Thiocyanate Pathway Map

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This pathway was contributed by Brian Brazeau, completed by Jun Ouyang, University of Minnesota.

Thiocyanate is one of the major constituents of waste water from factories for the gasification of coal, where various by-products are formed during the production of gas for fuel, coke, and substances for chemical industries. Cyanide is usually converted to thiocyanate by the addition reaction of sulfur since thiocyanate is less toxic than cyanide. The resultant thiocyanate is then treated to an activated sludge process, where microbes degrade this substance.

Thiobacillus thioparus, which is a chemolithoautotrophic sulfur bacterium, can obtain energy by degradation of thiocyanate. Thiocyanate hydrolase, a newly found enzyme from Thiobacillus thioparus catalyzes the conversion of thiocyanate to carbonyl sulfide and ammonia. A model for the sequential reaction steps for thiocyanate hydrolase was presented, which involves four intermediates (Katayama et al., 1992).

Carbonyl sulfide can be reduced by nitrogenase to carbon monoxide in Azotobacter vinelandii. The FeMo cofactor of nitrogenase's molybdenum-iron protein was proposed to participate in this reaction, which may involve two intermediates (Seefeldt et al., 1995).

In addition, carbon disulfide can be oxidized to carbonyl sulfide by an oxygenase in P. denitrificans. This process could be analogous to that seen in rat liver, in which carbon disulfide is oxidized by a P-450-containing NADPH-monoxygenase to produce monothiocarbonate (hydrated COS), rather than free COS itself, and "active sulfur" [S].

Carbon monoxide can be oxidized to carbon dioxide by methane monooxygenase in Methylococcus capsulatus, by carbon monoxide oxygenase in Pseudomonas aeruginosa, and by carbon monoxide dehydrogenase in many microbes.

Thiocyanate can also be utilized by the heterotrophic bacterium 26B. Thiocyanate is hydrolyzed to cyanate and sulfide by an inducible enzyme in reaction A. Cyanate is hydrolyzed further to carbon dioxide and ammonia by cyanase. Cyanase activity in isolate 26B is also inducible and has been shown to hydrolyze cyanate under anaerobic conditions. Sulfide is oxidized to produce tetrathionate via the formation of thiosulfate. The ability to convert thiosulfate to tetrathionate is constitutive in reactions B and C. No further oxidation of tetrathionate occurred in a variety of heterotrophic bacteria. However, in T. tepidarius, thiosulfate can be oxidized to sulfate with the obligatory formation of tetrathionate as an intermediate.

For other organosulfide metabolisms, see Dimethyl Sulfoxide & Organosulfide Cycle pathway.

The following is a text-format Phenanthrene pathway map. Organisms which can initiate the pathway are given, but other organisms may also carry out later steps. Follow the links for more information on compounds or reactions.

                    Graphical Map (7K)             Graphical Map (5K)
         |--------------------------------------| |-----------------|
           Thiocyanate       Carbon disulfide         Thiocyanate
           Paracoccus            Isolate 26B    
          denitrificans             | 
             THI 115                |                      |
                |                   |                      |
                |                   |                      | A
                | thiocyanate       | carbon               | 
                | hydrolase         | disulfide            |    
                |                   | oxygenase            |
                |                   |              +-------+-------+                   
                |                   |              |               |
                +---------+---------+              v               v
                          |                     Cyanate    +    Sulfide
                          |                        |               |
                          |                        |               |
                          v                        | cyanase       | 
                  Carbonyl sulfide                 |               | B
               Azotobacter vinelandii              |               | 
                          |                        |               | 
                          |                        v               v
                          | carbonyl             Carbon       Thiosulfate
                          | sulfide              dioxide           |
                          | nitrogenase                            |
                          |                                        |
                          |                                        | C
                          |                                        |
                          v                   from the             |                                     
                   Carbon monoxide <--------- Trichloroethylene    |
               Methylococcus capsulatus       Pathway              |  
                Pseudomonas aeruginosa                             v
                          |                                  Tetrathionate
                          | methane monooxygenase or
                          | carbon monoxide oxygenase or
                          | carbon monoxide dehydrogenase

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Page Author(s): Jun Ouyang

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