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

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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

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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

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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

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

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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

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Glycogenin stays bound to the single reducing
end of glycogen as glycogen synthase takes over
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Glycogen synthase and glycogen phosphorylase are
reciprocally regulated

• Details in text

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Starch synthesis

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

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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

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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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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

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Rubisco catalyses the addition of CO2 to RuBP and
cleavage to 3-phosphoglycerate
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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

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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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Transketolase reactions of the Calvin cycle
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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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An antiporter exchanges Pi with triose phosphates
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Regulation of the Calvin cycle (Rubisco)
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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

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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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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