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Glycogen Metabolism
• Purpose: Glycogen is a
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branched polymer of
glucose; it is the stored
form of G.
The many branches
each have a C#4 end
at which GP and GS
can act for rapid
response.
Glycogen is stored after
a meal for release:
From liver when blood
[G] is low to supply
brain; OR
In muscle for rapid
activity.
Main Enzymes of Glycogen Metabolism
• 1. Glycogen Phosphorylase (GP): releases G as
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G1P:
Gn + Pi  Gn-1 + G1P (no ATP cost)
GP removes G only from C#4 ends of chains that are
at least five G’s from a branch
G1P equilibrates with G6P; this is not regulated
G1P  G6P
2. Glycogen Synthase (GS): adds G (as UDP – G)
only to C#4 ends of chains.
a) Preliminary: G  G6P  G1P ; then: G1P +
UTP  PPi + UDP – G
b) GS rxn: Gn + UDP – G  UDP + Gn +1
• 1. Debranching
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enzyme: after GP
has removed all but
the last 4 G residues
from a branch, this
enzyme:
1) catalyses transfer
of 3 G residues to
the C#4 end of a
nearby branch and
2) catalyses
hydrolysis of the 1
 6 linkage,
producing G
Other Enzymes of
Glycogen Metabolism
Other Enzymes of Glycogen Metabolism
• 2. Branching Enzyme: transfers C#1 of a 7G residue
segment (from a branch at least 11 G long) to the
C#6 of a residue at least 4 G away.
Regulation of GP, GS
• 1. GP is designated by 2 systems a/b and m/o, which
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we will not use. Instead, we will refer to the enzymes
as: phosporylated (P) or dephosphorylated (DP) ( GS
is also P, DP).
2. GP and GS are phosphorylated in response to
glucagon (in the liver) (low blood [G]) and adrenalin
(muscle) (fight/flight), activating GP for release of G
and inactivating GS.
3. GP kinase (GPK): GP + ATP  ADP + GP – P.
4. They are dephosphorylated in response to insulin
inactivating GP, activating GS to store G.
Regulatory effectors of GP, GPK
• GP-DP is:
• 1) activated by AMP. MR: GP provides GlP  G6P for
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ATP production in glycolysis, and in OP via PDH,
TCA, ET, OP. ML: [AMP] is high when ATP use is rapid
and ATP production is needed.
2)
inhibited by ATP. MR in 1). ML: when [ATP] is high, GP
doesn’t need to release G to produce more
3) inhibited by G6P. MR: G6P is an indirect product of
GP. ML: when [G6P] is high, GP doesn’t need to make
more.
GP-P is inhibited by glucose. MR: G is indirect product:
GlP  G6P  G. ML: no need for more fuel when
plenty is available
GPK is activated by Ca2+. MR: GPK activates GP,
which provides fuel for ATP production. ML: Ca2+
triggers muscle contraction, ATP production is needed.
Regulatory effectors of GS
• GS-DP is activated by G6P: MR: G6P is
indirect substrate: G6P  GlP  UPD-G
(feed forward) ML: when G6P is plentiful,
it’s time to store G.
Hormonal Regulation of Glycogen Metabolism
• 1. The “hunger hormone”, glucagon is a signal to
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release G to blood from the liver via glycogenolysis
and gluconeogenesis. Liver GP is activated, GS
inhibited.
2. Adrenalin (epinephrine) is a signal to “break down”
muscle glycogen to produce G6P for ATP production
(in G’lys and in OP via PDH, TCA, ET and OP) for fightor-flight. Muscle GP is activated, GS inhibited.
When either one binds its cell-membrane receptor:
a. the hormone-receptor complex binds to adenylate
cyclase and activates it to catalyze: ATP  PPi +
cAMP
b. cAMP is the internal or “2nd” messenger. It binds
to protein kinase (PrK) regulatory (r) subunits,
dissociating them from the catalytic (c) subunits,
which are then active.
Hormonal Regulation of Glycogen Metabolism
• c. PrK catalyses the phosphorylation of a variety of
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proteins, including:
the tandem E (G’lys, G’neo)
pyruvate kinase (G’lys)
ACoA carboxylase (FA synthesis)
Glycogen synthase, inactivating it
Glycogen phophorylase kinase (GPK), activating it.
d. GPK-P catalyses phosphorylation of glycogen
phosphorylase (GP), activating it.
e. Net hormonal effect: GP activated: Gn  G; and
GS inactive, preventing opposition to GP.
