Sulfur is an essential nutrient that is incorporated into amino acids like cysteine and methionine. Sulfate is absorbed by plant roots and transported to leaves, where it undergoes a multi-step reduction process to form cysteine. First, sulfate is activated by ATP and converted to APS. APS is then reduced to sulfite and sulfide using electrons from glutathione and ferredoxin. Sulfide reacts with O-acetylserine to form cysteine. Cysteine can then be used to synthesize other sulfur-containing compounds like methionine and the antioxidant glutathione, which transports sulfur within the plant.

1. SULPHATE REDUCTION AND
INCORPORATION OF SULPHUR
INTO AMINO ACIDS
GLORY K S
2019520202

2. SULPHUR
 Sulfur is an essential macronutrient required for plant
growth. It is primarily used to synthesize cysteine,
methionine and numerous essential and secondary
metabolites derived from these amino acids.
 Disulfide bridges in proteins play structural and
regulatory roles.
 Sulfur participates in electron transport through iron–
sulfur clusters.
 The catalytic sites for several enzymes and
coenzymes, such as urease and coenzyme A, contain
sulfur.

3.  Secondary metabolites that contain sulfur range from
the rhizobial Nod factors antiseptic alliin in garlic and
anticarcinogen sulforaphane in broccoli.
 Sulfate enters a plant primarily through the roots by
way of an active uptake mechanism.
 Gaseous sulfur dioxide readily enters the leaves,
where it is assimilated.

4. SULPHATE ASSIMILATION
 Conversion of inorganic sulphur compounds
such as SO4
2- into sulfur-containing organic
compounds such as cysteine by plants is called
as sulfur or sulfate assimilation
 Sulfate assimilation takes place chiefly in leaves
in chloroplasts. The sulfate absorbed by roots
from soil solution is translocated through xylem
to shoots for assimilation. To some extent sulfate
assimilation may also occur in roots in
proplastids

5.  Sulfur in sulfate is present in highly oxidised state
with six positive charges while in cysteine it is present
in reduced state with four negative charges.
 Therefore, conversion of sulfate into cysteine is a
reduction process that is energy dependent and
requires ATP.
 It entails transfer of 10 electrons which are provided
by reduced glutathione (GSH), reduced ferredoxin
(Fd. red) and O-Acetylserine

7. SULPHATE REDUCTION

9. STEPS INVOLVED
 In the first step, sulfate is activated by ATP in the
presence of the enzyme ATP- sulfurylase to form
APS (Adenosine-5′-phosphosulfate) and
pyrophosphate (PPi). Mg++ ions are required in this
reaction
SO4
2- + Mg-ATP → APS + PPi
 PPi so formed is quickly hydrolysed by the enzyme
inorganic pyrophosphatase to yield two molecules of
inorganic phosphate (2Pi).
PPi + H2O → 2Pi

10.  The APS is now reduced . The enzyme APS-
reductase transfers two electrons from two molecules
of reduced glutathione (2 GSH) to produce sulfite
(SO3
2-). Glutathione is oxidised (GSSG).
APS + 2 GSH → SO3
2- + 2 H+ + GSSG + AMP

11.  Sulfite (SO3
2-) is now reduced to form sulfide (S2-) in
the presence of the enzyme sulfite reductase. This
reduction requires six electrons which are provided by
6 mols of reduced ferredoxin (Fd.red).
SO3
2- + 6 Fd.red → S2- + 6 Fd.oxi

12.  In the last step, sulfide reacts with O-acetylserine
(OAS) under the influence of the enzyme OAS-
thiolyase to form the amino acid cysteine and acetic
acid. There is transfer of two electrons in this
reduction, from OAS to sulfide.
OAS + S2- → cysteine + acetate
(The OAS is formed from serine and acetyl-CoA in
the presence of the enzyme serine acetyl transferase.)

13. Formation of other sulphur containing
compounds
 After the synthesis of cysteine, another sulfur-
containing amino acid methionine is synthesized from
it.
 Thereafter, other sulfur-containing organic
compounds are synthesized from these two amino
acids.

14. SYNTHESIS OF METHIONINE FROM CYSTEINE:

15. Various steps of synthesis of methionine
from cysteine are as follows:
(i) In the first step, cysteine reacts with O-
phosphohomoserine in the presence of the enzyme
cystathionine synthase to form cystathionine.
Inorganic phosphate (Pi) is released in the reaction.
(ii) In the next step, the enzyme cystathionine β-Iyase
splits cystathionine into homocysteine, NH3 and
pyruvate.
iii) Finally, homocysteine is methylated by the
enzyme methionine synthase to form methionine.

16. Glutathione
 GSH (y-glutamyl-cysteinyl-glycine) and GSSH,
reduced and oxidized forms of glutathione,
respectively, are readily interchangeable.
This tripeptide (y -Glu-Cys-Gly) is the dominant non-
protein thiol in plants and can play a role in regulating
the uptake of So4 by plant roots.
 Glutathione accumulates after excess feeding of sulfur
compounds if the normal regulatory control
mechanisms are circumvented , suggesting that
glutathione functions as a storage pool for excess
cysteine.

17. REDUCED GLUTATHIONE

18. BIOSYNTHESIS OF GLUTATHIONE
 GSH is synthesized by a γ-glutamyl-cysteine
synthase and has been characterized from Nicotiana
tabacum .
 This compound is condensed with glycine by the
glutathione synthase, forming GSH.

20.  The sulfur-containing organic compounds produced
after sulfur (sulfate) assimilation are exported to
other parts of the plant, such as root and shoot
apices and fruits, through phloem chiefly as
glutathione.
 It is believed glutathione also contribute absorption
of sulphate by roots and its assimilation by shoots.