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12.1: N and S assimilation from inorganic form

NITROGEN REDUCTION AND ASSIMILATION

Ammonia Synthesis in Plants, fungi, and some microorganisms:

Nitrate Reductase

\[\mathrm{NO_{3}^{-}+NADH+H^{+} \Rightarrow NO_{2}^{-}+NAD^{+} + H_{2}O}\]

Nitrite Reductase

\[\mathrm{NO_{2}^{-}+6Fd_{red}+8H^{+} \Rightarrow NH_{4}^{+}+2H_{2}O+6Fd_{ox}}\]

Biological Nitrogen Fixation by microorganisms:

Nitrogenase

\[\mathrm{N_{2}+6e^{-}+6H^{+}+~16ATP+16H_{2}O \Rightarrow 2NH_{3}+16ADP+16Pi+16H^{+}}\]

Major pathway for ammonia assimilation(=net assimilation in plants, fungi and microorganisms):

Glutamine synthetase (GS)

\[\mathrm{NH_{3}+Glu+ATP \Rightarrow Gln+ADP+Pi}\]

Glutamate synthase (= GOGAT) 

(not found in animals)

\[\mathrm{Gln+\alpha KG+NAD(P)H+H^{+} \Rightarrow 2Glu+ NAD(P)^{+}}\]

GS is present in animals as well.  GS is also the enzyme responsible for the synthesis of the Gln incorporated into protein in all organisms.  In some plants and in humans, Gln is used for intercellular transport of N in a non-toxic form.  Ala is also used in mammals for the transport of N from the muscle to the liver.