Sulphur metabolism

Sulphur metabolism is vital for all living organisms as it is a constituent of a number of essential organic molecules like cysteine, methionine, Coenzyme A, and iron sulphur clusters. These compounds are involved in a number of essential cellular processes like protein biosynthesis or the transfer of electrons and acyl groups. Sulphur, therefore, is an essential component of all living cells. The importance of sulphur is well-represented by the sulphydryl (thiol) functional group, lying at the centre of many chemical reactions in biology. Thiol-based reactions have diverse biological functions: thiols in thioredoxins provide reductive power for the synthesis of biological molecules; thiols in coenzyme A facilitate the oxidation of pyruvate and fatty acids to generate energy for living cells; and thiols in glutathione and mycothiol are involved in detoxifying hazardous molecules, as well as maintaining the redox balance of living cells. Additionally, sulphur containing molecules function as messengers in intracellular and intra-species communication. Sulphur is also a constituent of many other biomolecules like cysteine, methionine, biotin, lipoic acid, molybdopterin, thionucleosides in tRNAs, and thiamine.

Bacteria

Sulphur metabolic pathways of pathogenic bacteria, such as mycobacteria, hold importance both for its biological implications as well as discovering drug targets against enzymes in these pathways. In recent times, the endeavour to map the sulphur metabolic pathways has been greatly facilitated by the emerging information drawn from genome sequencing.cite book |author= Parish T, Brown A (editors)| year=2009 |title=Mycobacterium: Genomics and Molecular Biology | publisher=Caister Academic Press | id= ISBN 978-1-904455-40-0]

"Mycobacterium"

Sulphur metabolism in mycobacteria plays a role in the pathogenesis of the insidious human pathogen, "Mycobacterium tuberculosis". The other mycobacterial species are "Mycobacterium leprae", which causes leprosy in humans, "Mycobacterium bovis" which causes tuberculosis in cattle, "Mycobacterium avium" which causes disease in immunocompromised individuals' "M. bovis" bacille Calmette–Guérin (BCG), which is an attenuated strain of "M. bovis" used as a vaccine strain, and "Mycobacterium smegmatis", which is a saprophytic non-pathogenic species used extensively as a laboratory model for mycobacterial research.cite book |author= Senaratne RH, Dunphy KY|year=2009|chapter=Sulphur Metabolism in Mycobacteria|title=Mycobacterium: Genomics and Molecular Biology|publisher=Caister Academic Press|id= ISBN 978-1-904455-40-0]

"Corynebacterium"

An external supply of sulphur-containing compounds is essential for many eucaryotes and, due to their scarcity in many foods and feeds, their biosynthesis is of great industrial interest. Therefore, the metabolism of sulphur in "Corynebacterium glutamicum" has been studied. Besides the pathways to obtain and utilize sulphur from the environment, the reactions leading to and from the sulphur-containing amino acids cysteine and methionine have been analyzed in great detail, revealing a number of so far unique metabolic routes. In addition, the regulation of sulphur metabolism has been analyzed on the transcriptional as well as on the enzymatic level, revealing the presence of at least three transcriptional regulators and a high number of feed-back inhibitions of key enzymes.cite book | author = Burkovski A (editor). | title = Corynebacteria: Genomics and Molecular Biology | publisher = Caister Academic Press | year = 2008 | id = ISBN 978-1-904455-30-1 ]

"Treponema"

"Treponema denticola" is a species of bacterium that can become an opportunistic pathogen in the mixed microflora that colonizes the space between the teeth and inflamed gingival tissues (periodontal pocket). The generation of volatile sulphur containing compounds from amino acid metabolism by the enzyme cystalysin is cytotoxic and may be considered one of the virulence determinants of "T. denticola".cite book |author= Ellen RP|year=2006|chapter=Virulence Determinants of Oral Treponemes|title=Pathogenic Treponema: Molecular and Cellular Biology|publisher=Caister Academic Press|id= ISBN 978-1-904455-10-3]

Archaea

One of the hallmarks of living systems is their ability to use favorable redox reactions in the conversion of energy to forms that are useful to the cell. Microbes in the kingdom archaea contain many unique redox enzymes possibly because of the wide range of strategies they employ for energy conversion, the many extreme environments they inhabit, and the evolutionary separation of the archaea from bacteria that catalyze similar reactions. Sulphur metabolism is the subject of much reearch in archaea, including both sulphur oxidation and reduction and the hydrogenases frequently associated with sulphur reduction.cite book | author = Blum P (editor). | title = Archaea: New Models for Prokaryotic Biology | publisher = Caister Academic Press | year = 2008 | id = ISBN 978-1-904455-27-1 ]

Fungi

Fungi metabolize inorganic sulphate to make sulphur-containing organic compounds. Fungi have a sulphate assimilation pathway which transports sulphate into the cell, activates it with ATP and reduces it to sulphide which is a direct precursor of cysteine. When cysteine or methionine is available in the environment, the energy-consuming sulphate assimilation pathway is shut off by the sulphur metabolite repression system as sulphate assimilation is not required. All fungi have the same sulphate assimilation pathway but fungi differ in the organisation of sulphur amino acids metabolism and some species have alternative pathways of cysteine synthesis. All fungi can synthesize methionine from cysteine but only some can metabolize methionine to cysteine.

Plants

Sulphur is an essential element for the growth and physiology of plants. Sulphur metabolism varies between plant species. Sulphate taken up by the roots is the major source for growth; it is reduced to sulphide and then can be metabolized further and incorporated into cysteine. Cysteine is the precursor of most other organic sulphur compounds in plants.

References