PrK also phosphorylates phosphoprotein phosphatase
inhibitor 1 (PPI-1) causing it to bind to and inactivate
PP1, the enzyme that dephosphorylates GP, GS, etc .
Dephosphorylation of (GPK, GP, GS)
• Dephosphorylation of these enzymes (GPK, GP, GS) is
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catalyzed by phosphoprotein phosphatase 1 (PP1).
In muscle, phosphorylation of a regulatory glycogen
binding protein, GM in response to insulin (which
causes dephosphorylation of other Es) at site 1
activates PP1. This results in the opposite activities to
the above (GS active to store the plentiful G, GP not,
to prevent opposing GS).
Phosphorylation of GM at site 2 ( alone or in addition
to site 1) by PrK inactivates PP1, preventing it from
opposing PrK.
• In liver, the switch from the phospho- to the
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dephospho- state of GP, GPK, and GS cannot occur
without the accumulation of glucose. It is said that
“GP is the glucose sensor”:
a) In the phospho (active) form, the P’s on GP are
“buried” where PP1 can’t get at them.
b) When G binds to active GP-P, its conformation
changes, “exposing” the P’s so PP1 can “clip them” off.
c) PP1 binds strongly to GP-P in the R form; and is
not active toward other phosphoproteins in this state.
Only after GP-P binds G and PP1 dephosphorylates GP
is PP1 released and active.
d) PP1 has a much higher affinity for GP-P than for
GS, GPK, etc, so it must first “work its way through”
nearly all the GP-P, dephosphorylating it, before it has
much effect on GS.
Amplification cascade
• GP and GS are regulated by effectors like AMP, ATP, G,
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and G6P. So why has regulation by enzyme
phosphorylation evolved? The big advantage is speed
and magnitude of response: each enzymatic step
amplifies the signal, and subsequent ones multiply
previous ones:
1) each hormone-receptor complex activates one
adenylate cyclase, which can produce, say, 1000
cAMP/sec.
2) 4 cAMP can --> 2 active PrK which can produce,
say, 100 GPK-P/sec.
3) Each GPK-P can phosphorylate, say, 100 GP/s.
So, one H-R complex results in: 1000 cAMP x 2PrK/4
cAMP x 100 GPK-P/PrK x 100GP-P/GPK-P=5,000,000
activated GP/sec, rather than 1 Enzyme/1 effector.
That's why it’s called the" amplification cascade".
Glucokinase (GK)
• GK catalyzes G + ATP  G6P + ADP, same as HXK.
• GK is a liver enzyme; muscle doesn’t have it (has
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HXK).
The properties of GK are more suited to maximum
glycogen synthesis when [G] is high. HXK can also
support rapid glycogen synthesis, but not as well as
GK.
HXK is “designed” to keep up with extremely rapid
glycolysis (if that’s the pace PFK sets): when PFK
consumes F6P rapidly, G6P is also consumed rapidly,
(G6P F6P) so that HXK is not inhibited by G6P.
HXK has a very low Km (high affinity) for G, so it can
go at almost Vmax rate even if [G] is low, but high [G]
doesn’t increase rate.
• But GK is not
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inhibited by
G6P, so when
[G] is high it can
produce a much
higher [G6P]
which kinetically
pushes glycogen
synthase: via
G6P  GlP 
UDPG
GK has a much
higher Km (lower
affinity) for G so
its rate is nearly
proportional to
[G] across the
physiological [G]
range.
GK, HXK
FAs, Fatty Acids
• FAs are a much more efficient form of stored
fuel: 9kcal/g (9 Cal/g) vs. (4 Cal/gG); also
glycogen binds two times its weight of H2O. A
typical man would have to store ~ 90 kg of
glycogen (~200lbs) if he was to have the same
energy as in the ~15 kg fat he stores.
• Although glycolysis is a major fuel consuming
pathway, FAs are the main fuel (except in
brain, RBCs, rapid muscle activity).
• Because of the above, glycogen storage is
limited and “xs G” is converted to fat via
glycolysis, PDH, CS and FA synthesis.
