Sulfur dioxide-sulfate pathway

For each of the three precursors of hydrogen sulfide, i.e. sulfur dioxide/sulfite, sulfate, and cysteine, a different biosynthetic pathway has been established. Figure 1 gives an overall view of these three pathways a suggestion for a path of synthesis of hydrogen sulfide from carbonyl sulfide is included. 

Sulfate. As for the production of hydrogen sulfide from sulfur dioxide/sulfite at least three possible pathways for the light-dependent synthesis of hydrogen sulfide in response to sulfate can be assumed, i.e. first the light-dependent reduction of sulfate to carrier-bound sulfide followed by a release of the sulfide moiety from its carrier second the light-dependent reduction of sulfate to carrier-bound sulfide followed by an incorporation of the sulfide moiety into cysteine and subsequent degradation of cysteine third the release of sulfite from carrier-bound sulfite followed by reduction of free sulfite to sulfide (see Figure 1). 

Sulfite and sulfate both can be metabolized by plant tissue. Sulfite can be oxidized to sulfate, and this ability may be correlated with resistance (7). On the other hand, sulfate can be reduced all the way to sulfide (8). Sulfur dioxide is generally considered to generate a reducing type of air pollution, but in terms of the latter pathway of metabolism it should... 

Atmospheric reactions modify the physical and chemical properties of emitted materials, changing removal rates and exerting a major influence on acid deposition rates. Sulfur dioxide can be converted to sulfate by reactions in gas, aerosol, and aqueous phases. As we noted in Chapter 17, the aqueous-phase pathway is estimated to be responsible for more than half of the ambient atmospheric sulfate concentrations, with the remainder produced by the gas-phase oxidation of S02 by OH (Walcek et al. 1990 Karamachandani and Venkatram 1992 Dennis et al. 1993 McHenry and Dennis 1994). These results are in agreement with box model calculations suggesting that gas-phase daytime S02 oxidation rates are l-5% per hour, while a representative in-cloud oxidation rate is 10% per minute for 1 ppb of H202. 

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