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A Handbook of Biology
1
2
Nitrogenase is highly sensitive to the molecular oxygen. So, it
requires anaerobic conditions to protect from oxygen. For this,
leg-haemoglobin acts as an oxygen scavenger. (AIPMT 2011)
Ammonia synthesis needs high input energy (8 ATP for
each NH3). It is obtained from the respiration of host cells.
3
These microbes live as aerobes under free-living conditions
(where nitrogenase is not operational), but during N2 fixing
events, they become anaerobic (to protect nitrogenase).
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At physiological pH, the NH3 is protonated to form NH4
+ (ammonium) ion.
Most of the plants can assimilate nitrate and NH4
+. But NH4
+ is quite toxic
to plants and so cannot accumulate in them.
In plants, NH4
+ is used to synthesise amino acids by 2 ways:
a. Reductive amination: In this, ammonia reacts with α-ketoglutaric
acid to form glutamic acid.
Glutamate
dehydrogenase
!-ketoglutaric
acid
NH4
+
NADPH
y
g
Glutamate
H2O
NADP
b. Transamination: It is the transfer of amino group (NH2) from
one amino acid to the keto group of a keto acid in presence of
transaminase enzyme. Glutamic acid is the main amino acid from
which the transfer of NH2 takes place and other amino acids are
formed through transamination. Enzyme transaminase catalyses all
such reactions.
H
Amino-donor
Amino-acceptor
H
O
O
+
+
NH3
+
NH3
+
R1 — C — COO–
R2 — C — COO–
R2 — C — COO–
R1 — C — COO–
Asparagine & glutamine are
most important amides found in
plants.They are structural part of
proteins. They are formed from
2 amino acids (aspartic acid &
glutamic acid) by addition of
another amino group to each.
The hydroxyl part of the acid is
replaced by another NH2 radical.
Since amides contain more
nitrogen than the amino acids, they
are transported to other parts of the
plant via xylem vessels. In addition,
along with the transpiration stream
the nodules of some plants (e.g.,
soyabean) export the fixed nitrogen
as ureides. These compounds also
have particularly high nitrogen to
carbon ratio.