Sulfur 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-sulfur clusters. These compounds are involved in a number of essential cellular processes such as protein biosynthesis or the transfer of electrons and acyl groups. Sulfur, therefore, is an essential component of all living cells. The importance of sulfur is well-represented by the sulfhydryl (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, sulfur containing molecules function as messengers in intracellular and intra-species communication. Sulfur is also a constituent of many other biomolecules like cysteine, methionine, biotin, lipoic acid, molybdopterin, thionucleosides in tRNAs, and thiamine.
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Sulfur 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 sulfur metabolic pathways has been greatly facilitated by the emerging information drawn from genome sequencing.[1]
Sulfur 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.[2]
An external supply of sulfur-containing compounds is essential for many eukaryotes and, due to their scarcity in many foods and feeds, their biosynthesis is of great industrial interest. Therefore, the metabolism of sulfur in Corynebacterium glutamicum has been studied. Besides the pathways to obtain and utilize sulfur from the environment, the reactions leading to and from the sulfur-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 sulfur 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.[3]
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 sulfur-containing compounds from amino acid metabolism by the enzyme cystalysin is cytotoxic and may be considered one of the virulence determinants of T. denticola.[4]
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 domain 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. Sulfur metabolism is the subject of much research in archaea, including both sulfur oxidation and reduction and the hydrogenases frequently associated with sulfur reduction.[5]
Fungi metabolize inorganic sulfate to make sulfur-containing organic compounds. Fungi have a sulfate assimilation pathway which transports sulfate into the cell, activates it with ATP and reduces it to sulfide which is a direct precursor of cysteine. When cysteine or methionine is available in the environment, the energy-consuming sulfate assimilation pathway is shut off by the sulfur metabolite repression system as sulfate assimilation is not required. All fungi have the same sulfate assimilation pathway but fungi differ in the organisation of sulfur 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.
Sulfur is an essential element for the growth and physiology of plants with sulfur assimilation varying between plant species. Sulfate taken up by the roots is the major source for growth; it is reduced to sulfide and then can be metabolized further and incorporated into cysteine. Cysteine is the precursor of most other organic sulfur compounds in plants.