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Biosynthesis of carbohydrate polymers

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Biosynthesis of carbohydrate polymers Starch in plants, glycogen in vertebrates These polymerization reactions utilize sugar nucleotides as activated substrates – PowerPoint PPT presentation

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Title: Biosynthesis of carbohydrate polymers


1
Biosynthesis of carbohydrate polymers
  • Starch in plants, glycogen in vertebrates
  • These polymerization reactions utilize sugar
    nucleotides as activated substrates

2
Why sugar nucleotides?
  • Their formation is metabolically irreversible,
    contributing to the irreversibility of pathways
    in which they are intermediates
  • Nucleotide moiety provides potential interactions
  • The substrate is activated because the
    nucleotidyl group is a good leaving group
  • Tags the substrate, marking it for storage

3
Glycogen synthesis
  • Glucose 6-phosphate is isomerized to glucose
    1-phosphate by phosphoglucomutase
  • UDP-glucose pyrophosphorylase converts glucose
    1-phosphate to UDP glucose using UTP and
    producing pyrophosphate
  • Glycogen synthase attaches the UDP-glucose to the
    nonredcuing end of a branched glycogen molecule

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Making bonds in glycogen
  • Glycogen synthase requires as a primer an (a1-4)
    poly glucose chain or branch having at least
    eight glucose residues.
  • Glycogen synthase cannot make the (a1-6) bonds
    found at branch points these are formed by
    glycosyl (4-6) transferase

6
Branching glycogen
  • Glycosyl (4-6) transferase catalyzes the transfer
    of a terminal fragment of six or seven glucose
    residues from the non-reducing end of a glycogen
    branch (having at least 11 residues) to the C6
    hydroxyl group of a glucose residue at a more
    interior position of a glycogen molecule,
    generating a new branch

7
  • Branches can subsequently be modified by glycogen
    synthase
  • Branches increase solubility of glycogen

8
Where does the primer come from?
  • Glycogenin builds primers for glycogen synthase
  • Tyrosine-194 of this protein is the the site of
    covalent glucose attachment (via UDP-glucose)
  • This modified glycogenin binds to glycogen
    synthase, and the glycogen-bound glucose molecule
    is extended up to seven residues using UDP-glucose

9
Glycogenin stays bound to the single reducing
end of glycogen as glycogen synthase takes over
10
Glycogen synthase and glycogen phosphorylase are
reciprocally regulated
  • Details in text

11
Starch synthesis
  • Analogous mechanism to glycogen synthase, but
    starch synthase uses ADP-glucose

12
UDP-sugars are used in synthesis of other
biomolecules
  • UDP-glucose for sucrose synthesis
  • UDP-galactose for lactose synthesis
  • UDP-glucose for vitamin C
  • UDP-glucosamine for peptidoglycan

13
A discussion of carbohydrate biosynthesis must
encompass photosynthesis (chapter 20)
  • Photosynthetic organisms assimilate or fix CO2
    via the Calvin cycle
  • This cycle has three stages
  • Fixation making 3-phosphoglycerate
  • Reduction generating glyceraldehyde 3-phosphate
  • Regeneration making ribulose 1, 5 bisphosphate
    from triose phosphates

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15
Stage I is mediated by Rubisco
  • Rubisco is considered the most abundant protein
    on Earth (located in chloroplast)
  • Rubisco stands for ribulose 1,5-bisphosphate
    carboxylase/oxygenase

16
Rubisco catalyses the addition of CO2 to RuBP and
cleavage to 3-phosphoglycerate
17
Stage II
  • The first step is catalyzed by 3-phosphoglycerate
    kinase, which converts 3-phosphoglycerate to 1,3
    bisphosphoglycerate using ATP
  • This compound is reduced using NADPH by
    glyceraldehyde 3-phosphate dehydrogenase to
    glyceraldehyde 3-phosphate

18
Stage II (cont)
  • DHAP is formed by triose phosphate isomerase then
    a portion transported to the cytosol for either
    glycolytic metabolism or production of starch or
    sucrose as a storage and transport media

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Each CO2 fixed consumes a molecule of RuBP
  • Therefore, RuBP must be regenerated.
  • This is accomplished by a pathway including
    variable number carbon intermediates reminiscent
    of non-oxidative branch of PPP
  • Enzymes included in this stage include
    transaldolase and transketolase

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22
Transketolase reactions of the Calvin cycle
23
The result of the Calvin cycle
  • The net result is the conversion of three
    molecules of CO2 and one molecule of phosphate
    into a molecule of triose phosphate. (One
    molecule of glyceraldehyde 3-phosphate is the net
    product of this carbon assimilation pathway)
  • This result comes from (uses) 6 NADPH and 9 ATP
    supplied by photosynthesis (light)

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25
An antiporter exchanges Pi with triose phosphates
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27
Regulation of the Calvin cycle (Rubisco)
28
Four essential Calvin cycle enzymes are regulated
by light
  • Ribulose 5-phosphate kinase
  • Fructose 1,6-bisphosphatase
  • Sedoheptulose 1,7 bisphosphatase
  • Glyceraldehyde 3-phosphate dehydrogenase
  • Regulation mediated by disulfide bond formation
    and disruption

29
Rubisco is an oxygenase
  • Evolution has made Rubisco somewhat of an
    inefficient enzyme as it has a difficult time
    discriminating between O2 and CO2
  • Using oxygen results in a metabolically useless
    molecule, phosphoglycolate
  • Carbon is salvaged from phosphoglycolate by
    photorespiration

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32
Plants can minimize photorespiration
  • Photorespiration is wasteful
  • Tropical plants employ a more complex pathway for
    fixing CO2
  • This pathway fixes CO2 on PEP using PEP
    carboxylase and subsequently donates the CO2 to
    Rubisco
  • These are known as C4 plants, in contrast to C3
    plants which only use the Calvin cycle
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