Sulfur-specific pathway

The second Mycobacterium strain capable of DBT desulfurization was M. phlei WU-F1 [30], This strain was also reported to desulfurize naphtho[2,l-b]thiophene (NTH) and 2-ethyl-NTH to sulfur free products with the following intermediates for the latter molecule 2-ethyl-NTH sulfoxide, l-(2 -hydroxynaphthyl)-l-butene, and l-naphthyl-2-hydroxy-1-butene [94], Thus, this organism was reported to consist of a sulfur-specific pathway capable of desulfurization of broad range of sulfur compounds including symmetric and asymmetric molecules. 

The sulfur-specific pathway for desulfurization of benzothiophene (BT) has been reported in Gordonia sp. Strain 213E. The metabolites of BT conversion were determined by ethylacetate extraction of the culture medium followed by GC-MS analysis [33,34], The reaction mechanism was proposed to be very similar to that of DBT for the first two steps (Fig. 4) however, the third step was quite different. 

Gallagher, J. R. Olson, E. S., and Stanley, D. C., Microbial Desulfurization of Dibenzoth-iophene A Sulfur Specific Pathway. Ferns Microbiology Letters, 1993. 107 pp. 31-36. 

Characterization of the Sulfur-Specific Pathway. The enzymatic pathway for oxidative desulfurization of DBT and related organosulfur compounds has proven to involve unique biochemistry. 

Previous work (70) on the sulfur-specific pathway of a related organism revealed that NADH enhanced the conversion of DBT to HBP in cell-free extracts. The conversion of HBPSi" to HBP by DszB was not NADH dependent, and it was proposed that reaction was hydrol3d ic in nature, involving a nucleophilic attack of a base-activated water molecule on the sulfinate sulfur. On the basis of the accumulated information, they postulated that the overall reaction for the conversion of DBT to HBP is as follows ... 

The catalytic activities and substrates of the oxidative sulfur-specific pathway enzymes are unique, yet related enzymes and catalytic mechanisms can be found in a variety of enzymes operating on different substrates. 

Better understanding of the mechanism of biodesulfurization, as shown in Figure 8, may be gained from some recent studies ° Gallagher et al. reported a sulfur-specific pathway in microbial desulfurization of DBT. Rhodococcus rhodochrous strain IGTS8 metabolizes DBT in a sulflir-specific manner. Two routes of desulfurization have been identified. Under growth conditions, the intermediates are dibenzothiophene sulfoxide, dibenzothiophene sulfone, 2 -hydroxybipheny 1-2-sulfonate, and 2,2 -... 

Rhodococcus sp. Strain WU-K2R A Rhodococcus strain capable of sulfur-specific desulfurization of benzothiophene, naphthothiophene (NT), and some of their alkyl derivatives was reported [35]. The metabolites of BT desulfurization were BT sulfone, benzo[c][l,2]oxanthiin S-oxide, benzo[c][l,2]oxanthiin S,S-dioxide, o-hydroxystyrene, 2,(2 -hydroxyphenyl)ethan-l-al, and benzofuran. The NT metabolites were NT sulfone, 2 -hydroxynaphthyl ethene, and naphtho[2,l-b]furan [35], The exact biochemical pathway was not determined, however, part of the pathway for BT desulfurization was speculated to be similar to Paenibacillus All-2. 

A procedure for immobilization of a P. stutzeri UP-1 strain using sodium alginate was reported [133], This strain does not perform sulfur-specific desulfurization, but degrades DBT via the Kodama pathway. Nevertheless, the report discussed immobilization of the biocatalyst cells in alginate beads with successful biocatalyst recovery and regeneration for a period of 600 h. However, the immobilized biocatalyst did decrease in specific activity, although the extent of loss was not discussed. The biocatalyst was separated after every 100 h of treatment, washed with saline and a boric acid solution and reused in subsequent experiment. The non-immobilized cells were shown to loose activity gradually with complete loss of activity after four repeat runs of 20 hour each. The report does not mention any control runs, which leaves the question of DBT disappearance via adsorption on immobilized beads unanswered and likewise the claim of a better immobilized biocatalyst. 

In a patent on biological desulfurization [100] of petroleum/coal, only the use of whole cell biocatalysts was claimed. The biocatalysts included microorganisms belonging to the genus Pseudomonas, Flavobacterium, Enterobacter, Aeromonas, Bacillus or Corynebacterium. The desulfurization pathway (sulfur-specific vs. destructive) was not specified. The Japanese patents No. JP2071936C and JP7103379B seem to be equivalent patents. 

Since the isolation of IGTS8, many other Rhodococcus as well as Mycobacterium strains capable of sulfur-specific desulfurization via the 4S pathway have been isolated. Genetic analysis of some of these strains has shown that the dsz genes are almost identical in all these strains however, the strains still differ in their rate of desulfurization. It has been realized that this is due to the difference in non-desulfurizing traits of the strains. These traits are mostly physiological differences between the strains. These parameters play a secondary role in determining the rate of desulfurization in these strains. These include the ability to emulsify the oil phase, solvent tolerance and resistance to various... 

Plants absorb Se from soil primarily as selenate and translocate it to the chloroplast, where it follows the sulfur assimilation pathway. Se is reduced (enzymatically and non-enzymatically) to selenide, which reacts with serine to form selenocysteine (76). It can be further metabolized to selenomethionine (79) and methylated to form products such as. Se-methyl selenomethionine (89). Alternatively, selenocysteine-specific methyl transferase may form -methyl selenocysteine (83), allowing the plant to accumulate extraordinarily large amounts of Se. 

Sulfur-specific desulfurization of DBTs and other organosulfur compounds is best characterized in the bacterial genus Rhodococcus and exemplified by R. erythropolis strain IGTS8 and involves a series of oxidations of the sulfur moiety followed by a hydrolytic release of sulfite. This and related pathways have been shown to desulfurize a wide range of DBTs, BTHs, and sulfides. Moreover, deep desulfurization to low ppm levels of sulfur has been demonstrated with a variety of hydrotreated diesel range oils. 

Although sulfate is formed by hydrolysis of both alkyl and aryl sulfates, the pathway of degradation for aryl sulfates is controlled by the source of sulfur (Cook et al. 1999). The complex issues surrounding the hydrolysis of sulfate esters have been discussed (Kertesz 1999), and are illustrated by the number and substrate specificity of alkyl sulfatases for the surfactant 2-butyloctyl sulfate in Pseudomonas sp. strain AE-A (Ellis et al. 2002). 

Pseudomonas has been disclosed [233], The method consists of mutating the genes by UV irradiation or other methods and separating the desired protein. A specific amino acid sequence, present in that protein, shows the function for regulating the expression of benzothiophene oxidase gene. The fact that the protein is thought to be useful for both, desulfurization and for purification of sulfur contaminated soil or waste waters indicates a probable destructive pathway. 

Engineered biocatalysts with altered specificity have been developed. A Rhodococcus strain capable of DBT as well as BT desulfurization has been developed by cloning dsz genes into a strain-containing BT desulfurization pathway. The variety of sulfur compounds in petroleum feedstocks and complexity of the problem may require use of a consortium rather than a single bacterial strain. Alternately, use of multiple bioreactors each with a single dominant strain may be employed to achieve maximum desulfurization [299],... 

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