Nitrogen Assimilation

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Nitrogen Assimilation

• Ammonium taken up from the soil can be
  assimilated into amino acids in cytoplasm or
  transported into the plastid.
• Glutamine synthetase (GS) catalyzes the
  conversion of NH4 glutamate to glutamine
• NH4 ATP glutamate ? glutamine

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Glutamine Synthetase

• Isoforms present in both the cytoplasm and the
  plastid
• Function as an octomeric enzyme (plastid and
  cytosolic monomers can mix match)
• The plastid GS form is found predominately in
  leaves
• Cytosolic GS is main form in roots. Why?

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Glutamine Synthetase

• Km for GS is 50 mM
• Assures almost complete assimilation of cellular
  ammonium
• Why is this critical? (hint less than 20 of GS
  activity is associated with nitrogen
  assimilation)
• GS is inhibited by the herbicide glufosinate

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GS/GOGAT

• GS works in concert with a second enzyme in
  ammonium assimilation
• GOGAT glutamine-oxoglutarate amino transferase
• GOGAT catalyzes the transfer of an amino group
  from glutamine to a-ketoglutarate to
  form two molecules of glutamate

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GOGAT

• Two GOGAT isoforms exist.
• They are distinquished by different cofactors
• Ferridoxin and NADH are used by the different
  GOGAT isoforms
• Fdx-GOGAT is the major form in leaves, and
  NADH-GOGAT is found mainly in non-PS tissues.
  Why?

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Glutamate Dehydrogenase

• GDH exists in all living organisms
• GDH catalyzes both the synthesis and degradation
  of glutamate
• glutamate ? a-ketoglutarate NH4
• 2 forms exist
• NADH - mitochrondrial isoform
• NADPH - chloroplastic isoform

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Glutamate Dehydrogenase

• GDH is an octomeric protein
• Has a very large Km (10mM for NH4)
• In plants the major role for GDH is glutamate
  catabolism
• Involved in only under conditions of ammonium
  toxicity

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Aspartate Aminotransferase

• Aspartate is a major transport form of both
  carbon and nitrogen between cells in plants
• Glutamate OAA ? Aspartate a-KG
• AAT is found in cytoplasm, mitochondria,
  chloroplasts and peroxisomes.

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Regulation of Nitrogen Uptake

• Regulation is complex and is exerted at various
  stages in the process
• Nitrate/Ammonium presence in soil regulates the
  accumulation of transporters
• Nitrate in the root induces expression of genes
  encoding NR and NiR

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Regulation of Nitrogen Assimilation

• NR NiR are also regulated at the enzyme level
  (see handout)
• Regulation of GS/GOGAT/GDH/AAT is at multiple
  levels
• Photorespiration, carbon availability. lght