triphosphate (NTP) to form PPi is a
common synthetic strategy. The free energy of
PPi hydrolysis (by inorganic pyrophosphatase)
can be utilized together with the free energy of
NTP hydrolysis to drive an otherwise endergonic
reaction to completion. (We will see this over
and over and over this semester!)
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2) GLYCOGEN SYNTHASE MECHANISM (Fig. 15-10)
UDPG Glycogen(n units) lt---gt UDP
Glycogen(n1 units)
The glycosyl unit of UDPG is transferred to the
C(4)-OH on one of the non-reducing ends of
glycogen, forming an a(1-gt4) glycosidic bond.
Note that this step makes a-amylose, not the
branched structure of glycogen.
The DG for this reaction is -13.7 kJ/mol,
making this reaction spontaneous (exergonic)
under the same conditions that glycogen breakdown
is exergonic. Therefore, the rates of the two
reactions must be independently and tightly
controlled.
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For each molecule of GIP that is converted to
glycogen, one molecule of UTP is hydrolyzed to
UDP Pi. The UTP is replenished by the
enzyme NUCLEOSIDE DIPHOSPHATE KINASE UDP
ATP lt--gt UTP ADP (UTP
hydrolysis is energetically equivalent to
ATP hydrolysis.)
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GLYGOGENIN and Glycogen Priming
Glycogen synthesis can only occur by extending an
already existing a (1 4)-linked glucan chain.
Therefore, how can it get started in the first
place? Answer The first step in glycogen
synthesis is the attachment of a glucose residue
to the -OH group on Tyr-194 of GLYCOGENIN. This
attachment step is done by the enzyme TYROSINE
GLUCOSYLTRANSFERASE. Glycogenin then
autocatalytically extends the glucan chain by up
to 7 residues long (also donated by UDPG).
Glycogen synthase can then attach glucose
residues to this glycogen primer. Each
molecule of glycogen is associated with ONE
molecule each of glycogenin and glycogen
synthase.
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3) GLYCOGEN BRANCHING ENZYME (Fig. 15-11) or
AMYLO (1,4--gt 1,6) TRANSGLYCOSYLASE
Breaks a (1-gt 4) glycosidic bonds and forms a
(1-gt 6) linkages. Transfers terminal chain
segments of about 7 residues to the C(6)-OH
groups of glucose residues. Each transferred
segment must come from a chain of at least 11
residues, and the attachment point must be at
least 4 residues away from another branch point.
Segment can be moved to the same or a different
chain.
Note Not to be confused with Glycogen
Debranching Enzyme!
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• Control of glycogen metabolism is very complex.
• It involves
• allosteric regulation of both GS GP
• substrate cycles
• enzyme-catalyzed covalent modification of both
  GS GP
• covalent modifications are under hormonal
  control in the
• body, through their own enzymatic cascades
• In LIVER
• Glycogen metabolism is ultimately controlled by
  GLUCAGON a 29 amino acid-long polypeptide
  hormone that is secreted from the pancreas into
  the bloodstream (liver cells have glucagon
  receptors).
• In MUSCLES (and various other tissues)
• Is controlled by the adrenal hormones EPINEPHRINE
  (adrenalin) and NOREPINEPHRINE (noradrenalin).

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These hormones act at cell surfaces to stimulate
ADENYLATE CYCLASE, thus increasing cAMP. cAMP
acts inside cells as a second messenger for the
hormones. Cells have many cAMP-dependent PROTEIN
KINASES whose activities increase upon cAMP
binding. (Reminder Kinases catalyze the
transfer of phosphoryl groups between ATP and
other molecules, proteins in this case.)
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Liver maintains blood glucose at 5 mM if it
drops to half of this, a coma results. Upon
blood glucose decrease, the liver releases
glucose to the blood glucose triggers pancreas
to release glucagon, which causes increase cAMP
in liver, which in turn stimulates glycogen
breakdown. Glucose diffuses freely out of liver
cells, causing an increase in blood glucose.
High blood glucose causes release of INSULIN
from the pancreas to the blood. The rate of
glucose TRANSPORT across many cell membranes
increases in response to insulin.