Download Explain the biological transformation of sulphur

OJELADE, Tunmise
14/SCI03/012
BCH 413
Assignment: Explain the biological transformation of
sulphur
Microbial transformations of sulphur have counterparts in transformations of nitrogen. Both
sulphide and ammonia are products of decomposition of organic compounds. Both are oxidized
by autotrophic bacteria, as are other incompletely oxidized inorganic compounds of sulphur and
nitrogen. Sulphate and nitrate are reduced to sulphide and ammonia, respectively, by
microorganisms under anaerobic conditions. Elemental sulphur and nitrogen are transformed by
microorganisms but there is less specificity in the reactions of sulphur than those of nitrogen and
sulphur undergoes a greater variety of reactions. Furthermore, some microorganisms are
supported by the energy released from oxidation of sulphur but none has been reported that
similarly utilizes nitrogen. The major biological transformations of sulphur are:
1. Assimilation of sulphur
2. Decomposition of organic sulphur
3. Oxidation of sulphur compounds
4. Reduction of sulphate and other compounds inorganic sulphur compounds
Assimilation of Sulphur
Many microorganisms satisfy their sulphur requirements from sulphate, but some require
preformed sulphur-containing amino acids or vitamins. Most of the cell sulphur is in organic
compounds. From exhaustive studies of the sulphur metabolism of Escherichia coli, it was found
that the cells contained 1.12% sulphur and that 95% of the sulphur was in the amino acids
cysteine, cystine, and methionine. The methionine is entirely in the protein, whereas one half of
the cystine and cysteine is in protein and the other half is associated with glutamic acid and
glycine in glutathione. There were equal amounts of the sulphur in the amino acids. The culture
could meet its sulphur requirements from sulphate and also from other inorganic and organic
sulphur compounds. About one third of the sulphur converted to organic compounds was
released into the medium. The course of events in conversion of sulphate to amino acids is
obscure. The sulphate is first reduced to sulphite, then to sulphoxylate, dimerized to thiosulphate,
and finally became organically bound and transformed to cysteine. With certain microorganisms,
as well as with higher organisms, there may be an interconversion of cystine and methionine.
Decomposition of Organic Sulphur Compounds
Various sulphur products are formed in the breakdown of organic sulphur compounds. They vary
with the compounds attacked, the oganisms concerned, and the environmental conditions. In
animal metabolism, taurine, sulphate, and thiosulphate are produced from the sulphur amino
acids. The most commonly reported product from the bacterial breakdown of the sulphur of
cysteine and cystine is sulphide but other products were also detected. A single organic sulphur
compound, therefore, may yield different sulphur products on dissimilation by different
microorganisms. The aerobic decomposition of cystine by a mixed population such as that in
soils results in transformation of practically all of the sulphur to sulphate and there is an increase
in acidity proportional to the amount of cystine transformed. In poorly buffered substrates the pH
may drop sufficiently to inhibit growth of most bacteria. The acidity results not only from the
sulphuric acid produced, but also from the nitric acid that originates from the ammonia released
from the amino acid. Anaerobic decomposition of methionine by Clostridium tetanomorphum
resulted in formation of mercaptans.
Oxidation of Sulphur Compounds
Inorganic sulfur compounds including hydrogen sulfide, thiosulfate, polythionates, sulfites and
also elemental sulfur are produced by microorganisms in the aerobic and anaerobic
transformation of organic and inorganic compounds of sulfur or waters rich in waste materials.
These can be oxidized by various microorganisms such as photosynthetic sulfur bacteria,
colorless sulfur bacteria with relatively large cells sometimes producing filaments, and colorless
bacteria of the genus Thiobaczllus. These three groups of bacteria are autotrophic. Various
heterotrophic microorganisms, including bacteria, actinomycetes, and filamentous fungi also
oxidize some of the inorganic sulfur materials.
Reduction of Sulphate and Other Inorganic Sulphur Compounds
Of particular importance among the reductions of inorganic sulphur compounds is the reduction
of sulphate that is brought about principally by a specific group of bacteria. Reduction of
incompletely reduced materials such as sulphite, thiosulphate, tetrathionate, and elemental
sulphur is much more common and many bacteria and fungi are concerned in these
transformations. Sulphide is produced from thiosulphate and sulphite by thermophilic bacteria.
The reduction of tetrathionate to thiosulphate was sufficiently specific for certain intestinal
bacteria to serve as a diagnostic test. The reduction is essentially the reversal of the oxidation of
thiosulphate to tetrathionate common to heterotrophic microorganisms. Elemental sulphur is
reduced to sulphide by all microorganisms more or less rapidly. This is due in part to sulfhydryl
groups of the cell compounds and to other as yet undefined reactions.