• Glycogen catabolism
• Glycogen cynthesis
• Control of glycogen metabolism
• Metabolism of fructose, galactose and
• Biosynthesys of glucuronic acid
• Disorders of carbohydrate metabolism
Getting glucose from storage (or diet)
-Amylase is an endoglycosidase
• It cleaves amylopectin or glycogen to maltose,
maltotriose and other small oligosaccharides
• It is active on either side of a branch point, but
activity is reduced near the branch points
• Debranching enzyme cleaves "limit dextrins"
• Note the 2 activities of the debranching
• Glycogen is a polymer of glucose residues linked by
• (14) glycosidic bonds, mainly
• (16) glycosidic bonds, at branch points.
• Glycogen chains and branches are longer than shown.
• Glucose is stored as glycogen predominantly in liver
and muscle cells.
Metabolism of Tissue Glycogen
Digestive breakdown is unregulated - 100%!
• But tissue glycogen is an important energy
reservoir - its breakdown is carefully controlled
• Glycogen consists of "granules" of high M w
• Glycogen phosphorylase cleaves glucose from
the nonreducing ends of glycogen molecules
• This is a phosphorolysis, not a hydrolysis
• Metabolic advantage: product is a sugar-P - a
"sort-of" glycolysis substrate
• Glycogen Phosphorylase catalyzes
phosphorolytic cleavage of the (14)
glycosidic linkages of glycogen, releasing
glucose-1-phosphate as reaction product.
glycogen (n residues)
+ P i
glycogen (n–1 residues)
• This phosphorolysis may be compared to
• Hydrolysis: R-O-R' + HOH R-OH + R'-OH
• Phosphorolysis: R-O-R' + HO-PO 3
R-OH + R'-O-PO 3
Glycogen Phosphorylase (GP)
• Pyridoxal phosphate (PLP), a
derivative of vitamin B 6
as prosthetic group for GP.
• PLP is held at the active site by a
Schiff base linkage, formed by
reaction of the aldehyde of PLP
with the -amino group of a
• In contrast to its role in other
enzymes, the phosphate of PLP
is involved in acid/base catalysis
• A glycogen storage site on the surface of
the Phosphorylase enzyme binds the
• Given the distance between storage and
active sites, Phosphorylase can cleave
(14) linkages only to within 4 residues of
an (16) branch point.
• This is called a "limit branch".
• Debranching enzyme has 2 independent active sites,
consisting of residues in different segments of a single
• The transferase of the debranching enzyme
transfers 3 glucose residues from a 4-residue limit
branch to the end of another branch, diminishing
the limit branch to a single glucose residue.
• The (16) glucosidase moiety of the
debranching enzyme then catalyzes hydrolysis of
the (16) linkage, yielding free glucose. This is a
minor fraction of glucose released from glycogen.
• The major product of glycogen breakdown is
glucose-1-phosphate, from Phosphorylase activity.
• catalyzes the reversible reaction:
• serine -OH at the active site donates and accepts P i .
• the bisphosphate is not released
• Glucose-6-P may enter Glycolysis or (mainly in liver)
be dephosphorylated for release to the blood.
• Liver Glucose-6-phosphatase catalyzes the following,
essential to the liver's role in maintaining blood
glucose: glucose-6-P + H 2
O glucose + P i
• Most other tissues lack this enzyme.
Glucose units are activated for transfer by
formation of sugar nucleotides
• What are other examples of "activation"?
– acetyl-CoA, biotin, THF,
• Leloir showed in the 1950s that glycogen
synthesis depends on sugar nucleotides
• UDP-glucose pyrophosphorylase
– a phosphoanhydride exchange
– driven by pyrophosphate hydrolysis
• Uridine diphosphate glucose (UDP-glucose) is the
immediate precursor for glycogen synthesis.
• As glucose residues are added to glycogen, UDP-glucose is
the substrate and UDP is released as a reaction product.
• Nucleotide diphosphate sugars are precursors also for
synthesis of other complex carbohydrates, including
oligosaccharide chains of glycoproteins, etc.
• UDP-glucose is formed from glucose-1-phosphate:
• glucose-1-phosphate + UTP UDP-glucose + PP i
• PP i
+ H 2
O 2 P i
glucose-1-phosphate + UTP UDP-glucose + 2 P i
• Spontaneous hydrolysis of the ~P bond in PP i
(P~P) drives the overall reaction.
• Cleavage of PP i
is the only energy cost for glycogen
synthesis (one ~P bond per glucose residue).
4) glycosidic bonds in glycogen
• Glycogenin (a protein!) forms the core of a
• First glucose is linked to a tyrosine -OH
• Glycogen synthase transfers glucosyl units
from UDP-glucose to C-4 hydroxyl at a
nonreducing end of a glycogen strand.
• A glycosidic bond is formed between the
anomeric C1 of the glucose moiety derived
from UDP-glucose and the hydroxyl oxygen of
a tyrosine side-chain of Glycogenin.
• UDP is released as a product.
• Glycogen Synthase then catalyzes elongation
of glycogen chains initiated by Glycogenin.
• Question: Where would you expect to find
Glycogenin within a cell?
• Answer: Most of the Glycogenin is found
associated with glycogen particles (branched
glycogen chains) in the cytoplasm.
• Glycogen Synthase catalyzes transfer of the glucose
moiety of UDP-glucose to the hydroxyl at C4 of the
terminal residue of a glycogen chain to form an
(1 4) glycosidic linkage:
glycogen (n residues)
glycogen (n +1 residues)
• A branching enzyme transfers a segment from the
end of a glycogen chain to the C6 hydroxyl of a
glucose residue of glycogen to yield a branch with an
(1 6) linkage.
