The Lotus japonicus Genome

synthetase (EC 6.3.1.2) catalyzes the biosynthe-
sis of L-glutamine from L-glutamate, ATP, and
NH 4 +according to the following reaction, which
also requires Mg2+cations as cofactors:

LglutamateþATPþNH 4 þ!

GS

Lglutamine

þADPþPi

Subsequently, the glutamate synthase enzyme
(EC 1.4.7.1 or EC 1.4.1.14) catalyzes the transfer
of the amide group of glutamine into 2-oxoglu-
tarate, yielding two molecules of glutamate, a
reaction also requiring two electrons coming from
either reduced ferredoxin (Fd-GOGAT) or pyri-
dine nucleotides (NADH-GOGAT), as follows:

Lglutamineþ 2 oxoglutarateþ 2 e

þ 2 Hþ!

GOGAT

2 Lglutamate

The global balance of the consecutive action
of these two enzymes forms the GS-GOGAT
cycle by which one of the two molecules of
glutamate formed by the GOGAT would be used
by the reaction of GS. Consequently, the GS-
GOGAT pathway results in the net formation of
one molecule of L-glutamate at the expense
of one molecule of 2-oxoglutarate, one molecule
of NH 4 +and one molecule of ATP as follows:

2 oxoglutarateþNHþ 4 þ 2 eþ2Hþ

þATP!

GSGOGAT

LglutamateþADPþPi

11.2.2 Glutamate Dehydrogenase

In addition to GS and GOGAT, which catalyze
irreversible reactions, a third enzyme, glutamate
dehydrogenase (GDH; EC 1.4.1.2/4), catalyzes a
reversible amination/deamination reaction, which
could lead to either the synthesis or the catabo-
lism of glutamate, according to the following
equation:

2 oxoglutarateþNH 4 þþNADðÞPH

þHþ$

GDH

LglutamateþNADðÞPþ

The role of GDH in glutamate catabolism is

quite well established. However, the possible

anabolic role of GDH for the assimilation of

ammonium has been the subject of continuous

controversy because most lines of evidence

support the idea that glutamate biosynthesis takes

place through the GS-GOGAT pathway,

although a role for GDH under different plant

stress situations has been also reported. It has

been proposed that GDH has an important role in

terms of metabolic signaling in relation to the

partitioning of C and N assimilates, most likely

that GDH contributes to the control of the

homeostasis of leaf glutamate, a process of cru-

cial importance (Fontaine et al. 2012 ).

11.2.3 Asparagine Metabolism:

Asparagine Synthetase

and Asparaginase

In most temperate legumes, it is proposed that

asparagine, rather than glutamine, is the principal

molecule used to transport reduced nitrogen

within the plant, in contrast to many other plant

species (Credali et al. 2013 ). This is the case for

L. japonicus where it has been shown that

asparagine can account for almost 90 % of

the nitrogen transported from root to shoot

(Waterhouse et al. 1996 ).

Asparagine synthetase (AS, EC 6.3.5.4) is the

main enzyme in charge of asparagine biosyn-

thesis in plants. This enzyme catalyzes the

transfer of the amide group from glutamine to

aspartate in an ATP-dependent reaction:

LglutamineþLaspartate

þATP!

AS

LglutamateþLasparagine

þAMPþPPi

It has also been proposed that the enzyme can

use high concentrations of ammonia directly as

substrate, but this is not clearly demonstrated

(Lea et al. 2007 ).

Considering that asparagine is a nitrogen

transport compound inLotus, asparagine break-

down is also a process of crucial importance for

11 Genes Involved in Ammonium Assimilation 119