Signaling by sulfur-containing molecules. Quantitative aspects.

Willem H. Koppenol and Patricia L. Bounds, Archives of Biochemistry and Biophysics, online September 23, 2016

There is currently interest in sulfur-containing molecules that may or may not play a role in signaling. We have collected relevant thermodynamic data, namely standard Gibbs energies of formation and electrode potentials at pH 7, and used these to construct a Frost diagram. Thermodynamic data not available in the literature could be estimated with reasonable confidence. At pH 7, the electrode potential of the RSS/RSS couple is +0.68 V, 0.28 V less than that of the RS, H+/RSH couple. S2 is unstable with respect to HSS and S2. Generally, polysulfur compounds, with the exception of RSSR, are thermodynamically unstable with respect to disproportionation and ultimately lead to formation of kinetically inert S8(s). About thermoneutral is the formation of RSS- from RSSR and HS, but formation of HNO from HS and SNO, and from HS and RSNO, is unfavorable. The formation of SSNOin vivo is kinetically unlikely.

The scientific and technical impact of the study can be summarized as:

        • An oxidation state – or Frost – diagram is used to determine the electrode potentials, from which can be calculated the Gibbs energies of formation (ΔfG), of both organic and inorganic sulfur species, including some NO-containing compounds
        • From electrode potentials, ΔfG values in water for hydridodisulfide and HNO formation, as well as the ΔG for the transnitrosation of cysteine have been estimated
        • Reactions of RSS with RSSH, which would yield either RSSS and RSH or RSSSR and H2S, are chemically interesting but of limited physiological relevance
        • A role for SSNO in signaling is ruled out
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