Sulfur is an element essential to life. Sulfur-containing amino acids maintain the secondary, tertiary, and quaternary structures of proteins via disulfide linkages. Many enzymes are inhibited when treated with reagents that destroy sulfhydryl groups. The sulfhydryl groups are also involved in binding of substrates to enzymes. Reduced forms of S can serve as energy sources or source of reducing power for some prokaryotes. Oxidized forms, especially sulfate, can serve as terminal electron acceptors during anaerobic respiration.
Carbon-bonded S is mineralized through various pathways: (1) direct aerobic mineralization during the oxidation of C as an energy source, (2) anaerobic mineralization of organic matter (desulfurization), (3) incomplete oxidation of organic S into inorganic S compounds, (4) biological oxidation of H2S to sulfate via elemental S and sulfite, (5) biological oxidation of tetrathionate to sulfate via sulfide, (6) hydrolysis of cysteine by cysteine desulfhydrolase, and (7) indirect (enzymatic) mineralization when sulfate esters are hydrolyzed by sulfatases (Lawrence, 1987). The hydrolysis of ester sulfates occurs by splitting the O-S bond, through the action of sulfatase enzymes:
There are numerous sulfatases, characterized by high specificity. They include aryl-sulfatases and choline sulfatases, whose production is repressed in the presence of available SO43. The sulfatases are bound to the cell walls of fungi and gram-positive bacteria, while in gram-negative bacteria, they are found in the periplasm.
Inorganic S is usually assimilated into organic compounds as SO4- by plants and most microorganisms. This involves a series of enzymatic reactions called assimila-tory SO4- reduction. In the first step, a permease enzyme participates in the transfer of SO4- across the cell membrane. This step requires the input of energy via ATP to form adenosine 5'-phosphosulfate (APS) and is catalyzed by ATP sulfurylase:
Another ATP is then used to form PAPS using APS kinase:
From here, two pathways can be used to form cysteine. In one, reductive enzymes form the unstable intermediate SO3-. This is reduced by NADH to HS-, which reacts with serine to produce cysteine:
SO2- + 7H+ + 6e- SO32-redtictase,NiADH : HS- + 3H2O
This sequence has been observed in bacteria such as B. subtilis, Staphylococcus aureus, and Enterobacter aerogenes and the fungus A. niger. In the second pathway, glutathione is used to transfer a S group from APS to O-acetylserine to form cysteine:
serine + acetyl-SCoA ^ O-acetylserine + CoASH, O-acetylserine + H2S ^ cysteine + acetate + H+.
This latter sequence has been observed in Escherichia coli and Salmonella typhimurium. The presence of available cysteine regulates the SO4- permease and the enzymes involved early in this pathway. This ensures that these energy-consuming reactions occur only in the absence of available S. The other S-containing amino acid (methionine) is produced via the aspartate family of amino acids and trans-sulfuration, leading to homocysteine, which is then converted to methionine. In bacteria, methio-nine is synthesized from cysteine.
Microbial decomposition of plant residues with C:S ratios >400:1 (~0.1% S) results in immobilization of S. At C:S ratios <200:1 (0.2% S), S is released into the environment. Sulfur mineralization is less highly correlated with the degradation of C than is N mineralization. The proportion of HI-reducible and C-bonded S is not a good indicator of potentially mineralizable S, nor is the activity of the enzyme arylsulfatase.
In the presence of available electron acceptors, reduced forms of S are oxidized by both chemical and microbial pathways:
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