FA Use as Fuel
• FAs are released from storage (as triacylglycerol) by
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the hydrolytic action of hormone-sensitive lipase,
which is activated by phosphorylation by PrK in
response to adrenalin or glucagon, deactivated by
dephosphorylation by PP1 in response to insulin.
FA’s then travel from adipose cells (“cytosol” is mainly
a fat globule) via blood to cells that use them.
FA’s are prepared in the cytosol (cytoplasm) for
transport to the mitochondrial matrix, where they are
converted to ACoA in  oxidation.
FA activation: (costs 2 ATP)
a) FA + ATP  PPi + FA – AMP;
b) FA – AMP + CoASH  AMP + FA – SCoA
• CoA from cytosol doesn’t enter matrix (or vice-versa).
• Instead, on the outer surface of the inner membrane,
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FA is transferred to carnitine (releasing CoASH to the
cytosol) in a reaction catalyzed by carnitine
acyltransferase I (CATI) (aka Carnitine Palmitoyl
TransferaseI, or CPTI).
A transport protein in the inner membrane brings
fatty-acyl carnitine into the matrix (in exchange for
carnitine delivered outside).
CATII (CPTII) on inner surface transfers FAcyl group
from carnitine to CoASH.
(Palmitate is the 16C saturated FA)
 oxidation + TCA + ET + OP  ATP
•  oxidation converts the fatty acyl group to
ACoA. Net reaction for complete  oxidation
of palmitate:
• C15H31COSCoA + 7CoASH + 7FAD + 7NAD+ 
8ACoA + 7FADH2 + 7NADH
• ATP production from palmitate
• 8XTCA: +8GTP + 24NADH + 8FADH2
• ET: -31NADH – 15FADH2
• OP: [+3(31) + 2(15)] ATP = + 123 ATP
• 123ATP + 8GTP – 2 “ATP” (ATP  AMP in FA
acivation ) = 129ATP
Ketone Body Production
• The moderate rate of production of
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acetoacetate,  hydroxybutyrate and acetone
that occurs normally in the liver mitochondrial
matrix delivers “water soluble FA fragments” to
cells via blood for use as fuel.
Since this process involves unregulated
enzymes, the buildup of ACoA in diabetes
overproduces these compounds to toxic levels.
FA Synthesis
• FA synthesis is a liver pathway
• The net effect is to build up the CH2 chain by
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joining ACoAs’ acetyl gps. and reducing (and
hydrogenating) the C=O of ACoA.
The ACoAs for FA synth don’t come from 
oxidation. Rather it’s the “xs G” that enters liver
cells after a meal and goes through insulin
stimulated glycolysis and PDH.
• But PDH is in matrix, FA sythase is in cytosol:
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(ACoA doesn’t cross inner membrane)
1. high ACoA from PDH stimulates PC  high oxac.
2. (ACoA + oxac  citrate) in matrix; then transport
citrate to cytosol.
3. in cyto: citrate + ATP  ACoA + oxac + ADP + Pi
(catalyzed by citrate lyase)
4. oxac + NADH  malate + NAD+ then, malate can
enter matrix, OR
5. in cyto: mal + NADP+  NADPH + pyr + CO2 ;
(pyr goes to matrix). This rxn is catalyzed by the malic
enzyme.
6. The NADPH is needed for FA synthesis (below)
• 7. The cyto ACoA
is activated for
joining by
conversion to
malonyl CoA
(carboxylated)
by ACoA
carboxylase
(ACoAC):
ACoA + CO2
+ATP ---> ADP
+ Pi + malCoA
FA Synthase
• In E Coli, this consists of a number of separate
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enzymes, but in animals 2 identical subunits each
contain the enzymatic activities for all the rxns (
oxidation has a different enzyme for each step).
The substrate remains bound to the long
phosphopantethein prosthetic group (Fig. 25-29,
p931), which “carries” it to each of the various active
sites. This is on ACP (acyl-carrier protein)
Phases of FA Synthase Reaction “Cycle”
• Loading: the acetyl group of ACoA is
transferred to a cys-S (viaACP) and the malonyl
group of mal-CoA to ACP-S.
• Condensation, Reduction: C2 chains (of
malCoA from ACoA) are linked, releasing CO2,
then reduced to –CH2-CH2.
