4S Pathway

Rhodococcus sp. Strain KT462 This was the first strain discovered to be capable of BT and DBT desulfurization. The pathway of BT desulfurization is similar to that in Paenibacillus All-2 and Sinorhizobium sp. KT55 and that for DBT is the 4S pathway [37], The substrate specificity of this organism is different from R. erythropolis KA2-5-1 and Paenibacillus sp. All-2. It is able to desulfurize 2-methyl BT, 3-methyl BT, 5-methyl BT, 7-methyl BT, 1-methyl DBT, 3-methyl DBT, and 4-methyl DBT in addition to BT and DBT. 

The possibility of producing certain value-added compounds such as surfactants, which can be derived from intermediates produced in petroleum biodesulfurization processes, has been evaluated. HPBS is a molecule with amphiphilic characteristics desirable for surfactant applications [243], Several oxidation reactions, from the 4S pathway are considered before reaching the final product. The compounds of the invention include acyloxybiphenylsulfinates, acyloxybiphenylsulfonates, alkyl sulfinatobiphenyl ethers, and alkyl sulfonatobiphenyl ethers. The invention also provides methods of producing these compounds. 

Another Pseudomonas strain P. delafieldii R-8 was reported to remove 90.5% sulfur from highly desulfurized diesel oil [259], The biocatalyst achieved desulfurization via a pathway similar to the 4S pathway. The rate of desulfurization was reported to be 11.25 mmol sulfur/kg dcw/h, with the sulfur being reduced from 591 to 56 mg/L. This was achieved via two biocatalyst treatments lasting 20 hours each, although the biocatalyst was active only for first 6h in each treatment. Up to C4-DBTs were reported to be removed. Almost 100% of Q and C2 DBTs were removed and about 94% C3 DBTs and 97% C4 DBTs were removed. This strain of Pseudomonas thus appears to have a mechanism to uptake up to C4 DBTs through its cell membrane. 

The specificity of several of the enzymes identified in the 4S pathway of different organisms has been studied. In case of the DBT desulfurizing enzymes, little difference is expected in the specificity of the enzymes, say DszA, from different Rhodococcus strains found to date. This is essentially because the DNA sequence for the enzymes investigated so far has been the same. The difference in the specificity observed with whole cell assays is essentially due to the differences in substrate intake via the cell membrane and not necessarily due to a difference in the intrinsic enzyme specificity. It has been found that while isolated enzyme DszC (from KA2-5-1) can desulfurize up to 4,6 dipropyl DBT, whole cells cannot, indicating substrate transport as limiting factor. 

It has been claimed that biodesulfurization via the 4S pathway can also reduce viscosity. An application introduced by Environmental Bioscience Corporation included a method for reducing the viscosity of liquid-containing sulfur heterocycles [406] using biodesulfurization. The original patent application (Ser. No. 631642) was filed in US... 

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... 

Figure 11 shows the general steps in the biodesulfurization system based on studies of various bacterial species. " Attention has been given to the microbial chemical pathway which can remove sulfur from substituted dibenzothiophenes with alkyl groups that hinder chemical catalysis and that resist removal by hydrodesulfiirization. Monticello recently outlined sulfiir-specific Cx-DBT metabolism via the hydrocarbon-conserving (4S) pathway first proposed by Campbell and Kee Rhee The activity has been observed in many species of bacteria since the first confirmed isolation by Kilbane in 1988 In most cases, the bacteria have been closely related and catalyze the same reaction. 

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