The document discusses nitrogen metabolism in plants, focusing on ammonium assimilation and the pathways for incorporating ammonia into organic compounds. It describes two main routes for ammonia assimilation involving glutamine synthetase and glutamate dehydrogenase, detailing the enzymatic processes required for nitrogen incorporation and the energy dynamics involved. Furthermore, it highlights the importance of nitrogen sources in biosynthesis and the regulation of glutamine synthetase by various metabolites.

1. DR. VIBHA KHANNA
Asso. Prof. (Botany)
SPC GOVERNMENT COLLEGE
AJMER (Rajasthan)

2. PLANT BIOCHEMISTRY
• BLOCK 3. : Nitrogen Metabolism
– PRESENTATION 5:
Ammonium Assimilation
{Ammonium assimilation is defined as the
incorporation of ammonium ions into organic
compounds}

3. Nitrogen Assimilation in Plants
• N assimilation is energetically expensive: 2 to
15% of plant’s energy production
• Most plants can assimilate either nitrate or
ammonia
• NH4
+ formed as a result of nitrate reduction or
N2 fixation is first incorporated into amino
acids
• In the case of ammonium as N‐source, an
immediate incorporation into amino acids
occurs in the root, and no significant amounts
of ammonium have ever been discovered in
the xylem sap.

4. Nitrogen assimilation is a series of reactions that
coordinate C and N metabolism
N2 NO3
-Biologically unavailable! (via nitrification by
bacteria or fertilizer)
NH4
+
Biologically available but toxic!
ATP
ADP + Pi
NADPH
NADP + H+
dinitrogenase Nitrate reductase
+ nitrite reductase
Glutamate
α-ketoglutarate
(C skeleton from ______ )
Glutamate
Glutamate
synthase cycle
For export to N sinks
ATP + NADH
ADP + Pi + NAD+
• inhibits N2ase
• Uncouples ATP
synthesis from e-
transport
Thus, plants use the
Glu synthase cycle
to rapidly
assimilate N into
organic molecules

5. Routes Of Ammonia Assimilation
• Ammonia assimilation refers to conversion of ammonia to organic
nitrogen
• Two possible routes of ammonia assimilation have been postulated,
the first involving one enzyme, and the second involving two
enzymes:
• Route 1: The reductive amination of a keto acid to yield an amino
acid directly. The enzyme widely distributed in plant tissues is
glutamate dehydrogenase (E.C.1.4.1.3).
• Route 2: The initial incorporation of ammonia into the amide
position of glutamine by the enzyme glutamine synthetase
(E.C.6.3.1.2).
• The amide-amino group can then be transferred to the α position of
α-ketoglutarate by the enzyme glutamine: α -ketoglutarate amino
transferase NAD(P)H oxidizing (E.C.2.6.1.53) or reduced ferredoxin
oxidizing (E.C.1.4.7.1). The enzyme has been given the trivial name
glutamate synthase or the acronym GOGAT.

6. The two routes for Ammonium assimilation
i,e,. Ammonia can be
assimilated through either
• The glutamine
synthetase/glutamate
synthase cycle (GS/GOGAT).
{For each turn of the
GS/GOGAT cycle, one
molecule of ammonia is
assimilated into glutamate.}
• Or glutamate dehydrogenase
(GDH)

7. Routes Of Ammonia Assimilation
{coordination of C and N metabolism}

8. The Glutamine Synthetase (GS)/Glutamate
Synthase (GOGAT) Cycle
• The incorporation of ammonium into the pool of
N‐containing molecules is first catalysed by the glutamine
synthetase (GS)/glutamate synthase (GOGAT) cycle.
• In this metabolic pathway GS (EC 6.3.1.2) catalyses the
amidation of glutamate to generate glutamine at the
expense of ATP hydrolysis.
• The second enzyme, GOGAT (EC 1.4.7.1; 1.4.1.14), is
responsible for the reductive transfer of amide N to
2‐oxoglutarate for the generation of two molecules of
glutamate, one of which is recycled for glutamine
biosynthesis.
• This N assimilatory pathway is driven by energy and
reducing power derived from photosynthesis or from the
catabolism of protein and carbon reserves.

9. The Glutamine Synthetase (GS)/Glutamate
Synthase (GOGAT) Cycle
• Glutamate and glutamine
are the N donors for the
biosynthesis of major N
compounds in plants
• N can be channeled to the
biosynthesis of aspartate
and asparagine catalysed by
aspartate amino transferase
(AspAT) (EC 2.6.1.1) and
asparagine synthetase (AS)
(EC 6.3.5.4), respectively
• Fixed N is primarily
exported via xylem, in most
plants in form of the amides
glutamine or asparagine.

10. The Glutamine Synthetase (GS)/Glutamate
Synthase (GOGAT) Cycle

11. Ammonium Assimilation
*all organisms contain gln synthetase
*only in bacteria and plants
• Animals do not contain glutamate
synthase. They maintain high
glutamate levels via transamination
reactions with other amino acids
and some amounts of glutamate
from:

12. Fate of Glutamine & Glutamate
• Glutamate is the primary source of amino groups for amino acids
• Glutamine is the primary nitrogen source for other synthesis
processes

13. Allosteric regulation of glutamine synthetase
• The enzyme undergoes cumulative regulation by six end products of
glutamine metabolism. Alanine and glycine probably serve as
indicators of the general status of amino acid metabolism in the cell.

14. N assimilation is reliant on a steady supply of C !!
Presence of Nitrate Affects Carbohydrate Metabolism!!
• The carbohydrate metabolism is affected by the presence of
nitrate which shifts the relation between starch synthesis and
sucrose synthesis in favour of the latter. This finally results in
the production of organic acids such as oxoglutarate as an
acceptor for reduced N in the GS/GOGAT cycle or malate as a
counter‐ion in nitrate uptake and reduction.