• Reloading: existing chain transferred to cys-S;
next malonyl group to ACP-S (each mal of mal
CoA goes onto ACP, only acetyl group of ACoA
(and existing chain) go onto cys-S
• Release: FA hydrolyzed from ACP
Regulation of FA Metabolism
• ACoA Carboxylase (ACoA  mal CoA  FA synthesis)
• 1. Inhibited by palmitoyl CoA. MR: indirect product
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(mal CoA  palmitate pal CoA).
ML: If [pal CoA] is high, it is being produced faster
than it can be used, production can slow)
2. Activated by citrate. MR: indirect substrate; citrate
 oxac + AcoA, the substrate
ML: when[citrate] cyto is high, [citrate] mito is very
high, fuel is plentiful, time to store it
3. Inhibited by phosphorylation in response to
glucagon or adrenalin. These hormones promote fuel
mobilization to make fuel available, so they inhibit
storage.
4. Activated by dephosphorylation in response to
insulin. Insulin “signals fed state”, when [G] is high
it’s time to store C as glycogen and FAs.
Regulation of FA
Metabolism
• Phosphorylation shifts ACoAC
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from active polymer form to
inactive monomer form.
Carnitine Acyl Transferase I
(CATI) (transport of FAs into
matrix for  oxid’n).
Inhibited by mal CoA. MR: mal
CoA is the product of the
committed step in the opposing
pathway, FA synthesis.
ML: When [mal CoA] is high, FA
synthesis is rapid (in liver), with
the purpose of export of these
FA’s for storage. Inhibition of
CATI prevents consumption from
working against synthesis.
Active ACoAC
Page 942
Figure 25-41 All of cholesterol’s carbon
atoms are derived from acetate.
Amino Acid (AA) oxidation
• Introduction
• 1. Part of the C’s of some of the AAs are convertible to
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ACoA, either directly or via acetoacetate or pyr. (and
less directly, so are the others via TCA int  oxac 
PEP  pyr  ACoA.) (These AAs are “ketogenic)
So, these C’s of xs AA intake (in relation to need for
protein synth) are used as fuel, just like dietary
CH2O’s, fats.
2. Part (or all) of the C’s of 18 of the AAs can be
converted to TCA intermediates, which can be
converted to G (TCA int  oxac  PEP  G). These
are referred to as the “glucogenic” AAs.
AAs from digestion of muscle protein are the main
source of C for gluconeogenesis in CH2O starvation
Transaminations (trnsams)
• Each AA can be converted to the corresponding  keto
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acid by at least one transaminase. This AA is oxidized
in this rxn, but  kg is reduced to glutamate at the
same time so there’s not a net AA oxidation
The amino group transferred to kg (---> glu) is toxic
when released as NH3, this ammonia is detoxified
by conversion to urea in the urea cycle (NH3 can be
excreted). Net oxidation occurs by coupling of
trnsam with glutamate dehydrogenase (GDH).
GDH: glu + NAD+   kg + NADH + NH3
This rxn running in reverse when [NH3] is very high
depletes TCA ints, interferes with TCA +ET + OP in
brain cells and causes the delirium/dementia in liver
damaged patients.
GDH Regulation
• Inhibited by ATP and GTP; Activated by ADP
and GDP
• MR: GDH + trnsam  TCA ints   TCA 
ET  OP: ATP production
• ML: If [ATP] or [GTP] is high, more is not
needed; if [ADP] or [GDP] is high, ATP
synthesis is needed; TCA  ATP
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Carbamoyl Phosphate Synthetase I Regulation
CPSI is activated by N-acetylglutamate NAG
MR: NAG is produced from ACoA and glu:
ACoA + glu  NAG.
A high [ACoA] and/or a high [glu] increases the rate
of NAG production and the [NAG], so the [NAG]
indicates the levels of AcoA and glu.
MR, ML for ACoA: when [ACoA] is high there is a
need for oxac to react with ACoA in the CS rxn. GDH
+ trnsam can produce TCA ints from AAs at a high
rate only if CPSI consumes the ammonia product of
GDH.
MR, ML for glu: when glu is high it has been produced
by a high rate of trnsam and there is a need to
convert it to  kg in GDH to maintain [ kg] for TCA
and trnsam. CPSI must consume the ammonia
product of GDH.