Glyoxylate cycle

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Dr. Harisingh gour central university sagar
(M.P.)
Department of applied microbiology
20202022
Glyoxylate cycle
Presented to - Dr. Yogesh Bhargav, (Department
of applied microbiology)
Presented by- Priyanshi mishra Batch - Msc 2nd
semester
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SYNOPSIS

1. Introduction
2. Steps of glyoxylate cycle
3. Glyoxylate pathway
4. Relationship between Glyoxylate and TCA
5. Coordinate regulation between TCA and glyoxylate
   cycle
6. Significance of glyoxylate cycle.

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Glyoxylate cycle

• Discovered by A. Krebs and H. L.korenberg.
• In plants, certain invertebrates, and some
  microorganism ( including E.coli and yeast)
  acetate can serve both as an energy rich fuel and
  as a source Of PEP for carbohydrate synthesis.
• In these organisms, enzymes of the glyoxylate
  cycle catalyze the net conversion of acetate to
  Succinate or other 4 carbon intermediate of the
  citric acid cycle
• 2Acetyl-CoA NAD 2H2O
  Succinate 2CoA
  NADH H
• Definition The glyoxylate cycle, a variation
  of the tricarboxylic acid cycle, is an anabolic
  pathway occurring in plants, bacteria, protista
  and fungi. The glyoxylate cycle centers on the
  conversion of aacetyl-CoA to Succinate for the
  synthesis of carbohydrates.
• Utilizes five of eight enzymes associated with
  the tricarboxylic acid cycle-
• Citrate synthase,
• Aconitase,
• Succinate dehydrogenase,
• Fumarase,
• Malate dehydrogenase.
• It occurs in glyoxysome (a kind of peroxisome) .
• Bypass Consist of two reactions namely-
• Splitting of isocitrate into succinate and
  glyoxylate,
• The conversion of glyoxylate into Malate.

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Steps of glyoxylate cycle
Step 1)
Condensation of acetyl-CoA with oxaloacetate
Acetyl-CoA produced either from acetate or by
- oxidation of fatty acids enters into TCA cycle
and is condensed with oxaloacetate to form
citrate.
ß
O O
O
Coo-


C SCoA CCoo-
CSCoA
CH2 HSCoA
H

CH3
CH2
CH2
HOCCoo-


Coo-
HOCCoo-
CH2




CH2
Coo-


Coo-
Citrate synthetase
Hydrolysis
H2O
Condensation
Coenzyme-A
Acetyl-CoA
Oxaloacetate
Citrate
Citryl-CoA (Enzyme-bound)
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Isomerization of citrate into isocitrate
Step 2)
Citrate is isomerized to isocitrate in TCA cycle.
(Enzyme-bound)
Step 3)
Cleavage of isocitrate into succinate and
glyoxylate
This is the bypass reaction of TCA cycle. Unlike
the breakdown of isocitrate dehydrogenase in TCA
cycle, here isocitrate is cleaved by isocitrate
lease to form succinate and glyoxylate.
Coo-

Coo-

CH2

HCH CHO


HCCoo-

HCH Coo-

HCOH

Coo-
Coo-
Isocitrate lyase
Glyoxylate
Succinate
Isocitrate
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Step 4)
Conversion of glyoxylate into malate
Succinate is further metabolized by TCA cycle.
Glyoxylate then condenses with a second molecule
of acetyl-CoA to form Malate catalyzed by Malate
synthase.
CHO SCoA
Coo-
H2O
Coo-
C0
HOCH CoASH H


CH3
HOCH




Coo-
Malate synthase
Coenzyme-A
Glyoxylate
Acetyl-CoA
L-malate
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Step 5)
Dehydrogenation of Malate to oxaloacetate
Malate is oxidized subsequently to oxaloacetate
by the enzyme Malate dehydrogenase. Oxaloacetate
then starts another turn of the cycle.
Coo- CO
NADH H CH2
Coo-
Coo- HoCH NAD
HCH Coo-
L-malate dehydrogenase
L-malate
Oxaloacetate
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Net reaction

• Net conversion of 2 acetyl-CoA to OAA
  (or succinate) instead of 4 CO2 as in TCA.
•  
•  The transport of a succinate from glyoxysome to
  mitochondrion depletes 4 carbon atoms
  from glyoxysome.  The source of these 4 carbon
  atoms are 2 acetyl-CoA.

Glyoxylate cycle
Acetyl-CoA
Succinate coenzyme-A
Glyoxylate cycle
2CH3COSCoA NAD 2H2O
CH2Coo-


2CoASH
NADH 3H

CH2Coo-
Acetyl-CoA
Coenzyme-A
Succinate
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Glyoxylate cycle
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Relationship between TCA and glyoxylate cycle

• In germinating seeds, the enzymatic
  transformations of decarboxylic acid and
  tricarboxylic acid occur in there intracellular
  compartments.
• Mitochondria,
• Glyoxysome,
• Cytoplasm.
• There is a continuous exchange of metabolites in
  these compartments they linked with each other in
  this way.

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Coordinate regulation of TCA and glyoxylate cycle
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Control and regulation phenomenon
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Significance of glyoxylate pathway

• It is a bypass reaction of TCA cycle,
• Oxaloacetate formed in this pathway can be used
  to synthesize glucose via gluconeogenesis,
• It occurs In bacteria when they are cultured in
  acetate rich carbon source (acetate being the
  sole source of carbon).
• This pathway is very significant in germinating
  seeds where the stored triglycerols (fat) is
  converted into sugar to meet the energy needs.
• When higher fatty acids are oxidized into
  acetyl-CoA enters into glyoxylate cycle.
• Regulation of isocitrate dehydrogenase activity
  determines the partitioning of isocitrate between
  the glyoxylate and citric acid cycles.
• When the enzyme is inactivated by phosphorylation
  (by a specific protein kinase) , isocitrate into
  biosynthetic reactions via glyoxylate cycle,
• When the enzyme is activated by dephosphorylation
  ( by specific phosphatase), isocitrate enters the
  TCA cycle and ATP is produced.

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