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Feeder pathways for glycolysis
•Glycogen and starch
•Maltose, lactose, trehalose, sucrose
•Fructose, mannose, galactose
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Glycogen phosphorylase catalyzes an attack by Pi on
the (a1→4) glycosidic linkage that joins the last two
glucose residues at a nonreducing end, generating
glucose 1-phosphate and a polymer one glucose unit
shorter.
14-10
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 Glucose
1-phosphate produced by glycogen
phosphorylase is converted to glucose 6-phosphate
by phosphoglucomutase, which catalyzes the
reversible reaction
Glycolysis Pentose phosphate
pathway
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Dietary polysaccharides and
disaccharides
are hydrolyzed to monosaccharides by
various enzymes
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In galactokinase deficiency galactosemia ,
high conc. are found in blood and urine.
Infant develop cataracts, caused by
deposition metabolite galactitol in the lens.
Transferase deficiency galactosemia is more
serious; It is characterized by poor growth in
children, mental deficiency and liver damage
that may be fatal.
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To understand the function of the pentose
phosphate pathway in production of NADPH
and ribose precursors for nucleic acid
synthesis.
Provides NADPH for biosynthesis
NADPH is used in cytosol for fatty acid
synthesis
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The pentose phosphate pathway is an alternate
route for the oxidation of glucose.
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Oxidative non-reversible phase
Non-oxidative reversible phase
NADP+, not NAD +, is used as hydrogen acceptor
1st phase ; Oxidative Phase
This phase produces pentose phosphates and NADPH
- Glucose 6-phosphate undergoes dehydrogenation and
decarboxylation to give a pentose, ribulose 5-phosphate,
which is converted to its isomer, D-ribose 5-phosphate.
- Overall equation of 1st phase:
Glucose 6-phosphate + 2 NADP++ H2O
ribose 5-phosphate + CO2 + 2 NADPH + 2 H+
2nd PHASE; Non-Oxidative Phase
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Recycles pentose phosphate to glucose 6phosphate.
Two enzymes that act in the interconversion
of sugars are Transaldolase and
Transketolase.
Transketolase- it catalyzes the transfer of a
two carbon fragment from a ketose donor to
an aldose acceptor.
Transaldolase- they carry out transfer of
three carbon fragment from a keto donor to
an aldose acceptor.
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Genetic defects in any of the three enzymes
that catalyze conversion of galactose to
glucose 1-phosphate result in galactosemias.
Entry of glucose 6-phosphate either into
glycolysis or into HMP shunt is largely
determined by the relative conc. of NADP+ &
NADPH.