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 S_8(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 SSNO⁻in 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 (Δ_fG), of both organic and inorganic sulfur species, including some NO-containing compounds
    - From electrode potentials, Δ_fG 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