Control of Glycogen Metabolism
A highly regulated process, involving reciprocal control of
glycogen phosphorylase and glycogen synthase
• GP allosterically activated by AMP and
inhibited by ATP, glucose-6-P and
• GS is stimulated by glucose-6-P
• Both enzymes are regulated by
covalent modification - phosphorylation
Regulation of glycogene metabolism
• Both synthesis and breakdown of glycogen are
• If both pathways were active simultaneously in
a cell, there would be a "futile cycle" with
cleavage of one ~P bond per cycle (in forming
• To prevent such a futile cycle, Glycogen
Synthase and Glycogen Phosphorylase are
reciprocally regulated, by allosteric effectors
and by phosphorylation.
Regulation of GP
• Glycogen Synthase is allosterically activated by
glucose-6-P (opposite of effect on GP).
• Thus Glycogen Synthase is active when high
blood glucose leads to elevated intracellular
• It is useful to a cell to store glucose as glycogen
when the input to Glycolysis (glucose-6-P), and
the main product of Glycolysis (ATP), are
Phosphorylation of GP and GS
• Edwin Krebs and Edmond Fisher showed
in 1956 that a "converting enzyme"
converted phosphorylase „b“ to
phosphorylase „a“ (P)
• Nine Ser residues on GS are
Enzyme Cascades and
• Hormones (glucagon, epinephrine)
activate adenylyl cyclase
• cAMP activates kinases and phosphatases
that control the phosphorylation of GP and
• GTP-binding proteins (G proteins) mediate
the communication between hormone
receptor and adenylyl cyclase
Hormonal Regulation II
Glucagon and epinephrine
• Glucagon and epinephrine stimulate glycogen
breakdown - opposite effect of insulin!
• Glucagon (29 res) is also secreted by pancreas
• Glucagon acts in liver and adipose tissue only!
• Epinephrine (adrenaline) is released from
• Epinephrine acts on liver and muscles
• The phosphorylase cascade amplifies the
Epinephrine and Glucagon
• Both are glycogenolytic but for different
• Epinephrine is the fight or flight hormone
– rapidly mobilizes large amounts of energy
• Glucagon is for long-term maintenance of
steady-state levels of glucose in the blood
– activates glycogen breakdown
– activates liver gluconeogenesis
of Glycogen Synthesis and Degradation
• Insulin is secreted from the pancreas (to
liver) in response to an increase in blood
• Note that the portal vein is the only vein in
the body that feeds an organ!
• Insulin stimulates glycogen synthesis and
inhibits glycogen breakdown
Sources of Sugars
• Glucose: lactose (dairy products) and
sucrose (table sugar)
• Fructose: fruits and sucrose
• Galactose: lactose
• Mannose: polysaccharides and
Other Substrates for Glycolysis
Fructose, galactose, and mannose
• Fructose and mannose are routed into
glycolysis by fairly conventional means.
• Galactose is more interesting - the Leloir
pathway "converts" galactose to glucose
Metabolism of fructose
• Source - food – (saccharose, free)
• Synthese in cels – reduction of glucose
sorbitol oxidation fructose
• Metabolism occures in the liver (faster
than glucose – fructokinase have higher
• Target – glycolyse
O- -D-Glucopyranosyl-(1—>2)- -D-Fructofuranoside
Sucrose -D-glucose + -D-fructose -D-fructose
Muscle Metabolism of Fructose
Large Amounts of Hexokinase
• Formation fructose-6-P (muscle) alternative way
phosphorylation - limited, because activity of
hexokinase is 1/20 from activity for glucose
• Minority alternative way – reduction fructose to sorbitol
Liver Metabolism of Fructose I
• Obvious way transformation is fosforylation in the
liver (partially in intestine epithel and in kidney)
• Fosforylation can not effect hunger or insuline
compensate glucose for diabetics
Liver Metabolism of Fructose II
Liver Metabolism of Fructose III
• Too Much Fructose
– Fructose-1-P Aldolase is rate-limiting
– Depletion of P i
– Reduction in [ATP]
– Increase in glycolysis
– Accumulation of lactate (acid) in blood
• Fructose-1-P Aldolase Deficiency
Metabolism of galactose
• Source - lactose
• Occures in the liver
• Galactose is necessary for synthesis of
lactose, glycolipids, protheoglycans,
glycoprotheins or can converted to glucose
• UDP-galactose is high energy molecules,
galactose have ability bounded to other
saccharides(-OH compound) and formed O-
glycosidic bound (synthese
• Mutarotation of β-D-Galactose
• Glycolytic Enzymes are specific and do not
Phosphorylation of Galactose
Activation of Galactose
Epimerization of UDP-Galactose
Metabolism of manose
• Minority part of diet
• Important part of glycoproteines
The Role of Glucuronic acid
• Uridine-1-diphospho glucose can be used for:
• biosynthesis of glycogen
• biosynthesis of galactose
• after the oxidation at the 6th carbon atom is formed UDPglucuronic
acid, which is used for:
• conjugation (bilirubine etc.)
• biosynthesis of glycosaminoglycanes (hialuronic acid,
• biosynthesis of several pentoses (xylose, L-xylulose)
• biosynthesis of ascorbic acid
• Products capable be transformed in phosphopentose
Transformation of glucose-6-P
Glc-6-P Glucose (55 %)
6-P-Gluconate (2 %)
Glycolysis (25 %)
Glycogene (18 %)
Gulonate (vit. C)