Sulphur | its biochemistry || how the reduction of Sulphur occurs |||| plants and other microorganisms

1. SHRI SHIVAJI ARTS, COMMERCE AND SCIENCE COLLEGE, AKOLA
DEPARTMENT OF MICROBIOLOGY
BIOCHEMISTRY OF SULPHUR REDUCTION
GUIDED BY:
DR. MONIKA THAKARE MAM
Presented By:
Shraddha M. Madghe
M.Sc. First Year

2. CONTENT
 INTRODUCTION
 BIOCHEMISTRY OF SULPHUR REDUCTION
 SIMILARITIES BETWEEN ASSIMILATORY AND DISSIMILATORY SULPHATE REDUCTION
 DIFFERENCES BETWEEN ASSIMILATORY AND DISSIMILATORY SULPHATE REDUCTION
 ASSIMILATORY SULPHUR REDUCTION
 DISSIMILATORY SULPHUR REDUCTION

3. Introduction
 Sulphur is most abundant and widely distributed element in the nature and found both in free as well as
combined state.
 Sulfur occurs in all living matter as a component of certain amino acids. It is abundant in the soil , in
proteins and through a series of microbial transformations, ends up as sulfates usable by plants.
 Sulphur enters in soil in the form of plants and animals residue.
 It is also added as chemical fertilizer in the form of ammonium sulphate.
 Sulphur is taken by plants in most oxidized state i.e. 𝑆𝑂4
2−
.
 Due to this, it is added in the soil in the form of ammonium sulphate which supply both nitrogen as well
as sulphur.

4. This involves two types:
1. Assimilatory Sulphur Reduction
2. Dissimilatory Sulphur Reduction
Biochemistry Of Sulphur Reduction

5. Similarities Between Assimilatory and Dissimilatory
Sulphate Reduction
 Both processes take place under anaerobic conditions.
 Also, the starting compound of both processes is sulphate.
 Furthermore, sulphate acts as the final electron acceptor in both processes.
 Moreover, both reduction processes are ATP dependent.
 In addition, the activation of sulphate to adenosine 5’ – phosphosulphate is common to both processes.
 Besides, they are enzyme-catalyzed reactions.
 Both reduction processes are carried out by prokaryotes, fungi and photosynthetic organisms.

6. Difference Between Assimilatory and Dissimilatory Sulphate
Reduction

7.  Assimilatory Sulphur Reduction largely restricted to plants and microorganisms where it provides reduced Sulphur
for the formation of amino acids , proteins, nucleic acids, and various Sulphur-containing coenzymes, begins with the
activation of sulphate through reaction with ATP to form adenosine 5'-phosphosulphate (APS) and adenosine 3'-
phosphate 5'-phosphosulphate (PAPS).
 Adenosine 5'-phosphosulphate [APS] which is formed during reaction with ATP.
 Adenosine 3'-phosphate 5'-phosphosulphate (PAPS) which is formed by further phosphorylation of APS at 3’ position
 PAPS acts on high energy sulphate donor in many esterification reactions and it is usually the substrate participating
in sulphate reduction.
Assimilatory Sulphur Reduction

8.  APS acts as high energy sulphate donor for many esterification reaction and it is usually participating in assimilatory
sulphate reduction. Assimilatory sulphate reduction involves NADPH linked 2 electrons.
 PAPS reduced to give sulphite and 3-phosphoadenosine monophosphate followed by NADPH linked with reduction
of sulphite to sulphide catalysed by sulphide reductase .
 Sulphide is incorporated into biomass which by condensation with serine generating system.
 Thus, the various transfer reactions of the assimilation sulphate reduction can be summarized as follows:

9. 1.Activation of Sulphate with ATP
Organic sulphate reacts with ATP in the presence of enzyme
sulfurylase to form adenosine 5'-phosphosulphate (APS) and pyrophosphate [PPi].

10. 2. Phosphorylation of APS
PAPS
APS kinase
+
APS ATP
+
ADP
 Adenosine 5'-phosphosulphate (APS) reacts with Adenosine triphosphate (ATP) to form 3’-
phosphoadenosine 5’-phosphosulphate(PAPS) and ADP.

