Sulfur is metabolized by all organisms, from bacteria and archaea to plants and animals. Sulfur is reduced or oxidized by organisms in a variety of forms. The element is present in proteins, nucleic acids, sulfate esters of polysaccharides, steroids, phenols, and sulfur-containing coenzymes.
Reduced sulfur compounds are oxidized by most organisms, including higher animals and higher plants. Some organisms can conserve energy (i.e., produce ATP) from the oxidation of sulfur. Sulfur is the sole energy source for some lithotrophic bacteria and archaea. Reduced sulfur compounds, such as hydrogen sulfide, elemental sulfur, sulfite, thiosulfate, and various polythionates (e.g., tetrathionate), are used by various lithotrophic bacteria and are all oxidized by Acidithiobacillus.
Sulfur oxidizers utilize enzymes such as sulfur oxygenase and sulfite oxidase to oxidize sulfur compounds to sulfate. Lithotrophs that can produce sugars through chemosynthesis make up the base of some food chains. Food chains have formed in the absence of sunlight around hydrothermal vents, which emit hydrogen sulfide and carbon dioxide. Chemosynthetic archaea use hydrogen sulfide as an energy source for carbon fixation, producing sugars.
Sulfur reduction occurs in plants, fungi, and many bacteria. Sulfate can serve as an electron acceptor in anaerobic respiration and can also be reduced for the formation of organic compounds. Sulfate-reducing bacteria reduce sulfate and other oxidized sulfur compounds, such as sulfite, thiosulfate, and elemental sulfur, to sulfide.
Sulfate reduction can be dissimilatory or assimilatory. Sulfate reduction by sulfate-reducing bacteria, for example, is dissimilatory; the purpose of reducing the sulfate is to produce energy, and the sulfide is excreted. Dissimilatory sulfate reduction utilizes the enzymes ATP sulfurylase, APS reductase, and sulfite reductase. In assimilatory sulfate reduction, however, the sulfate is assimilated, or incorporated into organic compounds. In some organisms (e.g., gut flora, cyanobacteria, and yeast), assimilatory sulfate reduction is a more complex process that makes use of the enzymes ATP sulfurylase, APS kinase, PAPS reductase, and sulfite reductase.
Sulfur use by plants and animals
Plants take up sulfate in their roots and reduce it to sulfide (see sulfur assimilation). Plants are able to reduce APS directly to sulfite (using APS reductase) without phosphorylating APS to PAPS. From the sulfide they form the amino acids cysteine and methionine, sulfur lipids, and other sulfur compounds. Animals obtain sulfur from cysteine and methionine in the protein that they consume.
Sulfur is the third most abundant mineral element in the body. The amino acids cysteine and methionine are used by the body to make glutathione. Excess cysteine and methionine are oxidized to sulfate by sulfite oxidase, eliminated in the urine, or stored as glutathione (which can serve as a store for sulfur). The lack of sulfite oxidase, known as sulfite oxidase deficiency, causes physical deformities, mental retardation, and death.
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