MIC 428 Lecture

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MIC 428 - Lecture 9 Metabolism II
Outline
The Embden-Meyerhof (Parnas) pathway Products.
Regulation. Critical biosynthetic
intermediates. The Hexose monophosphate
pathway Significance. Intermediates
provided. Final products. Regulation. The
Entner-Doudoroff pathway Intermediates
provided. Energy yield.
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Embden-Meyerhof Pathway (EM, glycolysis)
Major pathway for the conversion of hexose sugars
into pyruvate.

• It results in the formation of
• two NADH
• two ATP

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(from Glyceraldehide-3-P to Pyruvate) Gain of 4
ATP
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The Embden-Meyerhof Pathway (EM, glycolysis)
It results in the formation of six of the
critical biosynthetic intermediates
Which ones? (look at the table provided)
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(from Glyceraldehide-3-P to Pyruvate) Gain of 4
ATP

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The EM pathway
Symmetrical pathway
Amphibolic pathway
Highly regulated pathway
Inhibited by ATP Inhibited by high levels of
glucose-6-P Inhibited by Ala, an intermediate
synthesis product from pyruvate.
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The EM pathway
It fails to provide some of the 12 key compounds.
Which ones?
Five carbon sugars required for DNA and RNA
formation. Erythrose-4-P NADPH, major source of
reducing power in biosynthesis.
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The Hexose Monophosphate (HM) Pathway (also known
as oxidative pentose, OM, or pentose phosphate
pathway)
It provides all the key intermediates not
provided by the EM pathway.
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The Hexose Monophosphate Pathway (HM, OM pathway)
The pathway begins with the formation of a
glucono lactone (internal ester) from glucose-6-P
and the conversion of NADP to NADPH.
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The lactone is then hydrolyzed yielding
6-phosphogluconic acid which is then
decarboxilated yielding more NADPH, carbon
dioxide and xylulose-5-P
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In one of the steps, this pathway produces
erythrose-4-P and glyceraldehyde-3-P that can get
into the EM pathway.

• When the complete OP operates, the products are
• Carbon dioxide
• NADPH and intermediates of biosynthesis such as
  erythrose-4-P and ribose-5-P.

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Is there any ATP formed?
How does this pathway operate without generating
any energy?
Regulation
NADPH regulates the pathway.
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The Entner-Doudoroff Pathway (pages 345-346,
Brocks book)
It may be considered an alternate hexose
monophsphate pathway.

• It provides a minimum of five of the critical
  biosynthetic intermediates
• glucose-6-P
• triose phosphate
• 3-phosphoglycerate
• phosphoenol pyruvate (PEP)
• pyruvate

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The Entner-Doudoroff Pathway
It begins the same as the HM pathway up to
phosphogluconic acid. Then, instead of being
converted to pentose and carbon dioxide, it is
dehydrated yielding 2-keto-3, dehydro, 6
phosphogluconic acid.
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The Entner-Doudoroff Pathway
Both the EM and the ED pathway convert a glucose
molecule to two molecules of pyruvate.
In the EM pathway, pyruvate arises by the
intermediate formation of glyceraldehyde-3-P. In
the ED pathway, from the top half of the molecule
of glucose.
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The Entner-Doudoroff Pathway
The energy yield by substrate-level
phosphorylation per molecule of hexose degraded
by the EM pathway is twice that obtained by the
ED pathway.