11. 3.Reduction of sulphate
 The active sulfate of PAPS is subsequently reduced sulfite and adenosine-3,5-diphosphate in presence
of PAPS Reductase.
 This involves transfer of electron from NADH/NADPH to PAPS.
PAPS
+ 𝟐 ⅇ−
𝑺𝑶𝟑
𝟐−
ADP
PAPS Reductase
NADH+𝐻+

12. 4. Reduction of Sulphite
 NADPH further reduces sulphite to sulphide i.e. immediately incorporated as amino acid
𝑆𝑂3
2−
+3 𝑒− +3𝐻+
𝐻2𝑆 + 𝐻2𝑂
NADPH NAD
S𝑂2
2− 𝑅𝐸𝐷𝑈𝐶𝑇𝐴𝑆𝐸
S𝑂2
2− 𝑅𝐸𝐷𝑈𝐶𝑇𝐴𝑆𝐸
NADPH NAD
𝐻2𝑆 + 𝟑𝐻2𝑂
𝐻2 𝑆𝑂3
2−
+ 6 𝑒− + 6𝐻+

13. 5. The sulphide is immediately used in the formation of cysteine with serine under the catalytic action of enzyme
Serine Sulfyhydrase
Serine
sulfyhydrase
Serine Hydrogen Sulphide Cysteine Water

14.  Certain organism utilizes sulphate as terminal electron acceptor in anaerobic respiration with production of 𝐻2𝑆 which
is released to environment.
 Anaerobic bacteria carry out dissimilarity sulphate reduction.
 The responsible organism are Desulfovibrio vulgaris, Desulfometaculum, etc.
 When an obligatory anaerobic bacteria carry out dissimilatory Sulphur reduction they are refers to as sulphate reducers.
 In addition to anaerobic sulphate reducing bacteria , some species of bacillus, pseudomonas and saccharomyces are
also found to liberate 𝐻2𝑆 from 𝑆𝑂4 but do not plays a major role in dissimilatory sulphate reduction.
 The most common electron donors are pyruvate, lactate and molecular hydrogen.
 𝑆𝑂4 + 𝐻2𝑂 𝐻2𝑆 𝑂2
 The reduction of sulphate results in production of Hydrogen Sulphide
 4𝐻2 + 𝐻2𝑆 2𝐻2𝑂 + 20𝐻−
Dissimilatory Sulphate Reduction
+
𝑆𝑂4
2− +

15. Mechanism Of Sulphate Reaction
 Dissimilatory sulfate reduction is a form of anaerobic respiration that uses sulfate as the
terminal electron acceptor to produce hydrogen sulfide.
 The mechanism of assimilatory sulphur reduction[ASR] is similar to that of dissimilatory sulphur
reduction[DSR].
 As activated sulphur participates in this process and APS reduction, Sulphide reduction, Thiosulphate
reduction are probably involved in cycle.
 The reduction of sulphate regenerated 𝑆𝑂3
2−
,thiosulphate i,.e. composite molecules containing sulphur
atoms of different valences and 𝐻2𝑆2.
 Although , 𝑆𝑂3reduction differ in mechanism in assimilatory and dissimilatory process. Sulphide
reductase is the similar enzyme present in ASR and DSR.
 In DSR process specific cytochrome donate electron at various reductase stage rather than NADPH and
ATP is generated by this electron transfer reduction.
 The exact mechanism is not completely understood.

17. REFERENCES
1. https://www.researchgate.net/figure/Prokaryotic-assimilatory-and-dissimilatory-
pathways-of-sulfate-reduction-adapted-from_fig1_342434176
2. https://www.differencebetween.com/difference-between-assimilatory-and-dissimilatory-
sulphate-reduction/
3. https://en.wikipedia.org/wiki/Dissimilatory_sulfate_reduction
4. https://pubmed.ncbi.nlm.nih.gov/398767/
5. https://www.sciencedirect.com/topics/chemistry/sulfur-cycle
6. https://www.britannica.com/science/sulfur-cycle

18. THANK YOU!